Lozenge was initially identified by mutation caused by a P-element insertion in the X chromosome. Because the P-element contained two copies of the sevenless enhancer, DNA adjacent to the site of insertion was expressed in cells eye disc precursor cells normally expressing sevenless (R7, the R3/R4 pair and cone cell precursors). The P-element caused a dominant mutant phenotype resembling loss-of-function mutations of seven-up. Consequently the dominant mutant was called Sprite. In Sprite/+ heterozygotes, 72% of the ommatidia show transformation of R4 into an R7 cell, and in 10% of ommatidia, both R3 and R4 become converted. The phenotype is more extreme in Sprite mutant homozygotes. One explanation for the mutant phenotype caused by the insertion is that the adjacent DNA codes for a protein that represses seven-up. A null mutation was used to test whether lozenge regulates seven-up. Whereas svp is normally expressed in the R1/R6 pair and the R3/R4 pair, in lozenge mutant flies, svp is expressed in R7 and the four cone cell precursors as well. It has been concluded that LZ negatively regulates svp in R7 cells, and in cone cells. In the absence of lozenge each of these cells develop an R7 fate. This transformation is partially dependent on the functioning of sevenless (Daga, 1996).

Lozenge is implicated in the regulation of Bar proteins, specifically required to specify R1/R6 cell fate. The expression of Bar in R1/R6 cells is dramatically reduced but not completely eliminated in lz mutants. The antibody used to detect Bar is raised against BarH1. Upon lz overexpression, Bar expression is no longer restricted to R1/R6, but ectopically staining cells are consistently detected in the developing cluster (Daga, 1996).

lozenge mutants do not express the two Bar genes, and the enhancer-trap O32 (associated with an unknown gene specific to cells R3/4 and R7) is expressed in too many cells. Thus the defective recruitment that occurs in lozenge mutants can be attributed to abnormalities in the expression of genes like Bar, the gene marked by O32, and seven-up, which are essential for establishing the correct cell fate for the final three photoreceptor cells, R1, R6 and R7. seven-up is derepressed in R7 cells in lozenge mutants. The derepression of seven-up is reminiscent of the derepression of svp in rough mutants. rough normally represses svp in R3/R4. Thus Lozenge both actively represses some genes and activates others (Crew, 1997).

Friend of GATA (FOG) proteins regulate GATA factor-activated gene transcription. During vertebrate hematopoiesis, FOG and GATA proteins cooperate to promote erythrocyte and megakaryocyte differentiation. The Drosophila FOG homolog U-shaped (Ush) is expressed similarly in the blood cell anlage during embryogenesis. During hematopoiesis, the acute myeloid leukemia 1 homolog Lozenge and Glial cells missing are required for the production of crystal cells and plasmatocytes, respectively. However, additional factors have been predicted to control crystal cell proliferation. Ush is expressed in hemocyte precursors and plasmatocytes throughout embryogenesis and larval development, and the GATA factor Serpent is essential for Ush embryonic expression. Furthermore, loss of ush function results in an overproduction of crystal cells, whereas forced expression of Ush reduces this cell population. Murine FOG-1 and FOG-2 also can repress crystal cell production, but a mutant version of FOG-2 lacking a conserved motif that binds the corepressor C-terminal binding protein fails to affect the cell lineage. Using an antibody directed against Ush synthetic peptides, Ush protein was detected in an expression pattern similar to that of the gene transcript. Around embryonic stage 8, both ush RNA and protein can be detected in blood cell precursors. By stage 10, Ush-positive hemocyte precursors have spread throughout the lateral and ventral head mesoderm. As embryogenesis progresses, Ush is detected in stage 13 plasmatocytes migrating throughout the head mesoderm and down the ventral midline. During the late stages of embryogenesis, Ush continues to be expressed in plasmatocytes circulating throughout the embryonic hemolymph. lz expression in crystal cells is detected first during stage 10 and is maintained in this lineage until the late stages of embryogenesis. Fluorescent antibody staining and confocal microscopy were used to determine whether Ush and lz are coexpressed in the crystal cell lineage. To detect lz expression in hemocyte precursors and crystal cells, the expression of a UASlacZ reporter gene driven by lzGal4 (lzlacZ) was monitored. This reporter is active in hemocyte precursors as early as stage 10 and is expressed in the crystal cell lineage throughout embryogenesis. During embryonic stage 10, a number of hemocyte precursors express both Ush and lz. Later, during stage 13, the number of cells that expressed both lz and Ush decreases. Finally, during the late stages of embryogenesis, Ush is not detected in crystal cell lineage, evidenced by its failure to colocalize with the lzlacZ crystal cell marker. These results are consistent with a role for ush as a repressor of crystal cell production and suggest that ush expression is down-regulated in hemocyte precursors during crystal cell lineage commitment (Fossett, 2001).

To demonstrate further that Ush represses crystal cell production, Ush was expressed in crystal cells by using the Gal4/UAS binary system. The lzGal4 driver was used to express UASUsh in crystal cells, and their production was monitored by using the lzlacZ marker. Embryos with forced expression of Ush in crystal cells have a significant reduction in the number of these cells. Compared with similarly staged wild-type controls, UASUsh stage 13 and 16 embryos had a 30% and 85% reduction in number of crystal cells, respectively. A sample of 40 stage 13-16 UASUsh embryos averaged a 30% reduction in the number of crystal cells compared with wild-type controls. The phenotype of individual embryos within this population ranged from being completely devoid of crystal cells to wild-type cell numbers. These results indicate that down-regulation of ush during crystal cell lineage commitment is required for development of these cells. Together with the observed increase in crystal cell number in ush loss-of-function assays, these findings suggest that Ush functions during hematopoiesis to limit the number of hemocyte precursors that enter the crystal cell lineage (Fossett, 2001).

In conclusion, Ush and Lz function antagonistically during crystal cell lineage commitment and Ush is required to limit the overproliferation of crystal cells. This demonstrates a possible intersection between the FOG and AML-1 gene pathways, which may prove important for understanding vertebrate hematopoiesis. Furthermore, this study expands the molecular characterization of the earliest events of hematopoiesis in Drosophila, identifying additional conserved genes that establish the fly as a model organism for hematopoiesis (Fossett, 2001).

Larval and Pupal

A P insertion into the lozenge gene, P-GawB, gives no overt lozenge phenotype and results in expression of the yeast GAL4 transcription factor under the control of lozenge promoter or enhancer elements. As development proceeds, reporter expression becomes evident in photoreceptor neurons R1/6 and later R7, and is detectable in axons from these cells, which grow into the brain. Expression is also present in the cone and pigment cells as they develop in the pupae. Another insertion element, lozenge1ArB1, appears to cause a severe cistron A phenotype. In strains carrying this insertion, ß-galactosidase expression occurs at low levels within the eye disc, with stronger expression evident along the posterior edge in what appears to be developing cone cells. There is also expression in basal glia, which are different from retinal glia, detected by an anti-repo serum. These glia may correspond to the subretinal glia. Expression is also seen within the developing optic ganglia of the brain in cells corresponding to the marginal and epithelial glia of the lamina and medulla. Altered glial expression of lozenge could account for some of the axon path finding and fasciculation defects in lozenge mutants. The different expression patterns of P-GawB and lozenge1ArB1 begin to unravel the complex basis of lozenge expression and the physical basis of the genetically defined multicistronic nature of lozenge (Crew, 1997).

In the developing Drosophila eye, individual cell fates are specified when general signaling mechanisms are interpreted in the context of cell-specific transcription factors. Lozenge, a Runt/AML1/CBFA1-like transcription factor, determines the fates of a number of neuronal and non-neuronal cells by regulating the expression of multiple fate-determining transcription factors. The Lozenge protein is expressed in the nuclei of the cells that it patterns and also in their undifferentiated precursors. Lz expression is visible basally in the nuclei of all undifferentiated cells posterior to the furrow. This is in contrast to the undifferentiated cells anterior to the furrow, in which Lz is not expressed. Immunolocalization of Lz and Decapentaplegic shows that Lz expression is initiated at the posterior edge of the morphogenetic furrow but not within it. An apical view reveals that Lz is also expressed in three cells within each ommatidium in positions consistent with their being R1/R6 and R7, as well as in cone cells. At 20-30 hours after puparium formation (APF), Lz expression is seen in primary pigment cells and, at 30-40 hours APF, Lz is expressed in secondary and tertiary pigment cells. At this late stage, Lz ceases to be expressed in the other cells of the eye disc. All cells that express Lz share the property that they arise from the synchronous round of cell division at the furrow. However, when this mitosis is prevented by ectopic expression of the human cell-cycle inhibitor p21 CIP1/WAF1, Lz is still expressed in a wild-type pattern, suggesting that Lz expression is not directly controlled by the cell cycle. The exclusion of Lz from photoreceptor cells R8, R2/R5 and R3/R4 is functionally important for proper cell fate specification. Indeed, when Lz is misexpressed in R3/R4 in the lz Sprite allele, the presumptive R3/R4 cells differentiate as R7s. This phenotype results from the repression of seven up transcription caused by the misexpression of Lz in R3/R4 (Flores, 1998).

amos is a new candidate Drosophila proneural gene related to atonal. Having isolated the first specific amos loss-of-function mutations, it has been shown definitively that amos is required to specify the precursors of two classes of olfactory sensilla. Unlike other known proneural mutations, a novel characteristic of amos loss of function is the appearance of ectopic sensory bristles in addition to loss of olfactory sensilla, owing to the inappropriate function of scute. This supports a model of inhibitory interactions between proneural genes, whereby ato-like genes (amos and ato) must suppress sensory bristle fate as well as promote alternative sense organ subtypes (zur Lage, 2003).

The transcription factor encoded by lozenge (lz) plays a number of roles in olfactory sensillum development, including activating amos expression. Mutants therefore show a loss of many amos-dependent sensilla. Interestingly flies mutant for both lz and amos (lz34; amos1/Df(2L)M36F-S6) have third antennal segments that bear only sensilla coeloconica, and so the ectopic bristles of amos mutants are dependent on lz function. Correlating with this, the expression of sc mRNA in the third antennal segment is much reduced in a lz mutant compared with wild type. Thus, lz appears at least partly responsible for the expression of sc in the antenna (zur Lage, 2003).

What does amos repress in the antenna? It appears that sc is expressed within the wild-type amos expression domain during olfactory SOP formation. Clearly amos must prevent the function of sc, since sc expression in ectoderm usually results in bristle specification. It may be significant that some of the sc RNA is in olfactory SOPs in the wild-type antenna, suggesting that the SOP may be a major location of repression by amos, as indicated by misexpression experiments. However, some bristle formation is maintained in ac/sc; amos mutants. This may be due to redundancy with other genes in the ASC: certainly wild-type bristle formation outside the antenna is not completely abolished in the absence of ac/sc. An alternative possibility is that some bristle SOPs result from other proneural-like activity in the antenna. Direct proneural activity of lz is a possibility, although misexpression of lz elsewhere in the fly (using a hs-lz construct) is not sufficient to promote bristle formation (zur Lage, 2003).

The Drosophila lymph gland as a developmental model of hematopoiesis

Drosophila hematopoiesis occurs in a specialized organ called the lymph gland. In this systematic analysis of lymph gland structure and gene expression, the developmental steps in the maturation of blood cells (hemocytes) from their precursors are defined. In particular, distinct zones of hemocyte maturation, signaling and proliferation in the lymph gland during hematopoietic progression are described. Different stages of hemocyte development have been classified according to marker expression and placed within developmental niches: a medullary zone for quiescent prohemocytes, a cortical zone for maturing hemocytes and a zone called the posterior signaling center for specialized signaling hemocytes. This establishes a framework for the identification of Drosophila blood cells, at various stages of maturation, and provides a genetic basis for spatial and temporal events that govern hemocyte development. The cellular events identified in this analysis further establish Drosophila as a model system for hematopoiesis (Jung, 2005).

In the late embryo, the lymph gland consists of a single pair of lobes containing ~20 cells each. These express the transcription factors Srp and Odd skipped (Odd), and each cluster of hemocyte precursors is followed by a string of Odd-expressing pericardial cells that are proposed to have nephrocyte function. These lymph gland lobes are arranged bilaterally such that they flank the dorsal vessel, the simple aorta/heart tube of the open circulatory system, at the midline. By the second larval instar, lymph gland morphology is distinctly different in that two or three new pairs of posterior lobes have formed and the primary lobes have increased in size approximately tenfold (to ~200 cells. By the late third instar, the lymph gland has grown significantly in size (approximately another tenfold) but the arrangement of the lobes and pericardial cells has remained the same. The cells of the third instar lymph gland continue to express Srp (Jung, 2005).

The third instar lymph gland also exhibits a strong, branching network of extracellular matrix (ECM) throughout the primary lobe. This network was visualized using several GFP-trap lines in which GFP is fused to endogenous proteins. For example, line G454 represents an insertion into the viking locus, which encodes a Collagen IV component of the extracellular matrix. The hemocytes in the primary lobes of G454 (expressing Viking-GFP) appear to be clustered into small populations within pockets or chambers bounded by GFP-labeled branches of various sizes. Other lines, such as the uncharacterized GFP-trap line ZCL2867, also highlight this branching pattern. What role this intricate ECM network plays in hematopoiesis, as well as why multiple cells cluster within these ECM chambers, remains to be determined (Jung, 2005).

Careful examination of dissected, late third-instar lymph glands by differential interference contrast (DIC) microscopy revealed the presence of two structurally distinct regions within the primary lymph gland lobes that have not been previously described. The periphery of the primary lobe generally exhibits a granular appearance, whereas the medial region looks smooth and compact. These characteristics were examined further with confocal microscopy using a GFP-trap line G147, in which GFP is fused to a microtubule-associated protein. The G147 line is expressed throughout the lymph gland but, in contrast to nuclear markers such as Srp and Odd, distinguishes morphological differences among cells because the GFP-fusion protein is expressed in the cytoplasm in association with the microtubule network. Cells in the periphery of the lymph gland make relatively few cell-cell contacts, thereby giving rise to gaps and voids among the cells within this region. This cellular individualization is consistent with the granularity of the peripheral region observed by DIC microscopy. By contrast, cells in the medial region were relatively compact with minimal intercellular space, which is also consistent with the smoother appearance of this region by DIC microscopy. Thus, in the late third instar, the lymph gland primary lobes consist of two physically distinct regions: a medial region consisting of compactly arranged cells, which was termed the medullary zone; and a peripheral region of loosely arranged cells, termed the cortical zone (Jung, 2005).

Mature hemocytes have been shown to express several markers, including collagens, Hemolectin, Lozenge, Peroxidasin and P1 antigen. The expression of the reporter Collagen-gal4 (Cg-gal4), which is expressed by both plasmatocytes and crystal cells, is restricted to the periphery of the primary lymph gland lobe. Comparison of Cg-gal4 expression in G147 lymph glands, in which the medullary zone and cortical zone can be distinguished, reveals that maturing hemocytes are restricted to the cortical zone. In fact, the expression of each of the maturation markers mentioned above is found to be restricted to the cortical zone. The reporter hml-gal4 and Pxn, which are expressed by the plasmatocyte and crystal cell lineages, are extensively expressed in this region. Likewise, the expression of the crystal cell lineage marker Lozenge is restricted in this manner. The spatial restriction of maturing crystal cells to the cortical zone was verified by several means, including the distribution of melanized lymph gland crystal cells in the Black cells background and analysis of the terminal marker ProPOA1. The cortical zone is also the site of P1 antigen expression, a marker of the plasmatocyte lineage. The uncharacterized GFP fusion line ZCL2826 also exhibits preferential expression in the cortical zone. Last, it was found that the homeobox transcription factor Cut is preferentially expressed in the cortical zone of the primary lobe. Although the role of Cut in Drosophila hematopoiesis is currently unknown, homologs of Cut are known to be regulators of the myeloid hematopoietic lineage in both mice and humans. Cells of the rare third cell type, lamellocytes, are also restricted to the cortical zone, based upon cell morphology and the expression of a msn-lacZ reporter (msn06946). In summary, based on the expression patterns of several genetic markers that identify the three major blood cell lineages, it is proposed that the cortical zone is a specific site for hemocyte maturation (Jung, 2005).

The medullary zone was initially defined by structural characteristics and subsequently by the lack of expression of mature hemocyte markers. However, several markers have been identified that are exclusively expressed in the medullary zone at high levels but not the cortical zone. Consistent with the compact arrangement of cells in the medullary zone, it was found that Drosophila E-cadherin (DE-cadherin or Shotgun) is highly expressed in this region. No significant expression of DE-cadherin was observed among maturing cells in the cortical zone. E-cadherin, in both vertebrates and Drosophila, is a Ca2+-dependent, homotypic adhesion molecule often expressed by epithelial cells and is a crucial component of adherens junctions. Attempts to study DE-cadherin mutant clones in the medullary zone where the protein is expressed were unsuccessful since no clones were recoverable. The reporter lines domeless-gal4 and unpaired3-gal4 are preferentially expressed in the medullary zone. The gene domeless (dome) encodes a receptor molecule known to mediate the activation of the JAK/STAT pathway upon binding of the ligand Unpaired. The unpaired3 (upd3) gene encodes a protein with homology to Unpaired and has been associated with innate immune function. These gal4 lines are in this study only as markers that correlate with the medullary zone and, at the present time, there is no evidence that their associated proteins have a role in lymph gland hematopoiesis. Other markers of interest with preferential expression in the medullary zone include the molecularly uncharacterized GFP-trap line ZCL2897 and actin5C-GFP. Cells expressing hemocyte maturation markers are not seen in the medullary zone. It is therefore reasonable to propose that this zone is largely populated by prohemocytes that will later mature in the cortical zone. Prohemocytes are characterized by their lack of maturation markers, as well as their expression of several markers described as expressed in the medullary zone (Jung, 2005).

The posterior signaling center (PSC), a small cluster of cells at the posterior tip of each of the primary (anterior-most) lymph gland lobes, is defined by its expression of the Notch ligand Serrate and the transcription factor Collier. During this analysis, several additional markers were identified that exhibit specific or preferential expression in the PSC region. For example, it was found that the reporter Dorothy-gal4 is strongly expressed in this zone. The Dorothy gene encodes a UDP-glycosyltransferase, which belongs to a class of enzymes that function in the detoxification of metabolites. The upd3-gal4 reporter, which has preferential expression in the medullary zone, is also strongly expressed among cells of the PSC. Last, three uncharacterized GFP-gene trap lines, ZCL2375, ZCL2856 and ZCL0611 were found, that are preferentially expressed in the PSC. This analysis has made it clear that the PSC is a distinct zone of cells that can be defined by the expression of multiple gene products (Jung, 2005).

The PSC can be defined just as definitively by the characteristic absence of several markers. For example, the RTK receptor Pvr, which is expressed throughout the lymph gland, is notably absent from the PSC. Likewise, dome-gal4 is not expressed in the PSC, further suggesting that this population of cells is biased toward the production of ligands rather than receptor proteins. Maturation markers such as Cg-gal4, which are expressed throughout the cortical zone, are not expressed by PSC cells. Additionally, the expression levels of the hemocyte marker Hemese and the Friend-of-GATA protein U-shaped are dramatically reduced in the PSC when compared with other hemocytes of the lymph gland. Taken together, both the expression and lack of expression of a number of genetic markers defines the cells of the PSC as a unique hemocyte population (Jung, 2005).

In contrast to primary lobes of the third instar, maturing hemocytes are generally not seen in the secondary lobes. Correspondingly, secondary lobes often have a smooth and compact appearance, much like the medullary zone of the primary lobe. Consistent with this appearance, secondary lymph gland lobes also express high levels of DE-cadherin. The size of the secondary lobe, however, varies from animal to animal and this correlates with the presence or absence of maturation markers. Smaller secondary lobes contain a few or no cells expressing maturation markers, whereas larger secondary lobes usually exhibit groups of differentiating cells. Direct comparison of DE-cadherin expression in secondary lobes with that of Cg-gal4, hml-gal4 or Lz revealed that the expression of these maturation markers occurs only in areas in which DE-cadherin is downregulated. Therefore, although there is no apparent distinction between cortical and medullary zones in differentiating secondary lobes, there is a significant correlation between the expression of maturation markers and the downregulation of DE-cadherin, as is observed in primary lobes (Jung, 2005).

The relatively late 'snapshot' of lymph gland development in the third larval instar establishes the existence of spatial zones within the lymph gland that are characterized by differences in structure as well as gene expression. In order to understand how these zones form over time, lymph glands of second instar larvae, the earliest time at which it was possible to dissect and stain, were examined for the expression of hematopoietic markers. As expected, Srp and Odd are expressed throughout the lymph gland during the second instar since they are in the late embryo and third instar lymph gland. Likewise, the hemocyte-specific marker Hemese is expressed throughout the lymph gland at this stage, although it is not present in the embryonic lymph gland (Jung, 2005).

To determine whether the cortical zone is already formed or forming in second instar lymph glands, the expression of various maturation markers were examined in a pair-wise manner to establish their temporal order. Of the markers examined, hml-gal4 and Pxn are the earliest to be expressed. The majority of maturing cells were found to be double-positive for hml-gal4 and Pxn expression, although a few cells were found to express either hml-gal4 or Pxn alone. This indicates that the expression of these markers is initiated at approximately the same time, although probably independently, during lymph gland development. The marker Cg-gal4 is next to be expressed since it was found among a subpopulation of Pxn-expressing cells. Finally, P1 antigen expression is initiated late, usually in the early third instar. Interestingly, the early expression of each of these maturation markers is restricted to the periphery of the primary lymph gland lobe, indicating that the cortical zone begins to form in this position in the second instar. Whenever possible, each genetic marker was directly compared with other pertinent markers in double-labeling experiments, except in cases such as the comparison of two different gal4 reporter lines or when available antibodies were generated in the same animal. In such cases, the relationship between the two markers, for example dome-gal4 and hml-gal4, was inferred from independent comparison with a third marker such as Pxn (Jung, 2005).

By studying the temporal sequence of expression of hemocyte-specific markers, one can describe stages in the maturation of a hemocyte. It should be noted, however, that not all hemocytes of a particular lineage are identical. For example, in the late third instar lymph gland, the large majority of mature plasmatocytes (~80%) expresses both Pxn and hml-gal4, but the remainder express only Pxn (~15%) or hml-gal4 (~5%) alone. Thus, while plasmatocytes as a group can be characterized by the expression of representative markers, populations expressing subsets of these markers indeed exist. It remains unclear at this time whether this heterogeneity in the hemocyte population is reflective of specific functional differences (Jung, 2005).

In the third instar, Pxn is a prototypical hemocyte maturation marker, while immature cells of the medullary zone express dome-gal4. Comparing the expression of these two markers in the second instar reveals an interesting developmental progression. A group of cells along the peripheral edge of these early lymph glands already express Pxn. These developing hemocytes downregulate the expression of dome-gal4, as they do in the third instar. Next to these developing hemocytes is a group of cells that expresses dome-gal4 but not Pxn; these cells are most similar to medullary zone cells of the third instar and are therefore prohemocytes. Interestingly, there also exists a group of cells in the second instar that expresses neither Pxn nor dome-gal4. This population is most easily seen in the medial parts of the gland, close to the centrally placed dorsal. These cells resemble earlier precursors in the embryo, except they express the marker Hemese. These cells are called pre-prohemocytes. Interpretation of the expression data is that pre-prohemocytes upregulate dome-gal4 to become prohemocytes. As prohemocytes begin to mature into hemocytes, dome-gal4 expression is downregulated, while the expression of maturation markers is initiated. The prohemocyte and hemocyte populations continue to be represented in the third instar as components of the medullary and cortical zones, respectively (Jung, 2005).

The cells of the PSC are already distinguishable in the late embryo by their expression of collier. It was found that the canonical PSC marker Ser-lacZ is not expressed in the embryonic lymph gland and is only expressed in a small number of cells in the second instar. This relatively late onset of expression is consistent with collier acting genetically upstream of Ser. Another finding was that the earliest expression of upd3-gal4 parallels the expression of Ser-lacZ and is restricted to the PSC region. Finally, Pvr and dome-gal4 are excluded from the PSC in the second instar, similar to what is seen in the third instar (Jung, 2005).

To determine whether maturing cortical zone cells are indeed derived from medullary zone prohemocytes, a lineage-tracing experiment was performed in which dome-gal4 was used to initiate the permanent marking of all daughter cell lineages. In this system, the dome-gal4 reporter expresses both UAS-GFP and UAS-FLP. The FLP recombinase excises an intervening FRT-flanked 'STOP cassette', allowing constitutive expression of lacZ under the control of the actin5C promoter. At any developmental time point, GFP is expressed in cells where dome-gal4 is active, while lacZ is expressed in all subsequent daughter cells regardless of whether they continue to express dome-gal4. In this experiment, cortical zone cells are permanently marked with ß-galactosidase despite not expressing dome-gal4 (as assessed by GFP), indicating that these cells are derived from a dome-gal4-positive precursor. This result is consistent with and further supports independent marker analysis that shows that dome-gal4-positive prohemocytes downregulate dome-gal4 expression as they initiate expression of maturation markers representative of cortical zone cells. As controls to the above experiment, the expression patterns of two other gal4 lines, twist-gal4 and Serrate-gal4 were determined. The reporter twist-gal4 is expressed throughout the embryonic mesoderm from which the lymph gland is derived. Accordingly, the entire lymph gland is permanently marked by ß-galactosidase despite a lack of twist-gal4 expression (GFP) in the third instar lymph gland. Analysis of Ser-gal4 reveals that PSC cells remain a distinct population of signaling cells that do not contribute to the cortical zone (Jung, 2005).

Genetic manipulation of Pvr function provides valuable insight into its involvement in the regulation of temporal events of lymph gland development. To analyze Pvr function, FLP/FRT-based Pvr-mutant clones were generated in the lymph gland early in the first instar and then examined during the third instar for the expression of maturation markers. It was found that loss of Pvr function abolishes P1 antigen and Pxn expression, but not Hemese expression. The crystal cell markers Lz and ProPOA1 are also expressed normally in Pvr-mutant clones, consistent with the observation that mature crystal cells lack or downregulate Pvr. The fact that Pvr-mutant cells express Hemese and can differentiate into crystal cells suggests that Pvr specifically controls plasmatocyte differentiation. Pvr-mutant cells do not become TUNEL positive but do express the hemocyte marker Hemese and can differentiate into crystal cells, all suggesting that the observed block in plasmatocyte differentiation within the mutant clone is not due to cell death. Additionally, Pvr-mutant clones were large and not significantly different in size from their wild-type twin spots. Thus, the primary role of Pvr is not in the control of cell proliferation. Targeting Pvr by RNA interference (RNAi) revealed the same phenotypic features, confirming that Pvr controls the transition of Hemese-positive cells to plasmatocyte fate (Jung, 2005).

Entry into S phase was monitored using BrdU incorporation and distinct proliferative phases were identified that occur during lymph gland hematopoiesis. In the second instar, proliferating cells are evenly distributed throughout the lymph gland. By the third instar, however, the distribution of proliferating cells is no longer uniform; S-phase cells are largely restricted to the cortical zone. This is particularly evident when BrdU-labeled lymph glands are co-stained with Pxn. Medullary zone cells, which can be identified by the expression of dome-gal4, rarely incorporate BrdU. Therefore, the rapidly cycling prohemocytes of the second instar lymph gland quiesce as they populate the medullary zone of the third instar. As prohemocytes transition into hemocyte fates in the cortical zone, they once again begin to expand in number. This is supported by the observation that the medullary zone in white pre-pupae does not appear diminished in size, suggesting that the primary mechanism for the expansion of the cortical zone prior to this stage is through cell division within the zone. Proliferating cells in the secondary lobes continue to be distributed uniformly in the third instar, suggesting that secondary-lobe prohemocytes do not reach a state of quiescence as do the cells of the medullary zone. These results indicate that cells of the lymph gland go through distinct proliferative phases as hematopoietic development proceeds (Jung, 2005).

This analysis of the lymph gland revealed three key features that arise during development. The first feature is the presence of three distinct zones in the primary lymph gland lobe of third instar larvae. Two of these zones, termed the cortical and medullary zones, exhibit structural characteristics that make them morphologically distinct. These zones, as well as the third zone, the PSC, are also distinguishable by the expression of specific markers. The second key feature is that cells expressing maturation markers such as Lz, ProPOA1, Pxn, hml-gal4 and Cg-gal4 are restricted to the cortical zone. The medullary zone is consistently devoid of maturation marker expression and is therefore defined as a region composed of immature hemocytes (prohemocytes). The finding of different developmental populations within the lymph gland (prohemoctyes and their derived hemocytes) is similar to the situation in vertebrates where it is known that hematopoietic stem cells and other blood precursors give rise to various mature cell types. Additionally, Drosophila hemocyte maturation is akin to the progressive maturation of myeloid and lymphoid lineages in vertebrate hematopoiesis. The third key feature of lymph gland hematopoiesis is the dynamic pattern of cellular proliferation observed in the third instar. At this stage, the vast majority of S-phase cells in the primary lobe are located in the cortical zone, suggesting a strong correlation between proliferation and hemocyte differentiation. Compared with earlier developmental stages, cell proliferation in the medullary zone actually decreases by the late third instar, suggesting that these cells have entered a quiescent state. Thus, proliferation in the lymph gland appears to be regulated such that growth, quiescence and expansion phases are evident throughout its development (Jung, 2005).

Drosophila blood cell precursors, prohemocytes and maturing hemocytes each exhibit extensive phases of proliferation. The competence of these cells to proliferate seems to be a distinct cellular characteristic that is superimposed upon the intrinsic maturation program. Based on the patterns of BrdU incorporation in developing primary and secondary lymph gland lobes, it is possible to envision at least two levels of proliferation control during hematopoiesis. It is proposed that the widespread cell proliferation observed in second instar lymph glands and in secondary lobes of third instar lymph glands occurs in response to a growth requirement that provides a sufficient number of prohemocytes for subsequent differentiation. The mechanisms promoting differentiation in the cortical zone also trigger cell proliferation, which accounts for the observed BrdU incorporation in this zone and serves to expand the effector hemocyte population. The quiescent cells of the medullary zone represent a pluripotent precursor population because they, similar to vertebrate hematopoietic precursors, rarely divide and give rise to multiple lineages and cell types (Jung, 2005).

Based on this analysis a model is proposed by which hemocytes mature in the lymph gland. Hematopoietic precursors that populate the early lymph gland are first distinguishable as Srp+, Odd+ (S+O+) cells. These will eventually give rise to a primary lymph gland lobe where the steps of hemocyte maturation are most apparent. During the first or early second instar, these S+O+ cells begin to express the hemocyte-specific marker Hemese (He) and the tyrosine kinase receptor Pvr. Such cells can be called pre-prohemocytes and, in the second instar, cells expressing only these markers occupy a narrow region near the dorsal vessel. Subsequently, a subset of these Srp+, Odd+, He+, Pvr+ (S+O+H+Pv+) pre-prohemocytes initiate the expression of dome-gal4 (dg4), thereby maturing into prohemocytes. The prohemocyte population (S+O+H+Pv+dg4+) can be subdivided into two developmental stages. Stage 1 prohemocytes, which are abundantly seen in the second instar, are proliferative, whereas stage 2 prohemocytes, exemplified by the cells of the medullary zone, are quiescent. As development continues, prohemocytes begin to downregulate dome-gal4 and express maturation markers (M; becoming S+O+H+Pv+dg4lowM+). Eventually, dome-gal4 expression is lost entirely in these cells (becoming S+O+H+Pv+dg4-M+), found generally in the cortical zone. Thus, the maturing hemocytes of the cortical zone are derived from prohemocytes previously belonging to the medullary zone. This is supported by lineage-tracing experiments that show cells expressing medullary zone markers can indeed give rise to cells of the cortical zone. In turn, the medullary zone is derived from the earlier, pre-prohemocytes. Early cortical zone cells continue to express successive maturation markers (M) as they proceed towards terminal differentiation. Depending on the hemocyte type, examples of expressed maturation markers are Pxn, P1, Lz, L1, msn-lacZ, etc. These studies have shown that differentiation of the plasmatocyte lineage requires Pvr, while previous work has shown that the Notch pathway is crucial for the crystal cell fate. Both the JAK/STAT and Notch pathways have been implicated in lamellocyte production (Jung, 2005).

Previous investigations have demonstrated that similar transcription factors and signal transduction pathways are used in the specification of blood lineages in both vertebrates and Drosophila. Given this relationship, Drosophila represents a powerful system for identifying genes crucial to the hematopoietic process that are conserved in the vertebrate system. The work presented here provides an analysis of hematopoietic development in the Drosophila lymph gland that not only identifies stage-specific markers, but also reveals developmental mechanisms underlying hemocyte specification and maturation. The prohemocyte population in Drosophila becomes mitotically quiescent, much as their multipotent precursor counterparts in mammalian systems. These conserved mechanisms further establish Drosophila as an excellent genetic model for the study of hematopoiesis (Jung, 2005).

Combinatorial signaling in the specification of primary pigment cells in the Drosophila eye

In the developing eye of Drosophila, the EGFR and Notch pathways integrate in a sequential, followed by a combinatorial, manner in the specification of cone-cell fate. This study demonstrates that the specification of primary pigment cells requires the reiterative use of the sequential integration between the EGFR and Notch pathways to regulate the spatiotemporal expression of Delta in pupal cone cells. The Notch signal from the cone cells then functions in the direct specification of primary pigment-cell fate. EGFR requirement in this process occurs indirectly through the regulation of Delta expression. Combined with previous work, these data show that unique combinations of only two pathways -- Notch and EGFR -- can specify at least five different cell types within the Drosophila eye (Nagaraj, 2007).

Unlike photoreceptor R cells, cone cells do not express Delta at the third instar stage of development. However, these same cone cells express Delta at the pupal stage. In addition, correlated with this Delta expression, the upregulation of phosphorylated MAPK was observed in these cells. This is very similar to the earlier events seen in R cells during larval development, in which the activation of MAPK causes the expression of Delta. Also, as in the larval R cells, the pupal upregulation of Delta in cone cells is transcriptional. A Delta-lacZ reporter construct, off in the larval cone cell, is detected in the corresponding pupal cone cells. To determine whether EGFR is required for the activation of Delta in the pupal cone cells, the temperature-sensitive allele EGFRts1 was used. Marked clones of this allele were generated in the eye disc using ey-flp at permissive conditions and later, in the mid-pupal stages, shifted the larvae to a non-permissive temperature. Cells mutant for EGFR, but not their adjacent wild-type cells, showed a loss of Delta expression. However, both mutant and wild-type tissues showed normal cone-cell development, as judged by Cut (a cone-cell marker) expression. As supporting evidence, ectopic expression of a dominant-negative version of EGFR (EGFRDN) in cone cells using spa-Gal4 after the cells have already undergone initial fate specification also causes a complete loss of Delta expression without compromising the expression of the cone-cell-fate-specification marker (Nagaraj, 2007).

Gain-of-function studies further support the role of EGFR signaling in the regulation of Delta expression in cone cells. Although weak EGFR activation is required for cone-cell fate, activated MAPK is not detectable in cone-cell precursors of the third instar larval eye disc. When spa-Gal4 (prepared by cloning the 7.1 kb EcoRI genomic fragment of D-Pax2) is used to express an activated version of EGFR in larval cone cells, detectable levels of MAPK activation in these cells were found and the consequent ectopic activation of Delta in the larval cone cells occurred. Taken together, these gain- and loss-of-function studies show that, during pupal stages, EGFR is required for the activation of Delta. However, this Delta expression is not essential for the maintenance of cone-cell fate (Nagaraj, 2007).

In larval R cells, the activation of Delta transcription in response to EGFR signaling is mediated by two novel nuclear proteins, Ebi and Sno. To determine the role of these genes in wild-type pupal-cone-cell Delta expression, sno and ebi function were selectively blocked in the pupal eye disc. A heteroallellic combination of the temperature-sensitive allele snoE1 and the null allele sno93i exposed to a non-permissive temperature for 12 hours caused a significant reduction in Delta expression. Similarly, a dominant-negative version of ebi also caused the loss of Delta expression. Importantly, pupal eye discs of neither spa-Gal4, UAS-ebiDN nor snoE1/sno93i showed any perturbation in cone-cell fate, as judged by the expression of Cut. Thus, as in the case of larval R cells, the loss of ebi and sno in the pupal cone cells causes the loss of Delta expression without causing a change in cone-cell fate (Nagaraj, 2007).

To test whether the expression of Delta in pupal cone cells is required for the specification of primary pigment cells, Nts pupae were incubated at a non-permissive temperature for 10 hours during pupal development and pigment-cell differentiation was monitored using BarH1 (also known as Bar) expression as a marker. Loss of Notch signaling during the mid-pupal stages caused a loss of Bar, further demonstrating the requirement of Notch signaling in the specification of primary pigment-cell fate. Similarly, when the 54CGal4 driver line, which is activated in pigment cells, was used to drive the expression of a dominant-negative version of Notch, pupal eye discs lost primary pigment-cell differentiation, again suggesting an autonomous role for Notch in pigment-cell precursors. In neither the Nts nor the 54C-Gal4, UAS-NDN genetic background was perturbation observed in cone-cell fate specification. It is concluded that Delta activation mediated by EGFR-Sno-Ebi in pupal cone cells is essential for neighboring pigment-cell fate specification (Nagaraj, 2007).

Delta-protein expression in pupal cone cells is initiated at 12 hours and is downregulated by 24 hours of pupal development. To determine the functional significance of this downregulation, the genetic combination spa-Gal4/UAS-Delta was used, in which Delta is expressed in the same cells as in wild type, but is not temporally downregulated. Whereas, in wild type, a single hexagonal array of pigment cells surrounded the ommatidium, in the pupal eye disc of spa-Gal4, UAS-Delta flies, multiple rows of pigment cells were observed surrounding each cluster. Furthermore, in wild type, only two primary pigment cells were positive for Bar expression in each cluster, whereas, in spa-Gal4, UAS-Delta pupal eye discs, ectopic expression of Bar was evident in the interommatidial cells. Therefore, the temporal regulation of Notch signaling and its activation, as well as its precise downregulation, are essential for the proper specification of primary pigment-cell fate (Nagaraj, 2007).

By contrast to the autonomous requirement for Notch signaling in primary pigment cells, the function of the EGFR signal appears to be required only indirectly in the establishment of primary pigment-cell fate through the regulation of Delta expression in the pupal cone cells. When a dominant-negative version of EGFR was expressed using hsp70-Gal4 at 10-20 hours after pupation, no perturbation was observed in the specification of primary pigment cells, as monitored by the expression of the homeodomain protein Bar. By contrast, the expression of dominant-negative Notch under the same condition resulted in the loss of Bar-expressing cells. Thus, in contrast to Notch, blocking EGFR function at the time of primary pigment-cell specification does not block the differentiation of these cells. Importantly, blocking EGFR function in earlier pupal stages caused the loss of Delta expression in cone cells and the consequent loss of pigment cells. Based on these observations, it is concluded that, in the specification of primary pigment-cell fate, the Notch signal is required directly in primary pigment cells, whereas EGFR function is required only indirectly (through the regulation of Delta) in cone cells (Nagaraj, 2007).

The Runt-domain protein Lz functions in the fate specification of all cells in the developing eye disc arising from the second wave of morphogenesis. At a permissive temperature (25°C), lzTS114 pupal eye discs showed normal differentiation of primary pigment cells. lzTS114 is a sensitized background in which the Lz protein is functional at a threshold level. When combined with a single-copy loss of Delta, a dosage sensitive interaction caused the loss of primary pigment cells. By contrast, under identical conditions, a single-copy loss of EGFR function had no effect on the proper specification of primary pigment-cell fate. This once again supports the notion that the specification of primary pigment cells directly requires Lz and Notch, whereas EGFR is required only indirectly to activate Delta expression in cone cells (Nagaraj, 2007).

This study highlights two temporally distinct aspects of EGFR function in cone cells. First, this pathway is required for the specification of cone-cell fate at the larval stage, and EGFR is then required later in the pupal cone cell for the transcriptional activation of Delta, converting the cone cell into a Notch-signaling cell. Delta that was expressed in the cone cell through the activation of the Notch pathway functioned in combination with Lz in a cell autonomous fashion and promoted the specification of the primary pigment-cell fate (Nagaraj, 2007).

Studies using overexpressed secreted Spitz have shown that ectopic activation of the EGFR signal in all cells of the pupal eye disc results in excess primary pigment cells. This study shows that EGFR activation in the pupal eye disc is required for the transcriptional activation of Delta in cone cells, but that the loss of EGFR function at the time when primary pigment cells are specified does not perturb their differentiation. It is concluded that the ectopic primary pigment cells seen in an activated-EGFR background result from the ectopic activation of Delta, which then signals adjacent cells and promotes their differentiation into primary pigment cells. Indeed, it has been shown that excessive Delta activity results in the over specification of primary pigment cells. The results are also consistent with the previous observation that the EGFR target gene Argos is not expressed in primary pigment cells in pupal eye discs. Additionally, loss of EGFR function in pupal eye discs does not perturb the normal patterning of interommatidial bristle development, which develop even later than the primary pigment cells (Nagaraj, 2007).

The elucidation of the Sevenless pathway for the specification of R7 led to the suggestion that different cell types within the developing eye in Drosophila will require combinations of dedicated signaling pathways for their specification. However, studies from several laboratories have suggested that the Sevenless pathway seems to be an exception, in that cell-fate-specification events usually require reiterative combinations of a very small number of non-specific signals. Cone-cell fate is determined by the sequential integration of the EGFR and Notch pathways in R cells followed by the parallel integration of the EGFR and Notch pathways in cone-cell precursors. This study shows that the most important function of EGFR in the specification of primary pigment cells is to promote the transcriptional activation of Delta in cone cells through the EGFR-Ebi-Sno-dependent pathway. The sequential integration of the EGFR and Notch pathways, first used in the larval stage for Delta activation in R cells, is then reused a second time in cone cells to regulate the spatiotemporal expression of Delta, converting the cone cells at this late developmental stage to Notch-signaling cells. Delta present in the cone cell then signals the adjacent undifferentiated cells for the specification of primary pigment cells. For this process, the Notch pathway functions directly with Lz but indirectly with EGFR. Through extensive studies of this system it now seems conclusive that different spatial and temporal combinations of Notch and EGFR applied at different levels can generate all the signaling combinations needed to specify the neuronal (R1, R6, R7) and nonneuronal (cone, pigment) cells in the second wave of morphogenesis in the developing eye disc (Nagaraj, 2007).

The EGFR and Notch pathways are sequentially integrated, in a manner similar to that described here, in multiple locations during Drosophila development. In the development of wing veins, EGFR that is activated in the pro-vein cells causes the expression of Delta, which then promotes the specification of inter-vein cells. Similarly, these two pathways are sequentially integrated in the patterning of embryonic and larval PNS, and during muscle development. Indeed, there are striking similarities between the manner in which the EGFR and Notch pathways are integrated in the developmental program in the C. elegans vulva and the Drosophila eye. During vulval fate specification in the C. elegans hermaphrodite gonad, anchor cells are a source of EGFR signal (Lin3), which induces the specification of the nearest (P6) cell to the primary cell fate from within a group of six equipotent vulval precursor cells (VPC). This high level of EGFR activation induces the transcriptional activation of Notch ligands in the primary cells in what can be considered sequential integration of the two pathways - the Notch signal from the primary cell both inhibits EGFR activity in the VPCS on either side of P6.p and also promotes the secondary cell fate. Thus, the reiterative integration of these two signals, in series and in parallel, can be used successfully to specify multiple cell fates in different animal species. Given that the RTK and Notch pathways function together in many vertebrate developmental systems, it is likely that similar networks will be used to generate diverse cell fates using only a small repertoire of signaling pathways (Nagaraj, 2007).

NR5A nuclear receptor Hr39 controls three-cell secretory unit formation in Drosophila female reproductive glands

Secretions within the adult female reproductive tract mediate sperm survival, storage, activation, and selection. Drosophila female reproductive gland secretory cells reside within the adult spermathecae and parovaria, but their development remains poorly characterized. With cell-lineage tracing, this study found that precursor cells downregulate lozenge and divide stereotypically to generate three-cell secretory units during pupal development. The NR5A-class nuclear hormone receptor Hr 39 is essential for precursor cell division and secretory unit formation. Moreover, ectopic Hr39 in multiple tissues generates reproductive gland-like primordia. Rarely, in male genital discs these primordia can develop into sperm-filled testicular spermathecae. Drosophila spermathecae provide a powerful model for studying gland development. It is concluded that Hr39 functions as a master regulator of a program that may have been conserved throughout animal evolution for the production of female reproductive glands and other secretory tissues (Sun, 2012).

In species where fertilization takes place internally, including mammals and insects, a sperm's long and obstacle-filled journey through the female reproductive tract culminates in the penetration of the egg. Prior to reaching its target, both paternal and maternal reproductive tissues deploy mechanisms that strongly influence an individual sperm's chances for success. In particular, specialized glands in female reproductive tracts produce mucus-rich secretions that capacitate sperm to fertilize successfully, inhibit infection, and provide nutritional, maintenance, and storage factors. The interactions of sperm and seminal fluid with the female reproductive tract and its secretions in Drosophila offer an opportunity to genetically analyze these complex processes (Sun, 2012).

Two paired glands, spermathecae (SPs) and parovaria (POs), are the primary sources of secretions encountered by sperm within the Drosophila female reproductive tract (see Structure and origin of Drosophila female reproductive glands). Messenger RNAs (mRNAs) encoding serine proteases, serpins, antioxidants, immune proteins, and enzymes involved in mucus production are found in SPs. Whereas two SPs arise from the engrailed (en) and en+ domains of the A8 segment, both POs originate in the en+ domain of the A9 segment in the female genital disc during pupal development. Both types of mature gland contain large, polyploid secretory cells (SCs). Each SC connects with the gland lumen via a specialized cuticular canal equipped with a secretion-collecting 'end apparatus'. Anatomically related secretory units are found in SPs from other species and in insect epidermal glands that produce pheromones, venoms, and many other products. Despite their ubiquity, insect epidermal gland development has not been well characterized at the molecular genetic level (Sun, 2012).

Studies of genital disc development and patterning have identified multiple genes important for reproductive gland formation. lozenge (lz), encoding a runt-domain transcription factor, is essential for both SP and PO formation and may be directly regulated by the sex determination pathway. Homologous to mammalian AML-1, Lz also supports developing blood precursors and prepatterns ommatidial cells in the developing eye. The dachshund (dac) gene also acts in multiple imaginal discs and is specifically needed for spermathecal duct development. Mutations that disrupt sphingolipid metabolism also cause abnormalities in spermathecal number and structure (Sun, 2012).

One of the most interesting genes needed to form reproductive glands encodes the nuclear hormone receptor Hr39, an early ecdysone-response gene. Hr39 and Ftz-f1 are the only two NR5A class nuclear hormone receptors in Drosophila, a class that in mammals includes steroidogenic factor 1 (SF-1) and liver receptor homolog 1 (LRH-1). All four of these proteins share 60%-90% sequence identity within their DNA binding domains and bind in vitro to identical sequences. SF-1 is a master regulator of steroidogenesis and sex hormone production, whereas LRH-1 is required in the ovary for female fertility, in embryonic stem cells for pluripotency and in endodermal tissues for metabolic homeostasis. Weak Hr39 mutations alter the production of some SP gene products, whereas LRH-1 directly controls major secretory proteins of the exocrine pancreas. Thus, NR5A class hormone receptors may play a conserved role controlling secretions from certain tissues, including female reproductive glands (Sun, 2012).

This study characterized the cell lineage of developing reproductive glands and clarify the roles of lz and Hr39. Hr39 is expressed sex-specifically in lz-positive female gland primordia beginning shortly after the ecdysone pulse that initiates prepupal development. When levels of Hr39 are reduced, lz-expressing precursors fail to protrude, divide, or remain viable, suggesting that Hr39 expression orchestrates reproductive gland development. Mouse LRH-1, but not SF-1, can partially replace Hr39 function in gland formation. Ectopic expression of Hr39 in male larvae can induce a pigmented SP-like structure containing sperm to develop in the male reproductive tract. It is proposed that Hr39 acts as a master regulator of reproductive gland development and that the production of sperm-interacting proteins in the female reproductive tract under the control of NR5A proteins has been conserved during evolution. These findings suggest new targets for controlling agriculture pests and human-disease vectors (Sun, 2012).

These studies reveal that lz and Hr39, despite their nearly identical loss-of-function phenotypes, have distinctive expression patterns during gland development. All gland precursors express both genes following puparium formation, but within 24 hr divide to produce lz+ epithelial precursors apically and lz SUPs basally. SUPs then differentiate according to a stereotyped program involving production of two transient accessory cells and a single polyploid secretory cell (Sun, 2012).

Reproductive secretory cells arise in a superficially similar manner to sensory bristles and multiple classes of mechanosensory and chemosensory sensilla. Both utilize short fixed-cell lineages that employ transient accessory cells to generate permanent extracellular structures (secretory canal, sensory bristle, etc.), but the three-cell secretory lineage analyzed in this study differs from the four asymmetric divisions producing five different cells typical of PNS differentiation (see Lineage Analysis of Secretory Unit Formation). Many other insect epidermal glands probably develop in a generally similar manner, but the precise cell lineages and mechanisms documented in this study for Drosophila reproductive glands (three cells, absence of ciliary involvement) differ from previous models (Sun, 2012).

Drosophila secretory units provide a powerful system for analyzing insect gland development. Studies in other insects suggested that an accessory cell utilizes a ciliary process to prevent the SCs from being sealed off by cuticle-secreting epithelial cells. This study found no morphological or genetic evidence that cilia are involved in forming Drosophila secretory units. However, the apical cell (AC) may fulfill this same role using normal microtubules, in much the same way that the anterior polar cells in egg chambers template the micropyle channel during oogenesis. Membranes from the basal cell (BC) likely surround this AC process, secrete the cuticular canal, and join it to the luminal cuticle. Concomitantly, the BC likely secretes the end apparatus around a large apical segment of the SC, which it surrounds (Sun, 2012).

The NR5A hormone receptor Hr39 plays multiple roles in reproductive gland development. Initially, Hr39 orchestrates gland protrusion and in the absence of Hr39 protrusion fails to occur. Among Drosophila imaginal discs, gland protrusion in genital discs is a unique process that leads to the differentiation of a gland capsule connected to the nascent reproductive tract by a tubular duct. When Hr39 is misexpressed, patches of cells within multiple imaginal discs that do not normally express Hr39 undergo changes reminiscent of early protrusion (Sun, 2012).

Hr39, a known member of the ecdysone response pathway, is likely to time reproductive gland cell divisions during pupal development. The initial Hr39 expression we observed in the genital disc was detected shortly after the prepupal ecdysone pulse. Several additional peaks of ecdysone titer during pupal development correspond closely with the timing this study measured of the secretory cell divisions. These observations suggest that external hormonal signals rather than internal autonomous mechanisms sometimes drive precise cell lineages. In addition to its requirement within cellular precursors, Hr39 mutations alter SP secretory gene mRNA levels (Allen, 2008), suggesting that Hr39 also regulates secretory gene expression within SCs (Sun, 2012).

Finally, Hr39 acts as a high level 'master regulator' by integrating individual pathways to elicit the production of an entire gland. Most cells expressing ectopic Hr39 could not progress past the initial stage of eversion, but in male genital discs Hr39-positive clones sometimes generated integrated structures that strongly resembled small spermathecae. They contained round heads with lumens, a pigmented layer, and rarely were connected to the male reproductive tract by ducts through which sperm were taken up. Thus, Hr39 (but not lz) can reprogram male genital cells to generate ectopic spermathecae that likely synthesize and secrete products attractive to sperm (Sun, 2012).

Drosophila reproductive gland development is unusually susceptible to perturbation. Rare adults in some wild strains contain an extra spermatheca, and females bearing weak alleles of either lz or Hr39 lose parovaria (POs) entirely and produce fewer spermathecae (SPs), which vary dramatically in size and cellular content. These effects probably result from the disparate sizes of the precursor pools for individual organs. PO pools are very small, whereas the exceptionally large posterior SP primordium may easily split in two under conditions where precursor proliferation is perturbed. The effects of dac mutations on duct structure are probably also due to altered precursor pools. Sphingolipids may affect gland development by serving as endogenous Hr39 ligands, consistent with reports that SF-1 can bind sphingolipids (Sun, 2012 and references therein).

In mammals, sperm interact with female secretory products at multiple locations. Glands within the uterine endometrium are hypothesized to govern selective passage through the cervix, uterus, and subsequently, the uterotubal junction. Following entry into the oviduct, sperm induce and interact with the products of specialized tubal secretory cells that likely mediate capacitation. In some species, these products also allow sperm to be stored in the oviduct while retaining their ability to fertilize an egg. Mammalian female reproductive glands continue to nurture preimplantation embryos and are likely essential for successful pregnancy (Sun, 2012).

Drosophila is emerging as a valuable model with which to study multiple aspects of reproductive physiology, some of which may have been conserved during evolution. The mouse lz homolog Aml1 (Runx1) is expressed in the Müllerian ducts and genital tubercle (Simeone, 1995), but its role in fertility is unknown. The murine Hr39 homolog LRH-1 is required for female fertility, but whether it plays a role in reproductive gland secretion has yet to be tested. However, LRH-1 is required for the development of several exocrine tissues and in the pancreas is directly involved in the transcription of major secretory products. Thus, LRH-1 and Hr39 may both govern the formation and secretory function of exocrine tissue (Sun, 2012).

These study studies provide further support for the idea that an NR5a-dependent program of secretory cell development has been conserved in evolution. Murine LRH-1 can partially replace Hr39 function in Drosophila reproductive gland formation. Similar rescue with two other NR5A members (mammalian SF-1 or Drosophila Ftz-F1) failed and instead suppressed all gland formation. This is consistent with previous findings that Hr39 and Ftz-F1 have opposing roles in alcohol dehydrogenase and EcR expression. Antagonistic roles in gene regulation by the two NR5A family members may be evolutionarily conserved. Further study of the roles of Hr39 and LRH-1 should help define a fundamental program of secretory cell development that may be widely used (Sun, 2012).

Effects of Mutation or Deletion

The R1, R6, R7 and cone cell precursors (see The Drosophila Adult Ommatidium: Illustration and explanation with Quicktime animation) appear to share a common developmental potential. These cells have been called the "R7 equivalence group", as they are all competent to realize the R7 fate. Normally, only one of these cells actually achieves R7 fate. Cone cells are prevented from becoming R7-like because they do not contact R8, whose expression of the BOSS protein leads to contact activation of the Sevenless protein in R7 precursors. R1/6 cells are normally prevented from becoming R7 by the expression of seven-up. In turn, activation of Bar transcription factors works to guide the differentiation of these cells. Whereas two R3/4 cells are added to each pre-cluster, in mutants of lozenge R1/6 cells are not recruited correctly. Extra cells are found surrounding each facet in lozenge mutants. Based on their position and morphology, the cells between ommatidia appear to be secondary and tertiary pigment cells (Crew, 1997).

lozenge mutants completely lack a faceted eye structure. The mutated smooth surface of the eye also lacks bristles, lenses and much of its pigment. Additionally, mutants have a partial or complete loss of tarsal claws in the leg, a reduction of the third antennal segment, and a partial loss of female fertility. lozenge gene controls at least five different functions in the antenna: the size of the third antennal segment, the overall number and density of sensilla, the proportions of the three types of sensilla and the generation of basiconic sensilla. The antenna of strong lz alleles is characterized by a lack of basiconic sensilla and by a significantly increased density of coeloconic sensilla. Intermediate alleles have a few basiconic sensilla; they exhibit a highly increased density of trichoid sensilla, but a normal coeloconic density. Basiconic sensilla on the maxillary palps are weakly affected even by strong lz alleles. (Stocker, 1993)

Analysis of the adult visual system phenotypes of lozenge mutation reveals two functional units necessary for normal eye development: cistrons A and B. Cistron A mutations map to the distal-most sub-locus, known as spectacle. Mutants of both cistrons perturb lens structure and eye pigmentation. Mutants in the second cistron, which fail to complement one another but complement mutants in cistron A, map to the spectacle sublocus and to the other three subloci: krivsheko, lozenge and glossy. Thus Cistron A mutations are confined to the spectacle sub-locus while cistron B mutations have been mapped to each of the four sub-loci and effect the development of eyes, antennae, tarsal claws, pulvilli, and crystal cells (hemocytes) in addition to female fertility and levels of phenol oxidase activity (Batterham, 1996).

Defects in the adult eye can be classified as eye surface phenotypes resulting in a mild phenotype such as disorganization of a few ommatidia and loss of some mechnosensory bristles at the posterior limbs to extreme phenotypes such as lack of any ommatidial array or structure. Various lozenge mutants exhibit different patterns of pigment distribution. Extreme cistron A alleles do make at least some lenses, albeit defective ones, while extreme cistron B spectacle mutants fail to make any lenses. Defects in the lens directly implicate defects in the underlying cone or pigment cells. As far as defects in photoreceptor and pigment cell defects, often there is loss of secondary pigment cells as well as the specific loss of R7-like neurons. Altered patterns in pigment distribution is indicative of defects in pigment cells. For the severe cistron A alleles, it is difficult to know the origination of the highly varied number of photoreceptor neurons that are evident in each cluster (Batterham, 1996).

In some instance there is severe disruption in the normally highly ordered lamina. The failure to form a fenestrated membrane is clearly responsible for some of this disorganization. The fenestrated membrane is normally a product of the secondary and tertiary pigment cells, which surround each cluster of photoreceptor neurons. While severe cistron A mutants always make some semblance of a fenestrated membrane, severe cistron B mutants may lack the structure altogether. Because the lens and fenestrated membrane phenotypes differ when comparing severe mutations of the two cistrons, it is concluded that the functions represented by the two cistrons affects cone and pigment cells differently. Finally, the optic lobe can be severly disrupted. Because of breakdown of the fenestrated membrane, cortex interneurons of the lamina often intermingle with photoreceptor cell bodies, causing the lamina neurons to be displaced and disorganized. In some cases, the lamina cortex and neuropil are lacking altogether. It is thought, in some cases at least, that deeper brain defects can be independent of the influence of photoreceptor neuron development (Batterham, 1996).

lozenge mutants express elav (a neural marker) cut and seven-up in cone cell precursors that normally do not express these genes. Therefore it is thought that lozenge is required to specify cone fate, and in the absence of lozenge, cone cells are reprogrammed to initiate a neuronal fate (Daga, 1996).

lozenge exhibits a significant interaction with ras1. Sprite, a gain of function allele of lozenge (see Targets of activity, above), was combined with ras1 mutation. Whereas only 5% of Sprite ommatidia contain fewer than the normal complement of R photoreceptor cells (albeit transformed into R7 fate), in a ras/Sprite mutant background, 83% of the ommatidia have lost at least one photoreceptor. It is unclear from this result whether both Lozenge and Ras pathway directly regulate Seven-up, or whether the Ras pathway acts on a downstream target of Seven-up (Daga, 1996).

The physiology and ultrastructure of the antenna in Drosophila melanogaster have been examined in wild-type and lozenge mutants. Scanning electron microscopy of sensilla on the antennal surface has revealed that in the wild-type, the basiconic sensilla contain linear arrays of pores connected by longitudinal furrows and transected by shorter furrows. Sensilla trichodea also are shown to have pores, as revealed by examining transverse sections by transmission electron microscopy; these data directly address a longstanding controversy. Coeloconic sensilla, previously described as "pit sensilla" and as "grooved" sensilla, are shown to rise directly from the antennal surface, as opposed to lying below the antennal surface in pits; the previously observed grooves correspond to the junctions between bundled, finger-like projections. This description of coeloconic sensilla is supported by analysis of lz mutants, in which the projections of coeloconic sensilla splay apart. Coeloconic sensilla are also shown to undergo duplication on the lz3 antenna. Physiological recordings from the antenna show that responses to all odorants tested are severely decreased in lz mutants. Measurements made from different parts of the antenna show similar defects. Evidence is provided that both the physiological and ultrastructural defects map to the lz locus (Riesgo-Escovar, 1997).

Sense organ development in the Drosophila antenna is initiated by the selection of a founder cell from an epidermal field. This cell is believed to recruit neighbours to form a cluster of cells, which then divides to form a mature sense organ. In most systems so far studied, sense organ type appears to be specified by the identity of proneural genes involved in the selection of precursors. The regulation of proneural gene expression is, in turn, controlled by the prepatterning genes. In the antenna, the only known proneural function is that of atonal, a gene that is involved in founder cell choice in the sensilla coeloconica, and no prepatterning gene function has yet been demonstrated. Lozenge, a protein that possesses a DNA binding domain similar to that of the Acute myeloid leukemia-1/Runt transcription factors, functions in a dose-dependent manner to specify the fate of the other two types of sense organs in the antenna: the sensilla trichoidea and the sensilla basiconica. The analysis of an allelic series of lz mutations leads to the proposal that high levels of lz+ activity specify the development of basiconic sensilla, while moderate levels result in sensilla trichoidea. This implies that, in the wildtype, there are at least two different regions in the disc, with different strengths of lz+ activity. In weak hypomorphic alleles, the reduction in lz+ activity only affects the threshold for sensilla basiconica, resulting in a reduction in the numbers of these sensilla. In moderate alleles, lz+ activity falls below the threshiold for sensilla basiconica development but the trichoid sensilla can still develop. This results in a change of fate of the founder cells (FCs) in the basiconic region to trichoidea. In strongly hypomorphic and null alleles, lz+ activity falls below the threshold for specification of both the trichoid and the basiconic sensilla. However, a small number of sensilla trichoidea can develop in these animals, suggesting the presence of other genes that can partially compensate for loss of lz during sense organ development. These results suggest that Lozenge may act on the epidermal field, resulting in founder cells acquiring specific cell fates that lead to the development of an appropriate type of sense organ (Gupta, 1998).

Afferent growth cone interactions control synaptic specificity in the Drosophila visual system: in lozenge mutants R3 and R4 are transformed into R7 cells

The pattern of connections between R1-R6 neurons and their targets in the lamina is one of the most extraordinary examples of connection specificity known. An interwoven set of connections precisely maps R cells in different ommatidia that 'see' the same point in space onto the same group of postsynaptic cells, the lamina cartridge. R1-R6 cells that see the same point in space are distributed over six neighboring ommatidia as a consequence of the curvature of the eye and the angular placement of their light-sensing organelles. Conversely, each of the R1-R6 axons from a single ommatidium sees a different point in space and connects to a different set of lamina target neurons arranged in an invariant pattern. Each cartridge is innervated by a complete set of R1-R6 neurons from six different ommatidia (i.e., an R1 from one ommatidium, an R2 from another, and so on). By superimposing multiple inputs from the same point in visual space upon a single synaptic unit, the signal-to-noise ratio of the response to a signal in the visual field is enhanced. This phenomenon is called neural superposition (Clandinin, 2000 and references therein).

The R1-R6 projection pattern develops in two temporally distinct stages. During the third larval stage, R cells extend axons into the brain, where they terminate between two layers of glia, forming the lamina plexus. These glia act as intermediate targets for R1-R6 neurons. R cell axons induce the differentiation and organization of lamina target neurons and glia. At this stage of development, R cell axons from the same ommatidium form a single fascicle. A column of lamina neurons forms above the lamina plexus, in tight association with a single R cell axon fascicle. By the sequential addition of ommatidial bundles and their associated columns of lamina neurons, a precise retinotopic map forms in which fascicles from neighboring ommatidia terminate adjacent to each other. As lamina neurons differentiate, they send axons along the surface of R cell axons through the plexus and fasciculate with R7 and R8 as they project into the medulla. Although lamina neurons are in close association with R cell axons at this early stage, no synaptic contacts are formed (Clandinin, 2000).

In the second phase of development, ~30 hr after reaching the lamina plexus, R cell axons defasciculate from each ommatidial bundle and project across the surface of the lamina to their synaptic partners, making the pattern of connections characteristic of neural superposition. Growth of R cell axons toward their targets occurs approximately simultaneously in all ommatidial bundles and is presaged by an invariant sequence of contacts between R cell growth cones. This reorganization of terminals converts a strictly anatomical retinotopic map that reflects neighbor relationships between ommatidia into a new topographic map that reflects R cell visual response and reconstructs visual space in the first layer of the optic ganglion (Clandinin, 2000 and references therein).

R cell projections from a single ommatidium display two prominent features. (1) Each R cell axon terminates in an invariant position relative to the other axons from the same ommatidial fascicle. (2) The projection is oriented with respect to the dorsoventral midline of the eye (i.e., the equator), with the R3 axon extending toward the equator -- as a result, the projection patterns on opposite sides of the dorsoventral midline of the eye are mirror images. Using mutations that eliminate specific subsets of R cells or alter ommatidial polarity, tests were performed to see whether R cell synaptic specificity requires interactions among neighboring afferent axons or reflects independent navigation of each axon to its target. It has been demonstrated that interactions between specific R cells are required for target selection, and it is proposed that the precise composition of R cell axons within a fascicle plays a critical role in target specificity (Clandinin, 2000).

Neural superposition was first noted 90 years ago and the R1-R6 connection pattern in the lamina was first described using serial reconstruction of electron microscopic images in 1965. This pattern is cited as a classic example of extreme connection specificity. However, mechanistic analysis of this pattern was prevented by the absence of a rapid method for assessing R cell projections. In particular, the complexity of the pattern precludes conventional approaches based on visualizing all R cell axons in the target region, yet the assessment of connection specificity requires visualization of all R cell axons from one ommatidium. A method has been developed to label individual ommatidia with DiI and visualize the projection pattern using confocal microscopy. R1-R6 axons form a single bundle as they project into the brain. They defasciculate, project across the surface of the lamina, and then turn 90° and extend into the lamina cartridge. R cell axons elaborate a complex en passant presynaptic structure with lamina interneurons within the lamina cartridge. The axons of R7 and R8 project through the lamina, into the medulla. The relative positions of lamina targets chosen by each R1-R6 growth cone are invariant between ommatidia. This labeling method facilitates analysis of R1-R6 projections in various genetic backgrounds and creates a unique experimental system in which synaptic partner choices made by identified neurons can be directly assessed (Clandinin, 2000).

Serial electron microscopic reconstruction studies have revealed that, during pupal development, individual R cell axons leave their original bundle and migrate outward, in the precise direction of their final targets. This process was visualized using confocal microscopy. Early in pupal development, each ommatidial bundle forms a compact mass of expanded growth cones in the lamina plexus. This spherical mass then flattens, as distinct filopodial extensions corresponding to individual R cell axons become visible. This pattern of connections forms within a spatially patterned environment containing lamina target neurons and glial cells, as well as R cell axons. Since extension from the bundle is not preceded by extensive filopodial exploration, interactions between axons within ommatidial bundles may specify the initial trajectory of each growth cone. To address whether cell intrinsic mechanisms or interactions between R cell growth cones or both control target specificity, R cell projections were examined in mutant animals lacking specific subsets of R1-R6 cells. R cell axons from single ommatidia were labeled with DiI and visualized by confocal microscopy. In this series of experiments, animals were analyzed in which the eye was genetically mutant and the lamina neurons and glia in the target were wild type. Three mutant backgrounds were examined: (1) phyllopod, in which R1, R6, and R7 are transformed into nonneuronal cone cells; (2) lozengesprite, in which R3 and R4 are transformed into R7 cells; and (3) seven-up, in which R1, R3, R4, and R6 are transformed into R7 cells (Clandinin, 2000).

The first step of lamina target innervation is the coordinated defasciculation of R cell axons from bundles comprising axons from the same ommatidium. To determine whether interactions between specific subsets of R1-R6 axons are necessary for this defasciculation, R cell projections were assessed in phyllopod, seven-up, and lozengesprite mutants. In all three of the R cell transformation mutants examined, R cell axons migrated outward from the bundle. In particular, 4 R cell fibers in the lamina of 14/15 phyllopod mutant animals (missing R1, R6, and R7) and 20/24 lozengesprite mutants (missing R3 and R4) defasciculated from the bundle and projected to local targets. Similarly, in 17/23 seven-up mutants (missing R1, R3, R4, and R6), it was observed that the two remaining R cell axons defasciculated from the ommatidial bundle and innervated separate cartridges. In some cases, additional R cell axons also defasciculated, consistent with the reported incomplete expressivity of cell fate transformations in these mutants. In each case, axons projected to lamina targets in the local environment of the fascicle terminus. It is concluded that each R cell subtype is programmed to initiate a search for targets in a local region of the lamina target, independent of interactions between other R cell subtypes. In the following sections, whether interactions between specific R1-R6 cells regulate target specificity is assessed (Clandinin, 2000).

In phyllopod mutants, R1, R6, and R7 are transformed into nonneuronal cone cells. The remaining R cells made normal projections: a single long projection corresponding to R3 was observed, and R2, R4, and R5 made projections of appropriate relative lengths compared to wild type. In 1 of these 15 phyllopod mutant ommatidia, an additional short R cell projection was also observed, consistent with an incomplete cell fate transformation of either R1 or R6. In phyllopod mutant animals, the pattern of targets chosen by R3 and R4 were invariably normal, while those chosen by R2 and R5 were usually correct. In 4/15 animals, R2 and R5 made projections of the appropriate length, but the targets they chose were misoriented with respect to the equator. Therefore, R3 and R4 do not require R1, R6, and R7 to target correctly, while in some cases R2 and R5 are affected by their loss. These effects are not caused by the loss of R7; a sevenless mutation that specifically eliminates R7 has no effect on R cell targeting in the lamina (Clandinin, 2000).

A gain-of-function mutation, lozengesprite, which transforms R3 and R4 into R7 cells was examined. In this mutant, the Lozenge gene product is ectopically expressed in R3 and R4. In such mutant animals, ~73% of ommatidia have both R3 and R4 transformed into R7 cells; in most of the remaining ommatidia (20% of the total), only R4 is transformed; the remaining ommatidia are missing one R cell. Since the reduction in the number of R cells projecting to specific cartridges roughly corresponds to the fraction of R3 and R4 cells transformed into R7, it is presumed that transformation is complete in ommatidia where four fibers are observed in the lamina. In cases in which five R cell axons are observed, it is inferred that R4 but not R3 was transformed into R7. In 20/24 lozengesprite ommatidia injected, four R cell projections were observed in the lamina, with R1, R2, R5, and R6 all making projections of appropriate length, while transformed R3 and R4 cells projected through the lamina into the medulla. In the remaining 4/24 cases, five projections were seen in the lamina, one of which was a long projection characteristic of R3. In completely transformed lozengesprite ommatidia, the relative positions of the targets chosen by R1, R2, R5, and R6 were frequently highly aberrant. In the remaining 9/20 fully transformed lozengesprite animals, the pattern of targets chosen was not grossly distorted, though minor irregularities were seen. The effects seen in lozengesprite do not result from defects in ommatidial orientation: ommatidia are normally oriented in this mutant (Clandinin, 2000).

In seven-up mutants, R1, R3, R4, and R6 are frequently transformed into R7, while R2 and R5 are unaffected. Moreover, the transformation of individual seven-up ommatidia is variable and complex, making detailed reconstruction of many ommatidia impossible. However, in the majority of seven-up ommatidia (17/23), two short R cell projections, characteristic of R2 and R5, are seen in the lamina, while the transformed R1, R3, R4, and R6 cells project into the medulla (as R7 cells normally do). The targets chosen by the presumptive R2 and R5 were invariably misoriented with respect to the equator. In 4/23 ommatidia, there were either three or four short R cell projections in the lamina, while the remaining R cells projected into the medulla. In 2/23 cases, a single, relatively long, R3-like projection was observed in the lamina, flanked by either two or three short projections. In summary, these data establish that R2 and R5 project to a local region within the lamina independent of R1, R3, R4, and R6 but require interactions with these neurons to specify their correct targets (Clandinin, 2000).

The defects in R cell projections seen in seven-up and lozengesprite animals are not due to effects on the differentiation of neurons in the target region as assessed using multiple markers; lamina neuron differentiation was not assessed in phyllopod. The defects seen in lozengesprite and seven-up are also not due to extra R7 cells; a gain-of-function mutation in the Raf gene recruits extra R7 cells to each ommatidium without affecting the differentiation and targeting of R1-R6 neurons. It is possible, however, that the effects seen in these mutants reflect altered composition of axons within the ommatidial fascicle caused by ectopic R7 axons in abnormal positions within the bundle (Clandinin, 2000).

Two models could explain the mechanisms that determine the precise projection of R3 and R4 axons toward the dorsoventral midline and, by extension, the relative orientations of the other R cell axons. The growth cones of R3 and R4 may respond to an orienting cue in the lamina that promotes extension toward the dorsoventral midline. Alternatively, the orientation of R cell bodies in the retina may determine the orientation of R cell growth cones in the lamina, independent of any environmental cues. To assess the role of ommatidial polarity on projection specificity, projections from misoriented ommatidia were assessed (Clandinin, 2000).

If a lamina cue can promote equatorial extension of the R3 and R4 axons, ommatidia that rotate incorrectly should project their axons normally, toward the equator. Alternatively, if ommatidial orientation determines the direction of axon projection in the lamina, incorrectly oriented ommatidia should project their R3 and R4 axons away from the equator (Clandinin, 2000).

In wild-type animals, ommatidia are mirror image reflected about the dorsoventral equator of the eye. R cell projections are also mirror image symmetric about the equator but are rotated 180° with respect to the retina. That is, while the R3 cell body is oriented toward the pole in each ommatidium, its axon projects toward the equator in the lamina. This rotation is generated by a twist in the axon fascicle that occurs between the retina and the lamina (Clandinin, 2000).

To test the effects of large changes in ommatidial orientation, two mutations, spiny legs (in homozygous animals) and frizzled (in somatic mosaic animals in which a mutant eye projects to a wild-type target), were examined. In these mutants, ommatidia frequently adopt orientations that are 180° rotated; that is, the R3 cell body is frequently oriented toward the equator in the eye. In these two mutant backgrounds, the orientation of projections from ommatidia that were correctly oriented was normal. Therefore, neither gene is required for R cell axons to respond to orienting cues in the target. However, almost 90% of the ommatidia that were ~180° misoriented in the eye made projections that were also 180° misoriented in the lamina. Rare, abnormal projections of single R cell axons in both of these mutant backgrounds were observed, irrespective of ommatidial orientation. Therefore, the orientation of R cell projections along the dorsoventral axis of the lamina is largely determined by the orientation of ommatidia in the retina (Clandinin, 2000).

Three exceptional cases, in which misoriented ommatidia projected axons toward the equator, were observed. Thus, a cue in the lamina may reinforce the ommatidial orientation cue to ensure the correct direction of outgrowth along the dorsoventral axis. To test whether such a cue contributes to directionality of R cell projections, a mutation that causes a more moderate defect in ommatidial orientation was examined. In nemo mutant animals, ommatidia are misoriented up to 45°. If ommatidial orientation directly determines the directionality of R cell projections, they would be misoriented 45° with respect to the equator; the angle between ommatidial orientation and the axon projection pattern would remain 180°. However, while ommatidial orientation was disrupted in nemo, R cell projections were normal with respect to the equator. This observation suggests that in addition to ommatidial polarity, a cue in the lamina can influence R cell projection orientation (Clandinin, 2000).

It is concluded that interactions between R cell afferents play a crucial role in target specification, and it is proposed that the spatial relationships between axons within a fascicle influence synaptic specificity. It is hypothesized that the interactions between R cell subtypes that are required for target specificity are mediated by direct contacts between specific growth cones. R3 and R4 are required for the remaining R cell axons to choose their normal targets. R1 and R6 are required for R2 and R5 projections but are not required for the projections of R3 and R4. These interactions could occur between growth cones from the same or neighboring ommatidial bundles. The characteristic morphological changes of these growth cones as revealed through electron microscopic reconstruction studies are consistent with the notion that precise spatial relationships between specific growth cones within the lamina plexus are required for these critical interactions to occur. This sequence of interactions determines the relative positions of targets chosen by R cell axons from the same ommatidium (Clandinin, 2000).

R cell transformation mutants could disrupt these interactions in two ways. First, transformation of specific R cells could directly disrupt the instructive signals between R cell growth cones within the plexus that determine growth cone trajectories. Alternatively, these mutations could affect the interactions indirectly, by disrupting the spatial relationships between the remaining R cell axons. That is, outgrowth trajectory could be determined passively by the position each growth cone occupies as it leaves the ommatidial fascicle. In this view, these mutant backgrounds alter the composition of axons within each ommatidial bundle and, hence, disrupt the precise packing of axons within the fascicle. The differential requirements for particular R cell subtypes would reflect their specific roles in directing the spatial relationships between growth cones within the fascicle, rather than interactions between specific growth cones in the target region (Clandinin, 2000).

The cellular mechanisms described here provide a conceptual framework for understanding the molecular basis of synaptic specificity. While the DiI method facilitates the analysis of R1-R6 specificity on a scale sufficient to analyze many mutants, it is too laborious to accommodate large-scale screening. Hence, a genetic screen based on visual behavior driven specifically by R1-R6 is required to extend these studies to the molecular level. A wealth of visual behaviors have been described in Drosophila, one of which, the optomoter response, is mediated by these cells. Techniques that generate mosaic flies in which only R cells are made homozygous for randomly induced mutations, while the rest of the fly is heterozygous, have recently been described. Currently, projects are underway, combining this specific behavioral screen with genetic mosaics, in order to screen for genes controlling R1-R6 synaptic specificity (Clandinin, 2000).

Induction and autoregulation of Bar during retinal neurogenesis

Neurogenesis in the Drosophila eye imaginal disc is controlled by interactions of positive and negative regulatory genes. The basic helix-loop-helix (bHLH) transcription factor Atonal (Ato) plays an essential proneural function in the morphogenetic furrow to induce the formation of R8 founder neurons. Bar homeodomain proteins are required for transcriptional repression of ato in the basal undifferentiated retinal precursor cells to prevent ectopic neurogenesis posterior to the furrow of the eye disc. Thus, precise regulation of Bar expression in the basal undifferentiated cells is crucial for neural patterning in the eye. Evidence is shown that Bar expression in the basal undifferentiated cells is regulated by at least three different pathways, depending on the developmental time and the position in the eye disc. (1) At the time of furrow initiation, Bar expression is induced independent of Ato by Hedgehog (Hh) signaling from the posterior margin of the disc. (2) During furrow progression, Bar expression is also induced by Ato-dependent EGFR (epidermal growth factor receptor) signaling from the migrating furrow. (3) Once initiated, Bar expression can be maintained by positive autoregulation. Therefore, it is proposed that the domain of Bar expression for Ato repression is established and maintained by a combination of non autonomous Hh/EGFR signaling pathways and autoregulation of Bar (Lim, 2004).

To identify activators of Bar expression in the basal undifferentiated cells, focus was placed on two different transcription factors, Lozenge (Lz) and Glass (Gl), as candidates. Both proteins are known to be required for normal Bar expression in R1/6 photoreceptor cells, but it has not been demonstrated whether they are also required for Bar expression in the basal undifferentiated cells. Lz is expressed in R1, 6 and 7 photoreceptor cells and is required for normal level of Bar expression in R1/6 cells. In the basal undifferentiated cells, Lz is co-expressed with Bar in a majority of Bar-expressing cells, except in a group of cells just posterior to the furrow. To test whether Lz is also required for Bar expression in the basal undifferentiated cells, Bar expression was examined in homozygous lzr15 mutants and loss-of-function (LOF) clones of lzr15, a null allele of lz. It was found that the expression level of Bar is strongly decreased but not completely eliminated in R1/6 photoreceptor cells within lzr15 mutant clones. However, Bar expression in the basal undifferentiated cells is little changed compared with its expression level in adjacent wild-type cells. These results suggest that Lz is necessary to activate Bar expression in R1/6 cells, but not in the basal undifferentiated cells behind the furrow (Lim, 2004).

Resolving embryonic blood cell fate choice in Drosophila: interplay of GCM and RUNX factors

The differentiation of Drosophila embryonic blood cell progenitors (prohemocytes) into plasmatocytes or crystal cells is controlled by lineage-specific transcription factors. The related proteins Glial cells missing (Gcm) and Gcm2 control plasmatocyte development, whereas the RUNX factor Lozenge (Lz) is required for crystal cell differentiation. The segregation process that leads to the formation of these two cell types, and the interplay between La and Gcm/Gcm2 was investigated. Surprisingly, Gcm is initially expressed in all prohemocytes but is rapidly downregulated in the anterior-most row of prohemocytes, which then initiates lz expression. However, the lz+ progenitors constitute a mixed-lineage population whose fate depends on the relative levels of Lz and Gcm/Gcm2. Notably, Gcm/Gcm2 play a key role in controlling the size of the crystal cell population by inhibiting lz activation and maintenance. Furthermore, prohemocytes are bipotent progenitors, and downregulation of Gcm/Gcm2 is required for lz-induced crystal cell formation. These results provide new insight into the mechanisms controlling Drosophila hematopoiesis and establish the basis for an original model for the resolution of the choice of blood cell fate (Bataille, 2005).

This study takes advantage of the relatively simple model provided by Drosophila embryonic hematopoiesis to attempt to unravel the mechanisms that underlie the choice of two blood cell fates in vivo. The data indicate that crystal cells and plasmatocytes develop from a pool of bipotential hematopoietic progenitors. The earliest detectable manifestation of the segregation of the two blood cell lineages occurs in the anterior row of prohemocytes with the downregulation of gcm and the induction of lz. Furthermore, the number of lz-expressing precursors, and their final differentiation into crystal cells or plasmatocytes, is regulated by gcm/gcm2 activity, which inhibits lz induction and maintenance. Thus, embryonic blood cell lineage segregation is revealed to be a highly dynamic process in which the interplay between the transcription factors gcm/gcm2 and lz plays a crucial role (Bataille, 2005).

gcm and gcm2 have been shown to be required for the proper differentiation of plasmatocytes, and Gcm and Gcm2 have been thought to be plasmatocyte-specific lineage transcription factors that are not involved in crystal cell development. By contrast, the current results clearly demonstrate that gcm and gcm2 inhibit crystal cell formation. Furthermore, the expression of gcm was detected in all of the prohemocytes. including the prospective crystal cell precursors, at stage 5, a result confirmed by tracing gcm-lacZ expression into early differentiating crystal cells. Thus, gcm and gcm2 participate in blood cell fate segregation by regulating not only plasmatocyte development but also that of crystal cells (Bataille, 2005).

gcm and gcm2 have been most intensively studied during neurogenesis, where they are required to promote glial cell development at the expense of neuronal cell fate. This study shows that they also regulate a binary cell fate choice during hematopoiesis. However, although their expression is restricted to glial precursors during neurogenesis, they are initially expressed in all prohemocytes irrespective of their subsequent fate. Furthermore, in the absence of gcm/gcm2, whereas almost all presumptive glial cells are transformed into neurons, only a small proportion of the presumptive plasmatocytes adopts a crystal cell fate. Therefore, the function and mechanism of action of gcm/gcm2 in regulating cell fate choice during neurogenesis and hematopoiesis are different (Bataille, 2005).

gcm and gcm2 control crystal cell formation by a two-step process. (1) gcm/gcm2 determines the number of crystal cell precursors by restricting the initiation of lz expression in the prohemocyte population. In the absence of gcm/gcm2, more lz+ progenitors are observed, correlating with a greater number of differentiated crystal cells at later stages. The data indicate that gcm is expressed early in the entire hematopoietic primordium but is rapidly downregulated in the prospective lz expression domain. Maintaining Gcm or Gcm2 expression in the lz+ progenitors inhibits crystal cell differentiation. Thus, repressing gcm/gcm2 expression in the anterior population of prohemocytes is most probably a prerequisite for the emergence of crystal cells (Bataille, 2005).

(2) gcm and gcm2 regulate the proportion of lz+ progenitors that ultimately differentiate in crystal cells: whereas 40% of these cells differentiate into plasmatocytes in wild-type embryos, all of them become crystal cells in the absence of gcm/gcm2. It has been noted that some lz-expressing cells differentiate into plasmatocytes and it was suggested that this might be due to the de novo activation of gcm expression in these cells. The current results extend their observations and demonstrate that gcm participates in this process, although it is not re-expressed in the lz+ cells. The data further suggest that the residual gcm activity present in the lz+ progenitors may contribute to the relative plasticity in the fate of these progenitors, allowing some of them to differentiate into plasmatocytes. In summary, compelling evidence is provided that gcm and gcm2 play a key role in regulating cell fate choice in prohemocytes and lz+ progenitors (Bataille, 2005).

This study yields new insight into the regulation and mode of action of lz during embryonic crystal cell development. Although plasmatocytes migrate through the embryo, crystal cells gather around the proventriculus. Strikingly, in the absence of gcm/gcm2, srp-driven high-level expression of lz induces the transformation of all of the hemocytes to authentic crystal cells that remain clustered. By contrast, when lz is expressed under the control of its own promoter, 40% of lz+ cells migrate through the embryo whether or not they express gcm/gcm2. Hence, the data suggest that high levels of lz are required for crystal cell clustering and lz induction in prohemocytes is heterogeneous. Below a certain threshold, lz+ progenitors retain the default migratory behaviour of hemocytes and, in the presence of gcm/gcm2, can differentiate into plasmatocytes. It is noteworthy that gcm/gcm2 participate in (but are not required for) hemocyte migration. Thus, lz and gcm/gcm2 appear to have opposite effects on blood cell migration, with gcm/gcm2 promoting a migratory behaviour that dominates the inhibitory effect of lz (Bataille, 2005).

lz function is continuously required to promote crystal cell development. This study has identified an enhancer of lz that is transactivated by the Srp/Lz complex. This observation suggests that, once initiated, lz expression can be maintained by a positive autoregulatory feedback loop, thereby providing a simple mechanism to stabilise crystal cell lineage commitment. This enhancer contains several RUNX-binding sites and the role of these sites in lz autoregulation is currently being investigated. Interestingly, the three mammalian homologues of the RUNX factor Lz contain several conserved RUNX-binding sites in their promoters. Furthermore, RUNX2 maintains its own expression through an auto-activation loop in differentiated osteoblasts, whereas RUNX3 inhibits RUNX1 expression in B lymphocytes. Thus, auto- or cross-regulation might be a common feature of the RUNX family. In addition, Gcm/Gcm2 repress lz expression. However, no consensus Gcm-binding sites are present in the lz crystal cell-specific enhancer. Interestingly, it was recently shown that zebrafish gcmb is expressed in macrophages. Yet, the putative functions of the two gcm homologues and their possible interplays with RUNX factors have not been investigated during vertebrate hematopoiesis (Bataille, 2005).

Because gcm is expressed early in the entire hematopoietic anlage, it is tempting to speculate that prohemocytes are primed to differentiate into plasmatocytes (i.e., macrophages). Thus, it appears likely that Drosophila blood cells progenitors are not 'naïve'. Similarly, mammalian stem and progenitor blood cells express low levels of lineage-affiliated genes and it has been suggested that they are primed for differentiation. Furthermore, from an evolutionary perspective, macrophages are certainly the oldest and most pervasive blood cell type, and it is remarkable that another hematopoietic cell type may have evolved from this lineage through the use of a conserved RUNX transcription factor (Bataille, 2005).

Acquisition of crystal cell fate involves both the repression of the primary fate (i.e. repression of gcm) and the activation of lz. The data show that these two steps are coordinated in space and time. Nonetheless, the induction of lz is not the mere consequence of relieving gcm/gcm2-mediated repression of lz but requires an active and localised process. How gcm transcription is repressed and lz is activated in the anterior row of prohemocytes is currently unknown. In the lymph gland, Notch/Serrate signalling is necessary and sufficient to induce crystal cell formation by activating lz expression. However the results demonstrate that, contrary to the situation in larvae, Notch is not required for crystal cell formation in the embryo. In this respect, it is interesting to note that neither gcm nor gcm2 is expressed in the lymph gland. Hence, the process that segregates crystal cells from plasmatocytes relies on different mechanisms in the embryo and in the larval lymph gland. Similarly, in vertebrates, primitive and definitive hematopoiesis may also depend on partially distinct programs. For instance, in mouse, the transcription factor PU.1 plays an essential role in the emergence of definitive macrophages but does not seem to be required for the formation of primitive macrophages in the yolk sac (Bataille, 2005).

The coincident repression of gcm and activation of lz between stages 6 and 7 in a row of prohemocytes is remarkable; it suggests that the head mesoderm is delicately patterned at this early stage of development. The hematopoietic primordium is located in the posterior head region, whose patterning involves several genes including buttonhead, empty spiracles, orthodenticle and collier. However, mutations of these genes do not specifically suppress crystal cell or plasmatocyte development. Further work will thus be required to understand the coordination permitting the silencing of gcm and the activation lz that triggers the choice of one fate at the expense of the other (Bataille, 2005).

It was shown that gcm can induce the differentiation of all of the prohemocytes into plasmatocytes. The data presented here demonstrate that, in the absence of gcm/gcm2, lz can transform all of the hemocytes to crystal cells. Thus, Drosophila prohemocytes are bipotent progenitors. However, the incapacity of lz to repress gcm (and thereby plasmatocyte fate) implies that the resolution of cell fate choice does not rely on reciprocal antagonism between two 'lineage-specific' transcription factors like between GATA1 and PU.1 during myeloid/erythroid cell fate choice in vertebrates. Instead, it is proposed that Drosophila embryonic blood cell fate segregation is a process that can be divided into two consecutive phases. A local cue triggers the process by downregulating gcm and activating lz in the anterior population of prohemocytes, whereas gcm expression is maintained in the remaining cells, which differentiate into plasmatocytes. Then, in the lz+ progenitors, the relative levels of LZ and Gcm will dictate lineage choice. If the ratio of LZ to Gcm is high enough to overcome Gcm-mediated repression of lz expression, Lz can elicit its autoregulatory activation loop and the progenitor will differentiate into a crystal cell. If not, Gcm inhibits lz autoactivation and the progenitor differentiates into a plasmatocyte. Such a mechanism of segregation could provide some plasticity, because the size of a population may be regulated at different times by physiological cues influencing either the initiation event or the feed-back loop required for its development (Bataille, 2005).

In conclusion, these data shed light on the transition in vivo from bipotent hematopoietic progenitors to lineage-restricted precursors. Interestingly, the embryonic Drosophila cell fate choice occurs though an original mechanism distinct from that observed during larval hematopoiesis. Moreover, this process does not seem to involve reciprocal negative regulation between two 'lineage-specific' transcription factors. Hence, the mechanisms leading to the resolution of hematopoietic lineages in vivo appear to be more complex and diverse than expected (Bataille, 2005).

A genetic screen in Drosophila for genes interacting with senseless during neuronal development identifies the importin moleskin: Alleles of msk are suppressors of lz

Senseless (Sens) is a conserved transcription factor required for normal development of the Drosophila peripheral nervous system. In the Drosophila retina, sens is necessary and sufficient for differentiation of R8 photoreceptors and interommatidial bristles (IOBs). When Sens is expressed in undifferentiated cells posterior to the morphogenetic furrow, ectopic IOBs are formed. This phenotype was used to identify new members of the sens pathway in a dominant modifier screen. Seven suppressor and three enhancer complementation groups were isolated. Three groups from the screen are the known genes Delta, lilliputian, and moleskin/DIM-7 (msk), while the remaining seven groups represent novel genes with previously undefined functions in neural development. The nuclear import gene msk was identified as a potent suppressor of the ectopic interommatidial bristle phenotype. In addition, msk mutant adult eyes are extremely disrupted with defects in multiple cell types. Reminiscent of the sens mutant phenotype, msk eyes demonstrate reductions in the number of R8 photoreceptors due to an R8 to R2,5 fate switch, providing genetic evidence that Msk is a component of the sens pathway. Interestingly, in msk tissue, the loss of R8 fate occurs earlier than with sens and suggests a previously unidentified stage of R8 development between atonal and sens (Pepple, 2007).

Sens, along with its homologs Gfi-1 and Pag-3, comprises a conserved family of proteins required for normal neural development. In Drosophila, sens is both necessary and sufficient for development of the PNS. In mice, loss of Gfi-1 leads to neurodegeneration of cerebellar Purkinje cells and sensoneural deafness due to loss of inner ear hair cells. Despite the obvious importance of the GPS proteins in normal neural development and their place near the top of the neuronal development cascade, few targets of these proteins in the process of neurogenesis are known. To identify members of this pathway required in neurogenesis, an F1 dominant modifier screen was performed using an ectopic Sens phenotype in Drosophila. Advantage was taken of a dominant, modifiable phenotype generated by ectopic expression of Sens in undifferentiated cells posterior to the morphogenetic furrow. This ectopic Sens led to the recruitment of undifferentiated cells to the bristle fate (Pepple, 2007).

Both known and novel genes have been identified as potential members of the sens pathway by their ability to modify an ectopic Sens phenotype. The Notch signaling pathway is known to regulate Sens function during the resolution of the proneural cluster. This interaction was identified in the screen by the ability of heterozygous loss of Dl to enhance the ectopic Sens phenotype. The nuclear import gene moleskin (msk) was able to strongly suppress the effect of ectopic Sens. msk plays a role in normal eye development and R8 photoreceptor differentiation. Identification of the genes that are represented in the remaining complementation groups will lead to a better understanding of the GPS pathway and normal neural development. It is likely that the remaining complementation groups represent components of the Sens pathway due to their specific effect on lz and not the secondary screens as well as their requirement for normal bristle development in adult thoracic clones. Further characterization of these genes will offer new insight into the highly conserved Sens pathway (Pepple, 2007).

Alleles of msk were found to be suppressors of lz (the expression of UAS-sens in undifferentiated cells by the lozenge-GAL4 driver) with the highest frequency of any complementation group in the EMS screen. Usually such high representation of alleles indicates that the gene has an important role in the phenotype being tested and/or is readily mutagenized. The results presented here suggest a model in which Msk plays a role in the sens pathway. Initial observations of the effect of Msk on the lz phenotype suggested that Msk was needed to maintain high levels of Sens expression. It is possible that in this ectopic situation, Msk contributes to Sens import, but more likely Msk contributes to Sens expression indirectly by importing another component of the pathway that regulates Sens expression. Characterization of the ey-GAL4, UAS-flp (EGUF); msk phenotype strongly suggests that Msk is not the only import factor involved in the Sens pathway during normal development. Clearly, there is functional redundancy with another importin since complete loss of Msk function during early eye development does not remove Sens expression in all R8 cells. In third instar discs, Msk appears to play a role in the maintenance of the R8 cell fate very early in development. Little is known about the early stages of R8 differentiation after specification by Atonal. Previous work on R8 specification and development outlined a hierarchy of events in which Atonal is expressed first and appears to simultaneously activate expression of the downstream targets sens and sca-lacZ. Work on the sens phenotype determined that sca-lacZ expression is still present in sens clones, thereby establishing an epistatic relationship between sca-lacZ expression and sens. The data indicate that there is yet another step in the relationship between Atonal and these two downstream factors. The data suggest that in the msk eye, after specification of the R8 by Atonal but before the onset of sca-lacZ expression, R8 development is disrupted in some clusters, leading to an R2,5 fate switch. This is the first genetic evidence for factors positioned between ato and sca-LacZ/sens (Pepple, 2007).

Nuclear transport is required for the viability of all cells. Interestingly, the loss or decrease in function of some importins can cause specific defects during development. For example, the nuclear exportin Dcas is required for the export of Importin α3 in Drosophila. While null mutants in dcas are not viable, hypomorphs lead to specific cell fate changes in mechanosensory bristles. This phenotype is likely due to extreme sensitivity of Notch signaling to disruption of nuclear transport of one of its pathway members by Importin α3. It is possible that the Msk/Sens interaction was detectable for a similar reason. In the Sens gain-of-function situation, the high level of Sens required to generate ectopic bristles is very sensitive to decreased Msk levels, while during wild-type SOP differentiation, Sens is far less sensitive to Msk levels and exhibits only sporadic effects (Pepple, 2007).

One question still remains: How does the EGUF; msk eye survive at all given the important cargo that Msk is known to transport? The functional redundancy in the Importin family likely provides the cell with enough transport for survival and development in the absence of Msk. However, this idea raises a new question: Why was only Msk identified in the screen and no other importins? A model is proposed in which Msk is the key importin utilized by the cell for high levels of signaling. The lz phenotype requires high levels of signaling to generate ectopic bristles, and this model would explain why an effect with Msk and no other importin was detected. The model does not preclude the ability of other importins to provide transport redundancy for Msk cargos, and in fact evidence is seen for this redundancy in the ability of the EGUF; msk eye to survive and produce some normal ommatidia. Another importin must have the ability to import some level of Sens, pMAPK, and other unidentified factors into the nucleus. Data existst that indirectly support such a model for the role of Msk. In the Atonal intermediate groups within the morphogenetic furrow, Msk must be sequestered away from the nucleus to prevent the very high levels of cytoplasmic pMAPK from entering the nucleus. Although whether other nuclear importins are also sequestered to block pMAPK nuclear entry was not tested, overexpression of Msk in the intermediate groups allows pMAPK to enter the nucleus and affect nuclear signaling. The fact that the cell needs to sequester Msk to prevent high levels of EGFR pathway signaling supports a model in which Msk is important for high levels of signaling (Pepple, 2007).

It has been suggested in other developmental systems that importins are part of a mechanism that regulates the nuclear protein composition of transcription factors and chromatin remodeling factors. In Drosophila, Msk has been shown to import two other developmentally significant cargos, pMAPK and Caudal. In addition to these previously defined roles, the additional data that Msk and nucleocytoplasmic transport play an important role in Sens expression and R8 development. Perhaps more importantly, the fact that abnormalities seen in msk mutant eye discs arise between Atonal and Senseless expression suggests roles for as-yet undiscovered factors and new modes of regulation in this critical pathway (Pepple, 2007).


Bataille, L., Auge, B., Ferjoux, G., Haenlin, M. and Waltzer, L. (2005). Resolving embryonic blood cell fate choice in Drosophila: interplay of GCM and RUNX factors. Development 132(20): 4635-44. 16176949

Batterham, P., et al. (1996). Genetic analysis of the lozenge gene complex in Drosophila melanogaster: adult visual system phenotypes. J. Neurogenet. 10: 193-220. PubMed Citation: 8923295

Behan, K. J., et al. (2002). Yan regulates Lozenge during Drosophila eye development. Dev. Genes Evol. 212: 267-276. 12111211

Bras, S., Martin-Lanneree, S., Gobert, V., Auge, B., Breig, O., Sanial, M., Yamaguchi, M., Haenlin, M., Plessis, A. and Waltzer, L. (2012). Myeloid leukemia factor is a conserved regulator of RUNX transcription factor activity involved in hematopoiesis. Proc Natl Acad Sci U S A 109: 4986-4991. Pubmed: 22411814

Bruhn, L., Munnerlyn, A. and Grosschedl, R. (1997). ALY, a context-dependent coactivator of LEF-1 and AML-1, is required for TCRalpha enhancer function. Genes Dev. 11:640-653. PubMed Citation: 9119228

Canon, J. and Banerjee, U. (2003). In vivo analysis of a developmental circuit for direct transcriptional activation and repression in the same cell by a Runx protein. Genes Dev. 17: 838-843. 12670867

Chatterjee, S. S., et al. (2011). The female-specific doublesex isoform regulates pleiotropic transcription factors to pattern genital development in Drosophila. Development 138(6): 1099-109. PubMed Citation: 21343364

Clandinin, T. R. and Zipursky, S. L. (2000). Afferent growth cone interactions control synaptic specificity in the Drosophila visual system. Neuron 28: 427-436. PubMed Citation: 11144353

Crew, J. R., Batterham, P. and Pollock, J. A. (1997). Developing compound eye in lozenge mutants of Drosophila: lozenge expression in the R7 equivalence group. Dev. Genes Evol. 206(8): 481-493

Daga, A., et al. (1996). Patterning of cells in the Drosophila eye by Lozenge, which shares homologous domains with AML1. Genes Dev. 10: 1194-1205. PubMed Citation: 8675007

Dyer, J. O., Dutta, A., Gogol, M., Weake, V. M., Dialynas, G., Wu, X., Seidel, C., Zhang, Y., Florens, L., Washburn, M. P., Abmayr, S. M. and Workman, J. L. (2017). Myeloid Leukemia Factor acts in a chaperone complex to regulate transcription factor stability and gene expression. J Mol Biol 429(13): 2093-2107. PubMed ID: 27984043

Flores, G. V., et al. (1998). Lozenge is expressed in pluripotent precursor cells and patterns multiple cell types in the Drosophila eye through the control of cell-specific transcription factors. Development 125(18): 3681-3687. PubMed Citation: 9716533

Flores, G. V., et al. (2000). Combinatorial signaling in the specification of unique cell fates. Cell 103: 75-85. PubMed Citation: 11051549

Fossett, N., et al. (2001). The Friend of GATA proteins U-shaped, FOG-1, and FOG-2 function as negative regulators of blood, heart, and eye development in Drosophila. Proc. Natl. Acad. Sci. 98: 7342-7347. 11404479

Fu, W. and Noll, M. (1997). The Pax2 homolog sparkling is required for development of cone and pigment cells in the Drosophila eye. Genes Dev. 11(16): 2066-2078. PubMed Citation: 9284046

Greene, M. M. (1990). The foundations of genetic fine structure: a retrospective from memory. Genetics 124: 793-796. PubMed Citation: 2108902

Gobert, V., et al. (2010). A genome-wide RNA interference screen identifies a differential role of the mediator CDK8 module subunits for GATA/ RUNX-activated transcription in Drosophila. Mol. Cell Biol. 30(11): 2837-48. PubMed Citation: 20368357

Golling, G., et al. (1996). Drosophila homologs of the proto-oncogene product PEBP2/CBF beta regulate the DNA-binding properties of Runt. Mol. Cell. Biol. 16: 932-942

Goulding, S. E., zur Lage, P. and Jarman, A. P. (2000). amos, a proneural gene for Drosophila olfactory sense organs that is regulated by lozenge. Neuron 25: 69-7

Gupta, B. P., et al. (1998). Patterning an epidermal field: Drosophila Lozenge, a member of the AML-1/Runt family of transcription factors, specifies olfactory sense organ type in a dose-dependent manner. Dev. Biol. 203(2): 400-11

Hayashi, T. and Saigo, K. (2001). Diversification of cell types in the Drosophila eye by differential expression of prepattern genes. Mech. Dev. 108: 13-27. 11578858

Jung, S. H., Evans, C. J., Uemura, C. and Banerjee, U. (2005). The Drosophila lymph gland as a developmental model of hematopoiesis. Development 132(11): 2521-33. 15857916

Kaminker, J. S., et al. (2001). Redundant function of runt domain binding partners, Big brother and Brother, during Drosophila development. Development 128: 2639-2648. 11526071

Lebestky, T., et al. (2000) Specification of Drosophila hematopoietic lineage by conserved transcription factors. Science 288: 146-149.

Li, L.-H. and Gergen, J. P. (1999). Differential interactions between Brother proteins and Runt domain proteins in the Drosophila embryo and eye. Development 126: 3313-3322

Lim, J. and Choi, K.-W. (2004). Induction and autoregulation of the anti-proneural gene Bar during retinal neurogenesis in Drosophila. Development 131: 5573-5580. 15496446

Mavromatakis, Y. E. and Tomlinson, A. (2013). Switching cell fates in the developing Drosophila eye. Development 140(21): 4353-61 PubMed ID: 24067351

Miller, M., Chen, A., Gobert, V., Auge, B., Beau, M., Burlet-Schiltz, O., Haenlin, M. and Waltzer, L. (2017). Control of RUNX-induced repression of Notch signaling by MLF and its partner DnaJ-1 during Drosophila hematopoiesis. PLoS Genet 13(7): e1006932. PubMed ID: 28742844

Milton, C. C., Grusche, F. A., Degoutin, J. L., Yu, E., Dai, Q., Lai, E. C. and Harvey, K. F. (2014). The Hippo pathway regulates hematopoiesis in Drosophila melanogaster. Curr Biol 24: 2673-2680. PubMed ID: 25454587

Nagaraj, R. and Banerjee, U. (2007). Combinatorial signaling in the specification of primary pigment cells in the Drosophila eye. Development 134(5): 825-31. Medline abstract: 17251265

Okuda, T, et al. (1996). AML1, the target of multiple chromosomal translocations in human leukemia, is essential for normal fetal liver hematopoiesis. Cell 84: 321-330

Pepple, K. L., et al. (2007). A genetic screen in Drosophila for genes interacting with senseless during neuronal development identifies the importin moleskin. Genetics 175: 125-141. Medline abstract: 17110483

Potier, D., Davie, K., Hulselmans, G., Naval Sanchez, M., Haagen, L., Huynh-Thu, V. A., Koldere, D., Celik, A., Geurts, P., Christiaens, V. and Aerts, S. (2014). Mapping gene regulatory networks in Drosophila eye development by large-scale transcriptome perturbations and motif inference. Cell Rep 9: 2290-2303. PubMed ID: 25533349

Protzer, C. E., Wech, I. and Nagel, A. C. (2008). Hairless induces cell death by downregulation of EGFR signalling activity. J. Cell Sci. 121(Pt 19): 3167-76. PubMed Citation: 18765565

Ray, A., van Naters, W. G., Shiraiwa, T. and Carlson, J. R. (2007). Mechanisms of odor receptor gene choice in Drosophila. Neuron 53(3): 353-69. Medline abstract: 17270733

Ray, K. and Rodrigues, V. (1995). Cellular events during the development of the olfactory sense organs in Drosophila melanogaster. Dev. Biol. 167: 426-438

Riesgo-Escovar, J. R., Piekos, W. B. and Carlson, J. R. (1997). The Drosophila antenna: ultrastructural and physiological studies in wild-type and lozenge mutants. J. Comp. Physiol. [A] 180(2): 151-160

Shi, Y. and Noll, M. (2009). Determination of cell fates in the R7 equivalence group of the Drosophila eye by the concerted regulation of D-Pax2 and TTK88. Dev. Biol. 331: 68-77. PubMed Citation: 19406115

Stocker, R. F., Gendre, N. and Batterham, P. (1993). Analysis of the antennal phenotype in the Drosophila mutant lozenge J. Neurogenetics 9: 29-53. PubMed Citation: 8295076

Stocker, R. F. (1994). The organization of the chemosensory system in Drosophila melanogaster: a review. Cell Tissue Res. 275: 3-26. PubMed Citation: 8118845

Sun, J. and Spradling, A. C. (2012). NR5A nuclear receptor Hr39 controls three-cell secretory unit formation in Drosophila female reproductive glands. Curr Biol 22: 862-871. PubMed Citation: 22560612

Swanson, C. I., Evans, N. C. and Barolo, S. (2010). Structural rules and complex regulatory circuitry constrain expression of a Notch- and EGFR-regulated eye enhancer. Dev. Cell 18: 359-370. PubMed Citation: 20230745

Swanson, C. I., Schwimmer, D. B. and Barolo, S. (2011). Rapid evolutionary rewiring of a structurally constrained eye enhancer. Curr. Biol. 21(14): 1186-96. PubMed Citation: 21737276

Wang, Q, et al. (1996). Disruption of the Cbfa2 gene causes necrosis and hemorrhaging in the central nervous system and blocks definitive hematopoiesis. Proc. Nat. Acad. Sci. 93: 3444-3449. PubMed Citation: 8622955

Wildonger, J., Sosinsky, A., Honig, B. and Mann, R. S. (2005a). Lozenge directly activates argos and klumpfuss to regulate programmed cell death. Genes Dev 19: 1034-1039. 15879554

Wildonger, J. and Mann, R. S. (2005b). The t(8;21) translocation converts AML1 into a constitutive transcriptional repressor. Development 132(10): 2263-72. 15829516

Yosef, N., et al. (2013). Dynamic regulatory network controlling TH17 cell differentiation. Nature 496: 461-468. PubMed ID: 23467089zur Lage, P. I., et al. (2003). The Drosophila proneural gene amos promotes olfactory sensillum formation and suppresses bristle formation. Development 130: 4683-4693. 12925594

lozenge: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation

date revised: 22 December 2017 

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