Abdominal-B

DEVELOPMENTAL BIOLOGY

Embryonic

The pattern of the Abd-B regulatory (r) protein expression, as deduced by analysis of Abd-B mutants, is restricted to ps14 and 15 in all germ layers and observes a parasegmental boundary at its anterior margin of expression. In contrast, the pattern of morphogenetic (m) protein expression is unusual as its level in the ectoderm increases from ps10 to ps13 in parasegmental steps. Its anterior margin of expression is highly dynamic shifting anteriorly across more than 3 parasegments during midembryonic development. Evidently, the control mechanisms of m and r protein expression are considerably different. M protein expression and regulation varies to some extent in individual germ layers (De Lorenzi, 1990a).

To gain further insights into homeotic gene action during CNS development, the role of the homeotic genes was characterized in embryonic brain development of Drosophila. Neuroanatomical techniques were used to map the entire anteroposterior order of homeotic gene expression in the Drosophila CNS. This order is virtually identical in the CNS of Drosophila and mammals. All five genes of the Antennapedia Complex are expressed in specific domains of the developing brain. The labial gene has the smallest spatial expression domain; it is only expressed in the posterior part of the tritocerebral anlage. This contrasts with previous reports that lab is expressed throughout the tritocerebral (intercalary) neuromere. The proboscipedia gene has the largest anteroposterior extent of expression, however, in contrast to other homeotic genes, pb is only found in small segmentally repeated groups of 15-20 cells per neuromere. These groups of pb-expressing cells range from the posterior deutocerebrum toward the end of the VNC. Since pb-expressing cells are found anterior to the lab-expressing cells in the brain, this is an exception to the spatial colinearity rule. (Spatial colinearity is conserved in the epidermis, where pb expression is posterior to lab expression). The Deformed gene is expressed in the mandibular neuromere and the anterior half of the maxillary neuromere and the Sex combs reduced gene is expressed in the posterior half of the maxillary neuromere and the anterior half of the labial neuromere. The Antennapedia gene is expressed in a broad domain from the posterior half of the labial neuromere toward the end of the VNC. The three genes of the Bithorax Complex are expressed in the VNC. Ultrabithorax gene expression extends in a broad domain from the posterior half of the T2 neuromere to the anterior half of the A7 neuromere, with highest expression levels in the posterior T3/anterior A1 neuromeres. The abdominal-A gene is expressed from the posterior half of the A1 neuromere to the posterior half of the A7 neuromere. For the above mentioned genes, the anterior border of CNS expression remains stable from stage 11/12 until the end of embryogenesis. In contrast, the anterior border of CNS expression for the Abdominal-B gene shifts at stage 14. Before this stage Abd-B expression extends from the posterior half of neuromere A7 to the end of the VNC; afterwards, it extends from the posterior half of neuromere A5 to the end of the VNC with the most intense expression localized to the terminal neuromeres. With the exception of the Dfd gene, the anterior limit of homeotic gene expression in the CNS is always parasegmental (Hirth, 1998).

Each of the somatic cell types of the gonad arises from mesodermal cells that constitute the embryonic gonad. Using markers for the precursors of the somatic cells of the gonad, five discrete steps have been identified in gonadal development:

1. First, somatic gonadal precursor cells are specified within the mesoderm in parasegments 10 through 12.
2. After pole cells traverse and exit the midgut they recognize and associate primarily with specific mesodermal cells laterally positioned in the mesoderm of parasegments 11 and 12. These are the migratory gonadal precursors that delaminate from the mesodermal cell sheet.
3. In a third step, gonadal precursors and pole cells migrate anteriorly, where they contact cells in parasegment 10.
4. Next, gonadal precursors and pole cells arrest migration at parasegment 10.
5. Finally, the mesodermal cells partially ensheath the arriving cells, and the cluster coalesces into the gonad.

The functions of the homeotic genes abdominal A and Abdominal B are both required for the development of gonadal precursors. Each plays a distinct role. abd A activity alone specifies anterior gonadal precursor fates, whereas abd A and Abd B act together to specify a posterior subpopulation of gonadal precursors. Once specified, gonadal precursors born within posterior parasegments move to the site of gonad formation. The proper regional identities, as established by homeotic gene function, are required for the arrest of migration at the correct position. abd A is required in a population of cells within parasegments 10 and 11 that partially ensheath the coalescing gonad. Mutations in iab-4, a distal enhancer element, abolish expression of abd A within these cells, blocking the coalescence of the gonad (Boyle, 1995).

The embryonic dorsal vessel in Drosophila possesses anteroposterior polarity and is subdivided into two chamber-like portions, the aorta in the anterior and the heart in the posterior. The heart portion features a wider bore as compared with the aorta and develops inflow valves (ostia) that allow the pumping of hemolymph from posterior toward the anterior. Homeotic selector genes provide positional information that determines the anteroposterior subdivision of the dorsal vessel. Antennapedia (Antp), Ultrabithorax (Ubx), abdominal-A (abd-A), and Abdominal-B (Abd-B) are expressed in distinct domains along the anteroposterior axis within the dorsal vessel, and, in particular, the domain of abd-A expression in cardioblasts and pericardial cells coincides with the heart portion. Evidence is provided that loss of abd-A function causes a transformation of the heart into aorta, whereas ectopic expression of abd-A in more anterior cardioblasts causes the aorta to assume heart-like features. These observations suggest that the spatially restricted expression and activity of abd-A determine heart identities in cells of the posterior portion of the dorsal vessel. Abd-B, which at earlier stages is expressed posteriorly to the cardiogenic mesoderm, represses cardiogenesis. In light of the developmental and morphological similarities between the Drosophila dorsal vessel and the primitive heart tube in early vertebrate embryos, these data suggest that Hox genes may also provide important anteroposterior cues during chamber specification in the developing vertebrate heart (Lo, 2002).

During early cardiogenesis at embryonic stages 10-11, peak levels of Abd-B are observed in parasegments (PS) 13 and 14, which express the m and r proteins of Abd-B, respectively. These two parasegments abut the region of PS 2-12 from which heart progenitors arise. Indeed, double stainings of stage 11 embryos for Abd-B (combined m+r) and Evenskipped (Eve), an early marker for pericardial cell and dorsal muscle progenitors, confirm that the previously known absence of mesodermal Eve cells in PS 13 coincides with the domain of peak expression of Abd-B in both ectoderm and mesoderm. This observed gap in Eve expression is compatible with the possibility that the Abd-B m variant is able to suppress the formation of Eve pericardial and somatic muscle cells in PS 13, whereas Abd-B r is not active in suppressing eve cells (with unknown fates) in PS 14. In agreement with this notion, Abd-B mutant embryos generate an additional cluster of mesodermal eve cells in PS 13. This observation suggests that Abd-B normally represses cardiogenesis, including the formation of pericardial cells, as well as the formation of somatic muscle #1, which is also derived from Eve-positive progenitor cells, in PS 13. This interpretation is further supported by the presence of supernumerary cardioblasts in the heart portion of the dorsal vessel of late stage embryos, as shown by anti-Mef2 stainings. In Abd-B mutant embryos, there are about 116 cardioblast nuclei as compared with the normal number of 104 in the wildtype. Although the heart does not appear significantly elongated in the mutant embryos, it is frequently much wider and extra cardioblasts are arranged in irregular clusters or double-rows within its posterior portion. Similar increases in the number of cardioblasts and pericardial cell within the heart portions were seen in anti-Tin stainings of late stage Abd-B mutant embryos. In addition to the observed increase in the number of heart cells, the somatic muscles in abdominal segment 8 (A8) in Abd-B mutant embryos show an increase in the number of nuclei and a Mef2 pattern that is more similar to the pattern normally found in A7. Together with the Eve expression data at earlier stages and in agreement with the known muscle pattern, this observation indicates that Abd-B functions also to suppress the formation of the majority of dorsal body wall muscles in A8, including the Eve-expressing muscle #1 (Lo, 2002).

The results of ectopic expression experiments with Abd-B are fully consistent with these proposed functions of Abd-B in early heart and somatic muscle development. Specifically, ectopic expression of Abd-B (m) that is driven by the twist promoter in the entire mesoderm completely suppresses the formation of cardioblast cells, as determined by anti-Mef2 staining. In addition, the number of Mef2-stained somatic muscle nuclei is reduced and more comparable to the number of somatic muscle nuclei normally found in A8. It appears therefore that Abd-B expression in the early mesoderm of those segments where it is not normally expressed is sufficient to suppress the development of the dorsal vessel as well as the formation of many somatic muscles (Lo, 2002).

Hox-controlled reorganisation of intrasegmental patterning cues underlies Drosophila posterior spiracle organogenesis

Hox proteins provide axial positional information and control segment morphology in development and evolution. Yet how they specify morphological traits that confer segment identity and how axial positional information interferes with intrasegmental patterning cues during organogenesis remains poorly understood. This study investigates the control of Drosophila posterior spiracle morphogenesis, a segment-specific structure that forms under Abdominal-B (AbdB) Hox control in the eighth abdominal segment (A8). The Hedgehog (Hh), Wingless (Wg) and Epidermal growth factor receptor (Egfr) pathways provide specific inputs for posterior spiracle morphogenesis and act in a genetic network made of multiple and rapidly evolving Hox/signalling interplays. A major function of AbdB during posterior spiracle organogenesis is to reset A8 intrasegmental patterning cues, first by reshaping wg and rhomboid expression patterns, then by reallocating the Hh signal and later by initiating de novo expression of the posterior compartment gene engrailed in anterior compartment cells. These changes in expression patterns confer axial specificity to otherwise reiteratively used segmental patterning cues, linking intrasegmental polarity and acquisition of segment identity (Merabet, 2005).

In the dorsal ectoderm of stage 10 embryos, hh and wg follow the same striped expression patterns in A8 as in other abdominal segments. rho expression, which marks cells secreting an active form of the Egf ligand, occurs in all primordia of tracheal pits, in A8 as in more anterior segments (Merabet, 2005).

Specification of posterior spiracle primordia occurs at early stage 11. The primordia can then be recognised by Cut expression in spiracular chamber cells and by Sal, the homogenous expression of which in A8 becomes restricted dorsally to stigmatophore cells (forming the external structure of the posterior spiracle) that form a crescent surrounding Cut-positive cells. From mid-stage 11, wg and rho adopt in the dorsal ectoderm expression patterns specific to A8, with wg transcribed in two cells only and rho in a second cell cluster, dorsal and posterior to the tracheal placode. To localise wg- and rho-expressing cells with regard to stigmatophore and spiracular chamber cells, co-labelling experiments for wg or rho transcripts and for Cut or Sal proteins were performed: the two wg cells lie between Cut- and Sal-positive cells; the second cell cluster expressing rho in A8 also expresses Cut but not Sal. This cluster is likely to produce the Egf ligand required for posterior spiracle development, since mutations that alleviate rho expression in the tracheal placodes do not abolish spiracles formation. At mid-stage 11, the hh pattern in A8, along a stripe lying posterior and adjacent to the spiracular chamber and overlapping stigmatophore presumptive cells, resembles expression in other abdominal segments. Analyses at later stages indicate that the relationships between posterior spiracle cells and hh, wg and rho patterns are maintained (Merabet, 2005).

Null mutations of wg, hh or Egfr result in the absence of posterior spiracles. The strong cuticular defects observed raise the possibility that the phenotypes result indirectly from early loss of segment polarity. Removing the Wg, Hh or Egfr signals from 5-8 hours of development using thermosensitive alleles causes strong segment polarity defects but allows filzkörpers, stigmatophores or even complete posterior spiracles to form. Thus, spiracular chamber and stigmatophore can develop in embryos that have pronounced segment polarity defects (Merabet, 2005).

It was next asked whether defects in primordia specification could account for posterior spiracle loss, and Cut and Sal expression was examined in the dorsal A8 ectoderm of hh, wg and Egfr mutant embryos. Expression of Cut and Sal is initiated at stage 11 in all of these mutants, although the somewhat disorganised patterns, especially from late stage 11, may reveal roles for these genes in signalling in sizing or shaping the posterior spiracle primordia. Alternatively, these defects may result from altered morphology of mutant embryos. In any case, the induction of the early markers Sal and Cut in A8 dorsal ectoderm of mutant embryos indicates that posterior spiracle primordia specification does occur in the absence of signalling by Wg, Hh or Egfr. Transcription of ems, another AbdB target that is activated slightly later than Cut, although not affected in hh mutants, is lost in wg or Egfr mutants. Thus, proper regulation of AbdB downstream targets activated following primordia specification appears dependent on signalling activities (Merabet, 2005).

The role was examined of Wg, Hh and Egfr signalling pathways in posterior spiracle organogenesis (i.e., after the specification of presumptive territories). Co-labelling experiments performed on embryos expressing GFP driven by ems-Gal4 or by sal-Gal4 indicate that whereas Cut and Sal are already expressed at early stage 11, GFP is detected from late stage 11 only. These two drivers, which promote expression approximately 1 hour after primordia specification, were used to express DN molecules for each pathway, counteracting Wg (DN-TCF), Egfr (DN-Egfr) or Hh [DN-Cubitus interuptus (Ci)] signalling from that time on. Blocking either pathway in spiracular chamber cells does not perturb stigmatophore morphogenesis, but specifically leads to the loss of differentiated filzkörpers. Conversely, blockade in stigmatophore cells provokes in each case its flattening, while differentiated filzkörpers do form (Merabet, 2005).

To ask how signalling inhibition interferes with the genetic modules initiated downstream of AbdB, expression of Sal and Cut was examined from stages 11 to 13. No major defects are seen until late stage 12. Strong deviation from the wild-type patterns is, however, observed slightly later, from stage 13 onwards: Sal expression in basal cells of the stigmatophore is lost and Cut expression remains in only a few scattered cells. The 2-hour delay seen between the onset of DN molecules expression and the detection of Sal and Cut could reflect the time required for shutting down the pathways. Alternatively, Sal and Cut expression may not require signalling activities before stage 13. To discriminate between these possibilities, an earlier expression of the DN molecules was forced, using the 69B-Gal4, known to promote protein accumulation by the onset of stage 11 (i.e., slightly before posterior spiracle primordia specification). Strong defects in Sal and Cut expression were again seen only in stage 13 embryos, supporting the notion that signalling activities are dispensable before the end of stage 12, but are required from stage 13 onwards to maintain Sal in basal stigmatophore cells and Cut in the spiracle chamber (Merabet, 2005).

A8-specific modulation of rho and wg patterns at mid-stage 11 suggests a regulation by AbdB. In AbdB mutants, rho expression in the spiracle-specific cell cluster is lost, and wg transcription does not evolve towards an A8-specific pattern. In embryos expressing AbdB ubiquitously, ectopic posterior spiracle formation in the trunk can be identified as ectopic sites of Cut accumulation. In such embryos, rho and wg are induced in trunk segments following patterns that resemble their expression in A8: rho in a cluster that overlaps the Cut domain, and wg in few cells abutting ectopic Cut-positive cells. These transcriptional responses to loss and gain of function of AbdB indicate that the Hox protein controls the A8-specific expression patterns of wg and rho. The lines gene (lin), which is known to be required for Cut and Sal activation by AbdB, also controls wg and rho patterns respecification (Merabet, 2005).

In contrast to wg and rho, hh does not adopt an A8-specific expression pattern at mid-stage 11. At that stage, hh expression pattern is not affected upon AbdB mutation. The hh stripe in A8 lies posterior and adjacent to spiracular chamber cells and overlaps stigmatophore cells, suggesting that Hh signalling may participate in the regulation of rho and wg transcription by AbdB. In support of this, it was found that the AbdB-dependent aspects of rho and wg transcription patterns are missing in hh mutant embryos. Thus, inputs from both Hh and AbdB are required to remodel Wg and Egfr signalling in A8 (Merabet, 2005).

The dependence of wg and rho A8 expression patterns on Hh, and the loss of ems expression in wg and rho but not in hh mutants, suggest that transcription of ems requires Wg and Egfr signalling prior to wg and rho pattern respecification by AbdB and Hh. To explore this point further, the time course of ems, wg and rho expression was comparatively analyzed. Embryos bearing an ems-lacZ construct stained for ß-Gal and for wg or rho transcripts show that ems expression precedes wg pattern respecification, and occurs at the same time as rho acquires an A8-specific pattern. Importantly, A8-specific rho clusters were never observed before the onset of ems expression. Thus, ems transcription starts before wg and at the same time as rho pattern respecification, supporting that signalling by Wg and Egfr is required prior to mid-stage 11. These observations also indicate that respecification of the wg pattern occurs slightly later than that of rho, which could not been concluded from changes in embryo morphology (Merabet, 2005).

To determine whether signalling by Wg and Egfr from local sources is important for posterior spiracle organogenesis, the production of Wg and Spi^S (the mature form of Spi) ligands was forced from domains broader than normal in A8 dorsal ectoderm. This was performed after posterior spiracle specification, using the ems-Gal4 and sal-Gal4 drivers. Ectopic signalling results in abnormally shaped posterior spiracles: stigmatophores are reduced in size and filzkörpers do not elongate properly. Ectopic signalling from all presumptive stigmatophore cells results in stronger defects than those produced when ectopic signals emanate from all spiracular chamber cells. This can be correlated to the fact that sal-Gal4 drives expression in a pattern that more strongly diverges from the wild-type situation than ems-Gal4 does. Thus, restricted delivery of Wg and Spi^S signals is required for accurate posterior spiracle organogenesis (Merabet, 2005).

It was next asked whether, downstream of Hh, the Wg and Egfr pathways provide separate inputs for posterior spiracle organogenesis. Two sets of experiments were conducted and it was found that: (1) in embryos respectively mutant for Egfr or wg, wg and rho acquire A8-specific patterns; (2) epistasis experiments performed by forcing in spiracular or stigmatophores cells the activity of one pathway while inhibiting the other indicate that loss of one pathway could not be rescued by the other. Thus, Egfr and Wg pathways do not act as hierarchically organised modules, but provide independent inputs for posterior spiracle organogenesis (Merabet, 2005).

The expression of the posterior compartment selector gene engrailed (en) until stage 12 follows a striped pattern identical in all trunk segments. Later on, En adopts a pattern that is specific to A8: it is no longer detected in the ventral part of the segment; dorsally, the En stripe has turned to a circle of cells that surround the future posterior spiracle opening and express the stigmatophore marker Sal. The transition from a striped to a circular pattern depends on AbdB. This transition could result either from a migration of en posterior cells towards the anterior, or from transcriptional initiation in cells that were not expressing en before stage 12, and that can therefore be defined as anterior compartment cells (Merabet, 2005).

To distinguish between the two possibilities, en-Gal4/UAS-lacZ embryos were simultaneously stained with anti ß-Gal and anti-En antibodies. If circle formation results from cell migration, one would expect ß-Gal and En to be simultaneously detected in all cells of the circle since the two proteins are already co-expressed in the posterior compartment stripe earlier on. Conversely, if the circle results from de novo expression, one would expect anterior cells in the circle to express En before ß-Gal, since ß-Gal production requires two rounds of transcription/translation compared with one for En. It was found that cells from the anterior part of the circle express En but not ß-Gal in stage 13 embryos, which demonstrates that de novo expression of En occurs in anterior compartment cells. Further supporting En expression in anterior compartment cells, it was found that precursors of anterior spiracle hairs that do not express En at stage 12 do so at stage 13. Engrailed function in A8 is essential for posterior spiracle development, since stigmatophores do not form in en mutants, and are restored if En is provided in stigmatophore cells (Merabet, 2005).

It was also found that although identical in all abdominal segments at stage 11, hh transcription adopts an A8-specific pattern from stage 12 onwards: transcripts are then localised only at the anterior border of the En stripe. This expression of hh is lost in AbdB mutants and still occurs in en mutant. The uncoupling of hh transcription from En activity in the dorsal A8 ectoderm correlates with the distinct phenotypes seen for en mutants, which do differentiate filzkörper like structures, and for hh mutants, which do not (Merabet, 2005).

Data in this paper allow the distinguishing of four phases in functional interactions between AbdB and signalling by Wg, Hh and Egfr during posterior spiracle formation. The first phase corresponds to the specification of presumptive territories of the organ. The signalling activities are not involved in this AbdB-dependent process, since they are not required for the induction of the earliest markers of spiracular chamber and stigmatophore cells, Cut and Sal, in the dorsal ectoderm of A8. The second phase, which immediately follows primordia specification, concerns the regulation of AbdB target genes activated slightly later. Inputs from the Hox protein and the Wg and Egfr pathways are then simultaneously needed, as seen for transcriptional initiation of the ems downstream target. This function of Wg and Egfr signalling precedes and does not require the reallocation of signalling sources in A8-specific patterns; impairing A8-specific expression of wg and rho by loss of hh signalling does not affect ems expression. Within the third phase, AbdB and Hh activities converge to reset wg and rho expression patterns. The three phases take place in a narrow time window, less than 1 hour during stage 11, and could only be distinguished by studying the functional requirements of Wg, Hh and Egfr for transcriptional regulation in the posterior spiracle (Merabet, 2005).

The fourth phase is referred to as an organogenetic phase. Data obtained using DN variants to inhibit the pathways in cells already committed to stigmatophore or filzkörper fates, indicate that Wg, Egfr and Hh pathways are required for organ formation after specification and early patterning of the primordia. Their roles are then to maintain the AbdB downstream targets' expression in posterior spiracle cells as development proceeds, as shown for Cut and Sal at stage 13 (Merabet, 2005).

A salient feature of AbdB function during posterior spiracle development is to relocate Wg and Egfr signalling sources in the dorsal ectoderm at mid-stage 11. wg and rho then adopt expression patterns that differ from expressions in other abdominal segments, conferring axial properties unique to A8 to otherwise segmentally reiterated patterning cues. Resetting Wg and Egfr signalling sources into restricted territories is of functional importance for organogenesis, as revealed by the morphological defects that result from the delivery of Wg or Spi^S signals in all spiracular chamber or stigmatophore cells after the specification phase. During stage 12, AbdB also relocates the Hh signalling source by inducing En-independent expression of hh in the dorsal ectoderm. Thus, later than Wg and Egfr signalling, the Hh signal also acquires properties unique to A8. In generating this pattern, AbdB plays a fundamental role in uncoupling hh transcription from En activity, providing a context that prevents anterior compartment En-positive cells to turn on hh transcription, and that allows hh expression in the absence of En in other cells. Slightly later, at stage 13, AbdB modifies the expression of the posterior selector gene en, initiating de novo transcription in anterior compartment cells. In these cells, En fulfils different regulatory functions than in posterior cells, as discussed above for hh regulation. Changes in En expression and function can be interpreted as a requisite to loosen AP polarity in A8 and gain circular coordinates required for stigmatophore formation (Merabet, 2005).

Larval

The level of polyteny of the Drosophila salivary gland chromosomes was determined throughout the chromosome region 89E1-4, the locus of the Bithorax Complex. A zone of underreplication spans the 300 kb of DNA from the Ubx to Abd-B loci. From the centromere proximal end of the complex, a 70-kb-long gradual decrease of polytenization starts with the Ubx transcription unit and, after a floor corresponding to the abd-A locus, raises gradually back to the maximum over 70 kb in the region of the Abd-B transcription unit. The maximum relative level of underreplication is about 10-fold. The level of polyteny of chromosomes in a gland is estimated at about 1,000. Therefore, even at the lowest point of polyteny, the number of DNA duplexes assuring the continuity of the chromosomes can be estimated at 100 and certainly not limited to a unique double helix. In flies carrying the mutation Suppressor of DNA Underreplication [Su(UR)ES], the underreplication of the Bithorax Complex is fully suppressed. In the wild type, the Bithorax Complex forms a weak point featuring thinner bands separated by clefts or constrictions. In Su(UR)ES strain in contrast, the 89E1-4 band looks like a single solid band consisting of homogenous dense material. It is speculated that the wild-type Su(UR)ES protein hampers DNA replication of silenced domains and leads to their underreplication in salivary gland polytene chromosomes (Moshkin, 2001).

The expression of homeotic Bithorax Complex proteins in the fat bodies of Drosophila larvae was analyzed by staining with specific antibodies. These proteins are differentially expressed along the anteroposterior (AP) axis of the fat body, with patterns parallel to those characterized for the larval and adult epidermis. Since fat body nuclei have polytene chromosomes, it was possible to identify the BX-C locus and show that it assumes a strongly puffed conformation in cells actively expressing the genes of the BX-C. Immunostaining of these polytene chromosomes provided the resolution to cytologically map binding sites of the three proteins: Ubx, Abd-A and Abd-B. The results of this work provide a system with which to study the positioning of chromatin regulatory proteins in either a repressed and/or active BXC at the cytological level. In addition, the results of this work provide a map of homeotic target loci and thus constitute the basis for a systematic identification of genes that are direct in vivo targets of the BX-C genes (Marchetti, 2003).

Ubx is intensely expressed in a contiguous region, with an anterior limit distal to, but near, the anterior crossbridge in the third thoracic segment (T3). The domain includes the gonad, and the posterior limit falls in a region corresponding approximately to segments A6/A7. The Abd-A protein is expressed anteriorly in a longitudinal line of cells in a region corresponding to the A2 segment. From that point posteriorly it is accumulated in almost all of the cells in a region that is co-extensive with abdominal segments A3-A7. Finally, the Abd-B protein is expressed to the posterior end of the fat body with an anterior limit in the middle of A4. It is interesting to note that although Ubx is detected in all the nuclei of its domain, Abd-A and Abd-B are only expressed in subsets of nuclei in their respective domains. However, in the region corresponding to segments A4-A6 all of the proteins are co-expressed in most nuclei. These observations demonstrate that the protein products of the BX-C are differentially expressed along the AP axis of the fat body in a manner reminiscent of their accumulation patterns in the epidermis. However, the similarity of expression patterns of the proteins between the two tissues is more evident at their anterior limits than in their posterior extent. Perhaps the most striking result is the overlap of the three proteins in the region around the gonads. It will be interesting to determine if this overlap of domains has some operational significance, or if it is functionally irrelevant (Marchetti, 2003).

Pupal

The cuticle of the adult abdomen of Drosophila is produced by nests of imaginal histoblasts, which proliferate and migrate during metamorphosis to replace the polyploid larval epidermal cells. In this report, a detailed description is presented of the expression of four key patterning genes, engrailed (en), hedgehog (hh), patched (ptc), and optomotor-blind (omb), in abdominal histoblasts during the first 42 h after pupariation, a period in which the adult pattern is established. In addition, the expression is described of the homeotic genes Ultrabithorax, abdominal-A, and Abdominal-B, which specify the fates of adult abdominal segments. The results indicate that abdominal segments develop in isolation from one another during early pupal stages, and that some patterning events are independent of hh, wg, and dpp signaling. Pattern and polarity in a large anterior portion of the segment are specified without input from Hh, and evidence is presented that abdominal tergites possess an underlying symmetric pattern upon which patterning by Hh is superimposed. The signals responsible for this underlying symmetry remain to be identified (Kopp, 2002).

The dorsal cuticle of a typical abdominal segment contains a stereotyped sequence of pattern elements. At the anterior edge of each segment is the acrotergite, a narrow strip of naked sclerotized cuticle (a1). The remainder of the tergite is covered by trichomes, and can be subdivided into four regions. From anterior to posterior these regions are: a lightly pigmented region with no bristles (a2 fate); a lightly pigmented region that contains two to three rows of microchaetes (a3); a darkly pigmented region with one to two rows of microchaetes (a4); and a darkly pigmented region with a single row of macrochaetes (a5). The tergite is followed by the unpigmented posterior hairy zone (PHZ), which is composed of both anterior (a6) and posterior (p3) compartment cells. All trichomes and bristles in the segment are oriented uniformly from anterior to posterior. Finally, at the posterior edge of the segment is a zone of thin, naked intersegmental membrane (ISM), which can be subdivided into anterior smooth (p2) and posterior crinkled (p1) regions (Kopp, 2002).

The adult abdominal pattern is established in the first 2 days of pupal development, concurrent with the proliferation and migration of histoblasts and the destruction of the larval epidermal cells (LECs.) The spatial and temporal evolution of en, hh, ptc, and omb expression is followed during this critical period. The cuticle of each abdominal hemisegment is formed by three major histoblast nests. The anterior dorsal nest (aDHN) is composed of anterior compartment histoblasts and produces the tergite and part of the PHZ (a1-a6), whereas the posterior dorsal nest (pDHN) is composed of posterior compartment cells and produces the intersegmental membrane and the remainder of the PHZ (p1-p3). The ventral histoblast nest, which produces the sternite and pleura, contains both anterior and posterior compartment cells. en, hh, ptc, and omb are expressed in similar patterns in dorsal and ventral histoblasts, and the description is limited to the dorsal abdomen (Kopp, 2002).

Segment identities in the abdomen are specified by the Ubx, abd-A, and Abd-B genes of the bithorax complex (BX-C). More precisely, BX-C genes control the development of parasegments (ps), which are composed of the posterior compartment of one segment and the anterior compartment of the following segment. Ubx controls the identity of ps6, which includes the anterior compartment of the first abdominal segment (A1); abd-A functions primarily in ps7-ps9 (A2-A4), although it also contributes to the identities of ps10-ps12; and Abd-B is the main determinant of the identities of ps10-ps12. In the pupal abdomen, Abd-B is expressed strongly in ps12 (A7) (in females; the last abdominal segment is rudimentary in males), weaker in ps11 (A6), and at very low levels in ps10 (A5). This pattern is consistent with the view that different levels of Abd-B expression promote distinct segment identities in the posterior abdomen. abd-A is expressed in ps7 (A2) through ps12 (A7), at levels gradually increasing from the anterior to the posterior parasegments. Ubx is expressed only in the anterior compartment of A1 (ps6) in the abdominal epidermis. Double staining for Ubx and hh-lacZ shows that the posterior boundary of Ubx expression coincides precisely with the ps6/ps7 boundary. Thus, Ubx and abd-A are expressed in adjacent nonoverlapping domains, contrasting sharply with their overlapping expression in the embryo. Ubx expression is eliminated from A1 in the abd-A gain-of-function mutant Uab⁵, suggesting that abd-A represses Ubx during the pupal stage (Kopp, 2002).

Effects of Mutation or Deletion

The proteins responsible for m and r activities were ectopically expressed in fly embryos. The resultant larval cuticular transformations are consistent with the genetically defined role of each protein during normal embryogenesis. Both ABD-B proteins activate ectopic expression of transcripts encoding the m protein, but the levels of Antennapedia, Ultrabithorax and abdominal-A transcripts are differentially repressed (Kuziora, 1993).

To determine when the homeotic genes are required for specific developmental events Ultrabithorax, abdominal-A and Abdominal-B proteins were expressed at different times during development using the GAL4 targeting technique. Early transient homeotic gene expression has no lasting effects on the differentiation of the larval epidermis, but it switches the fate of other cell types irreversibly (e.g. the spiracle primordia). One cell type in the peripheral nervous system makes sequential, independent responses to homeotic gene expression. There is also an in vivo competition between the bithorax complex proteins for the regulation of their down-stream targets (Castelli-Gair, 1994).

The metameric organization of the Drosophila melanogaster tail is obscured by developmental events that partially suppress or fuse some of its regions. engrailed patterns in different bithorax complex mutants ( Abd-B morphogenetic (m) and regulatory (r) mutants) demonstrate that Abd-B acts to suppress embryonic ventral epidermal structures on the posterior side of A8 to A9 (Kuhn, 1995).

The tumorous-head-3 (tuh-3) mutation has been associated with the insertion of mobile element Delta 88 at +200 on the bithorax complex (BX-C) DNA map, 5' of all Abdominal-B (Abd-B) transcripts. Different phenotypes of tuh-3 are regulated by the tumorous-head-1 (tuh-1) maternal effect locus. In the presence of the recessive tuh-1h maternal effect, tuh-3 offspring produce homeotic abdominal and genital tissue in the head. In the presence of the dominant tuh-1g maternal effect, tuh-3 offspring have normal heads but now show genital defects. One other mutant, I127B, produces flies with identical defects to that of tuh-3 in the presence of both maternal effects. Molecular analysis of I127B reveals the insertion of mobile element 297 in the Abd-B gene, approximately 25 kb downstream of the Delta 88 insertion in tuh-3. No other abnormalities are detected. Reexamination of the tuh-3 strain reveals a 297 insertion in an identical region to that of I127B, in addition to the Delta 88 insertion. Recombinants of tuh-3, carrying 297 only, produce homeotic head defects and genital defects in the presence of the tuh-1h and tuh-1g maternal effects, respectively. Recombinants of tuh-3, carrying Delta 88 only, fail to produce any defects in the presence of either maternal effect. Based upon these results, it is proposed that it is the 297 insertion in the Abd-B gene, not Delta 88, that is responsible for the tuh-3 mutation (Mack, 1997).

The genital disc of Drosophila, which gives rise to the genitalia and analia of adult flies, is formed by cells from different embryonic segments. To study the organization of this disc, the expressions of segment polarity and homeotic genes were investigated. The organization of the embryonic genital primordium and the requirement of the engrailed and invected genes in the adult terminalia were also analysed. The three primordia, the female and male genitalia plus the analia, are composed of an anterior and a posterior compartment. In some aspects, each of the three primordia resemble other discs: the expression of genes such as wingless and decapentaplegic in each anterior compartment is similar to that seen in leg discs; the absence of engrailed and invected causes duplications of anterior regions, as occurs in wing discs. The absence of lineage restrictions in some regions of the terminalia and the expression of segment polarity genes in the embryonic genital disc suggest that this model of compartmental organization evolves, at least in part, as the disc grows. The expression of homeotic genes suggests a parasegmental organization of the genital disc, although these genes may also change their expression patterns during larval development (Casares, 1997).

Mutations in Abd-B transform female genitalia into abdomen, suggesting that the activity of Abd-B is a prerequisite for the specification of the terminalia by the sex-determing genes. abd-A is expressed only in female genital discs, in the region corresponding to the female genital primordium, particularly in the prospective internal female genitalia. abd-A expression is coincident with engrailed in the central region of the female genital primordium engrailed band. Abd-B transcripts are located in the genital disc. The Abd-B protein is present in the male and female primordia in both male and female discs, leaving unstained the region where the analia map. Abd-B expression is coincident with en bands 1 and 2. In female discs, Abd-B m transcript is present only in the female genital primordium: transcript levels are strong in the prospective external genitalia and faint in the prospective internal genitalia. In the male disc, only the repressed male primordium is labelled. Abd-B r transcript is expressed in the repressed male primordium of female discs and the male genetal primordium of male discs. caudal is located in the analia primordium of the genital disc, overlapping with the third engrailed band. However, caudal and enoverlap in only a few, dorsally located, epidermal nuclei of stage 14 embryos. This overlap is not seen in the ventrally located embryonic genital disc where caudal expression is observed in its posterior region. This suggests that en expression in anal primordium of mature genital discs appears during larval development. The perianal ring corresponds to the terminal band of en, and the co-expression of en and cad is maintained from the third instar disc until the adult stage (Casares, 1997)

The genital disc consists of three primordia: moving from anterior to posterior they are the female genital primordium, the male genital primordium and the anal primordia. Only one of the two genital primordia develops, depending on the individual's sex, whereas the anal primordium develops in both sexes. It is proposed here that the genital disc, which is of ventral origin, is organized in a manner similar to the antennal and leg discs: the expression domains of decapentaplegic and wingless are mostly complementary and abut engrailed expression. An analysis was made of the roles of the genes hedgehog, patched, dpp and wg in the development of the three primordia that form the genital disc. The morphogenetic alterations produced by ectopic expression of hh mimic a lack of ptc function. Both genetic conditions cause derepression of dpp and wg. Ectopic expression of either of these genes causes non-autonomous duplications and/or reductions of genital and anal structures. Some of these alterations are explained by the mutual repression of wg and dpp. In the development of the genital disc, the functional relationships between these genes seem to be analogous to those described for leg and antennal discs: dpp and wg are induced in the anterior compartment by Hh protein blocking the repressive effect of Ptc, and the mutual repression of dpp and wg restrict one another to their respective domains. It may be concluded that dpp and wg act as general organizers for development of the genital disc (Sanchez, 1997).

The gene doublesex controls which genital primordium of the genital disc will grow and which will be repressed. The female genital primordium develops from A8 and the male genital primordium develops from A9. Therefore, the gene doublesex must act in concert with another regulatory gene(s) to determine the genital primordium that develops in each sex. A possible candidate for this additional regulatory element is the homeotic gene Abd-B, since this gene is reponsible for specification of posterior segments. Under normal conditions, the female genital primordium can develop either into genitalia or remain in the repressed state, producing no adult structures. In mutant conditions for Abdominal-B m transcript, it can develop into an abdominal tergite plus sternite. Similarly, under normal conditions, the male genital primordium can develop into either normal genitalia or remain in the repressed state, forming no adult strucures. In Abd-B r transcript mutants it develops into rudimentary genitalia (Sanchez, 1997 and references).

The development of the genital primordia is based on two processes: cell proliferation and sexual differentiation. Cell proliferation refers to the capacity of each genital primordium to grow or to be kept in the repressed state. Sexual differentiation refers to the type of adult structure formed by each genital primordium. It is proposed that the control of cell proliferation in the male and female genitalia requires the concerted action of Abd-B and doublesex, either directly or indirectly, through the expression of the genes dpp and wingless. Thus, in female genital discs, the repressed male primordium does not express dpp whereas the repressed female primordium of the male genital discs expresses a reduced level of dpp. This reduced level seems to be insufficient to stimulate cell growth. In contrast, when strong dpp levels are obtained in the repressed female primordium of male discs, repressed female primordia overproliferate in mutants for patched or costal-2, as well as in the discs where uniform ectopic expreession of hedgehog is produced. The genes dpp and wg, however, do not participate in the sexual differentiation process, which depends on sexual cytodifferentiation genes. Thus the growth of repressed female primordia of the patched mutant male discs would give rise to no adult female genital structures since the genetic sex is male (Sanchez, 1997 and references).

The Drosophila kismet gene is related to chromatin-remodeling factors and is required for both segmentation and segment identity

The Drosophila trithorax group gene kismet (kis) was identified in a screen for extragenic suppressors of Polycomb (Pc) and subsequently shown to play important roles in both segmentation and the determination of body segment identities. One of the two major proteins encoded by kis (Kis-L) is related to members of the SWI2/SNF2 and CHD families of ATP-dependent chromatin-remodeling factors. To clarify the role of Kis-L in gene expression, its distribution on larval salivary gland polytene chromosomes was examined. Kis-L is associated with virtually all sites of transcriptionally active chromatin in a pattern that largely overlaps that of RNA Polymerase II (Pol II). The levels of elongating Pol II and the elongation factors SPT6 and CHD1 are dramatically reduced on polytene chromosomes from kis mutant larvae. By contrast, the loss of Kis-L function does not affect the binding of PC to chromatin or the recruitment of Pol II to promoters. These data suggest that Kis-L facilitates an early step in transcriptional elongation by Pol II (Srinivasan, 2005).

The Drosophila kismet gene was identified in a screen for dominant suppressors of Polycomb, a repressor of homeotic genes. kismet mutations suppress the Polycomb mutant phenotype by blocking the ectopic transcription of homeotic genes. Loss of zygotic kismet function causes homeotic transformations similar to those associated with loss-of-function mutations in the homeotic genes Sex combs reduced and Abdominal-B. kismet is also required for proper larval body segmentation. Loss of maternal kismet function causes segmentation defects similar to those caused by mutations in the pair-rule gene even-skipped. The kismet gene encodes several large nuclear proteins that are ubiquitously expressed along the anteriorposterior axis. The Kismet proteins contain a domain conserved in the trithorax group protein Brahma and related chromatin-remodeling factors, providing further evidence that alterations in chromatin structure are required to maintain the spatially restricted patterns of homeotic gene transcription (Daubresse, 1999).

The genetic interactions between kis and Pc provided the first clue that kis plays an important role in the determination of body segment identity. kis mutations suppress the adult Pc phenotype by preventing the ectopic transcription of homeotic genes. Thus, kis is a member of the trithorax group of homeotic gene activators. Mosaic analyses reveal that loss of kis function causes homeotic transformations, including the transformation of first leg to second leg and the fifth abdominal segment to a more anterior identity. These phenotypes are identical to those associated with loss-of-function Scr and Abd-B mutations, respectively. Taken together, these findings suggest that kis acts antagonistically to Pc to activate the transcription of both Scr and Abd-B. It is intriguing that kis mutations alter the fate of only the fifth abdominal segment, since the identities of the fifth through ninth abdominal segments are determined by a single homeotic gene, Abd-B (Daubresse, 1999).

Variations in the levels of Abd-B protein result in the differences between these abdominal segments, with Abd-B expression being lowest in the fifth abdominal segment. Parasegment-specific cis-regulatory regions, termed infra-abdominal (iab) regions control Abd-B expression. Each iab region is named for the segment that it affects (iab-5 through iab-9). Mutations in both iab-5 and kis affect the identity of only the fifth abdominal segment, suggesting that the Kis protein may interact specifically with the iab-5 cis-regulatory element of Abd-B (Daubresse, 1999).

kis probably interacts not only with Scr and Abd-B, but with other homeotic genes as well. For example, the isolation of kis mutations as enhancers of loss-of-function Deformed (Dfd) mutations suggests that kis is probably also required to activate transcription of this ANTC homeotic gene. Furthermore, kis duplications strongly enhance the transformation of wing to haltere in Pc heterozygotes, a phenotype caused by the ectopic transcription of Ubx in the wing imaginal disc. However, kis mutations do not cause haltere-to-wing transformations due to decreased Ubx transcription. A possible explanation for the lack of homeotic transformations in kis clones in segments other than the prothoracic and fifth abdominal segment is that the mutations used in these studies are not null alleles. kis¹ is a strong loss-of-function mutation. It has not been characterized at the molecular level, however, and may not completely eliminate kis function. It is also possible that sufficient levels of Kis protein persist in homozygous mutant tissue following mitotic recombination to support normal development. Further genetic studies, including the analysis of conditional kis alleles, will be necessary to distinguish between these possibilities (Daubresse, 1999).

Germline clonal analysis has revealed an unanticipated role for kis in segmentation. Embryos from mosaic kis^S females exhibit a deletion or alteration of every other segment, while mutant embryos from mothers bearing germline clones of the stronger kis¹ allele usually develop only half of the normal number of segments. This variation in phenotypic severity is closely correlated with the extent to which en expression is disrupted. The phenotypes associated with loss of maternal kis function resemble those caused by mutations in pair-rule segmentation genes that cause the deletion of the odd-numbered parasegments. kis thus appears to be necessary for the expression (or function) of one or more pair-rule genes. Recent genetic studies have suggested that kis may also be involved in the Notch signaling pathway. Thus it appears that kis plays roles in addition to the regulation of homeotic genes (Daubresse, 1999).

What pair-rule genes might require kis for their activity? Based on the kis mutant phenotype, perhaps the best candidates are eve and hairy (h), both of which are required for the formation of odd-numbered parasegments. Unlike eve, h and most other segmentation genes, kis is uniformly expressed in the early embryo. This raises the possibility that Kis functions as an essential cofactor or modifier of Eve or other pair-rule proteins. It is also possible that loss of kis function might result in pair-rule genes being transcribed outside of their normal expression domains. Additional work will be necessary to determine the molecular basis of the segmentation defects resulting from loss of maternal kis function (Daubresse, 1999).

Abdominal-B is essential for proper sexually dimorphic development of the Drosophila gonad

Sexual dimorphism requires the integration of positional information in the embryo with the sex determination pathway. Homeotic genes are a major source of positional information responsible for patterning along the anterior–posterior axis in embryonic development, and are likely to play a critical role in sexual dimorphism. The role of homeotic genes in the sexually dimorphic development of the gonad has been investigated in Drosophila. Abdominal-B (ABD-B) is expressed in a sexually dimorphic manner in the embryonic gonad. Furthermore, Abd-B is necessary and sufficient for specification of a sexually dimorphic cell type, the male-specific somatic gonadal precursors (msSGPs). In Abd-B mutants, the msSGPs are not specified and male gonads now resemble female gonads with respect to these cells. Ectopic expression of Abd-B is sufficient to induce formation of extra msSGPs in additional segments of the embryo. Abd-B works together with abdominal-A to pattern the non-sexually dimorphic somatic gonad in both sexes, while Abd-B alone specifies the msSGPs. These results indicate that Abd-B acts at multiple levels to regulate gonad development and that Abd-B class homeotic genes are conserved factors in establishing gonad sexual dimorphism in diverse species (DeFalco, 2004).

The homeotic genes initially work to specify the distinct types of somatic cells that will contribute to the gonad. Abd-B is necessary for the specification of msSGPs in PS13, and is sufficient to induce msSGP clusters in ectopic positions. Thus, Abd-B appears to restrict msSGP development to PS13. Consistent with this idea, the anterior limit of Abd-B expression is initially in PS13, and only later extends into more anterior regions (DeFalco, 2004).

In a similar manner, abd-A is required for the specification of SGPs in PS10-12. abd-A acts to promote SGP development by blocking srp and fat body development in these PS. abd-A is also sufficient to induce ectopic SGPs when expressed in more anterior regions. Thus, the first stage where the homeotic genes act in patterning the somatic gonad is in restricting SGP and msSGP development to their proper PS (DeFalco, 2004).

The homeotic genes next act to pattern distinct identities within the somatic gonad. abd-A alone specifies anterior SGP identity, a combination of abd-A and Abd-B specifies posterior SGP identity, and Abd-B alone is required to specify msSGP identity. This role for the homeotic genes is greatly facilitated by the fact that the cells of the somatic gonad are originally specified in four different PS of the embryo, allowing these cells to acquire unique homeotic gene expression profiles, or Hox codes, that will determine A-P identities. These Hox codes are maintained as the SGPs and msSGPs move anteriorly and coalesce with the germ cells to form a gonad in PS10, as clearly evidenced by the maintenance of Abd-B expression in the msSGPs and posterior SGPs in the coalesced gonad (DeFalco, 2004).

The precursors for the dorsal vessel, the Drosophila heart, are similarly specified in separate PS (4-13), allowing distinct identities to be patterned along the A-P axis by Ultrabithorax, abd-A, and Abd-B. This is also true in other tissues, such as the visceral mesoderm and fat body. Thus, it is a common theme that organ precursors are specified in a spatially segregated manner, allowing the cells to acquire distinct identities that are preserved during organogenesis (DeFalco, 2004).

The last stage where homeotic genes act is in the development of sexual dimorphism in the gonad. The unique identity of the msSGPs, provided in part by Abd-B, allows these cells to behave differently in males and females. In males these cells join the posterior of the coalescing gonad, while they are removed by programmed cell death in the female. Furthermore, the anterior SGPs also behave differently in males vs. females, indicating that the unique SGP identity conferred by abd-A also allows cells to respond differently to distinct sexual identities. How cell identity in the gonad, regulated by the homeotic genes, interacts with the sex determination pathway to produce distinct outputs is a fascinating area for future study (DeFalco, 2004).

There appears to be a common regulatory link between cell types specified by Abd-B and sex-specific regulation by the sex determining gene dsx. Abd-B is critical for specifying msSGP identity, and dsx is critical for causing these cells to behave differently in males and females. The head involution defective (hid) gene is essential for female-specific programmed cell death of the msSGPs, and is a candidate for being differentially regulated by Abd-B and dsx in the two sexes (DeFalco, 2004).

A similar relationship between Abd-B and dsx has been observed in several other examples. It has been shown that these genes interact to control the pattern of sex-specific pigmentation in the Drosophila abdomen, and that bric à brac (bab) integrates positional and sexual inputs in this tissue. The combination of Abd-B and female identity allows bab to act in blocking pigment formation, whereas in males, Abd-B can repress bab in order to allow pigment formation to occur (DeFalco, 2004).

Abd-B and dsx also cooperate in sex-specific development of the genital disc, which gives rise to the non-gonadal structures that must eventually join with the gonad to form the functional adult reproductive system. Abd-B and dsx act through the signaling molecules Wingless and Decapentaplegic to pattern the genital disc, and through the FGF ligand Branchless to regulate mesodermal cell migration into the disc. The expression of a key regulator of genital disc development, dachshund, has been shown to be affected by both Abd-B and dsx (DeFalco, 2004).

Thus, Abd-B and dsx are used in combination to pattern several independent tissues during development. Other cell-type-specific factors must be involved, since these tissues exhibit distinct responses to Abd-B and dsx. However, Abd-B and dsx clearly form a common regulatory network used multiple times in development to create sexual dimorphism (DeFalco, 2004).

Data from studies on Caenorhabditis elegans and mice suggest that regional identities conferred by homeotic genes are required for the proper development and sexual dimorphism of the gonad in these species. An Abd-B homolog in C. elegans, egl-5, is expressed in the somatic gonad and is required for SGP development. Furthermore, in a certain percent of egl-5 mutant males it appears as if the somatic gonad takes on a hermaphrodite-like morphology. This sex-specific phenotype may be analogous to what is seen in Drosophila, in which Abd-B mutant male gonads take on a partial female phenotype (as characterized by an absence of msSGPs). Due to a great deal of gene expansion in the mammalian homeotic complex resulting in potential gene redundancy or overlapping function, it may prove difficult to find a single mouse gene with a similar phenotype to Abd-B or egl-5. However, Hoxa10 male knockout mice exhibit blocks in spermatogenesis, while the female gonad can produce functional eggs, demonstrating a sexually dimorphic role for posterior Hox genes in mouse gonad development (DeFalco, 2004).

In addition, studies of the Polycomb (Pc) group of homeotic regulators are also consistent with a role for homeotic genes in establishing sexual dimorphism. C. elegans Pc homologs mes-2, mes-3, and mes-6 have been shown to regulate homeotic gene expression, in particular egl-5 and mab-5, the latter of which is necessary for sexually dimorphic male V-ray sense organs. Knockouts of the mouse Pc homolog M33 have altered expression of Hox genes resulting in sterility and male-to-female sex reversal (DeFalco, 2004).

These results indicate that the regulation of homeotic gene expression is important for gonad development and sexual dimorphism in diverse organisms. Although methods of initial sex determination have widely diverged among animal species, many lines of evidence strongly suggest that mechanisms to promote sexual dimorphism in the gonad are conserved. Positional information provided by the homeotic genes is likely to be a key conserved element in creating sexual dimorphism (DeFalco, 2004).

Temporal and spatial expression of homeotic genes is important for segment-specific neuroblast 6-4 lineage formation in Drosophila

Different proliferation of neuroblast 6-4 (NB6-4) in the thorax and abdomen produces segmental specific expression pattern of several neuroblast marker genes. NB6-4 is divided to form four medial-most cell body glia (MM-CBG) per segment in thorax and two MM-CBG per segment in abdomen. Since homeotic genes determine the identities of embryonic segments along the A/P axis, whether temporal and specific expression of homeotic genes affects MM-CBG patterns in thorax and abdomen was ivestigated. A Ubx loss-of-function mutation was found to hardly affect MM-CBG formation, whereas abd-A and Abd-B caused the transformation of abdominal MM-CBG to their thoracic counterparts. In contrast, gain-of-function mutants of Ubx, abd-A and Abd-B genes reduced the number of thoracic MM-CBG, indicating that thoracic MM-CBG resembled abdominal MM-CBG. However, mutations in Polycomb group (PcG) genes, which are negative transregulators of homeotic genes, did not cause the thoracic to abdominal MM-CBG pattern transformation although the number of MM-CBG in a few per-cent of embryos were partially reduced or abnormally patterned. These results indicate that temporal and spatial expression of the homeotic genes is important to determine segmental-specificity of NB6-4 daughter cells along the anterior-posterior (A/P) axis (Kang, 2006).

In the Drosophila embryonic central nervous system (CNS), about 30 glia are produced in a stereotyped pattern in each hemisegment, and certain of these glia are arranged in different patterns between segments along the A/P axis. Thus, it is important to understand how the regional specificity of certain glia is determined and maintained during nervous system development. repo is essentially required for the differentiation and maintenance of glia. Moreover, some of these repo expressing cells, MM-CBG, show different patterns along the A/P axis. In the present study, MM-CBG pattern abnormalities were examined in BX-C and its negative transregulator, PcG mutant embryos (Kang, 2006).

The data showed that Ubx loss-of-function mutation did not cause the homeotic transformation of the abdominal MM-CBG pattern to the thoracic one. However, a loss-of-function mutation in the abd-A gene caused the transformation of abdominal MM-CBG into a thoracic pattern. Abd-B mutant embryos also showed transformation of MM-CBG in its functional domain. These results indicate that unlike Ubx, abd-A and Abd-B genes are involved in the segment-specific MM-CBG pattern formation. The role of BX-C on MM-CBG formation was confirmed using gain-of-function BX-C mutation. Ectopic expression of BX-C with sca-GAL4/UAS system caused thoracic MM-CBG to follow the abdominal pattern of MM-CBG. Unlike the result shown in Ubx loss-of-function mutant embryos, four thoracic MM-CBG were frequently reduced to two or three MM-CBG in Ubx gain-of-function mutant embryos, suggesting that Ubx might be involved in MM-CBG pattern formation. The Abd-A and Abd-B proteins driven by sca-GAL4 driver changed the thoracic MM-CBG pattern to the abdominal one. It was suggested that Abd-A and Abd-B proteins repress the proliferation of MM-CBG through inhibition of CycE in the abdomen, which makes two MMCBG per abdominal segment and four MM-CBG per thoracic segment (Kang, 2006).

PcG mutation causes the ectopic expressions of abd-A and Abd-B genes in the anterior of their functional domains. It is presumed that the ectopic thoracic expressions of abd-A and Abd-B genes would transform thoracic MM-CBG to an abdominal one as shown in the gain-of-function BX-C mutation, because the thoracic pattern of the epidermis and central nervous system are transformed to the abdominal segments in these two mutants. However, PcG mutant embryos showed little evidence of an abnormal MM-CBG pattern in the thorax because most PcG mutant embryos showed wild type thoracic MM-CBG pattern. This was confirmed using a gcm enhancer trap line. A gmc driven reporter was expressed only in the MM-CBG of the abd-A domain. Although Pc zygotic, esc and pho maternal effect mutations caused the ectopic expressions of abd-A and Abd-B in the CNS from head to tail, the anterior boundary of gcm-lacZ expression did not move to more anterior segments. In addition, thorax-specific eg expression pattern was unchanged in PcG mutant embryos (Kang, 2006).

These observations indicate that temporal and spatial homeotic gene expression is important in MM-CBG pattern formation. The homeotic gene products driven by sca- GAL4 driver are present in the neuroectoderm from embryonic stage 8, which clearly changes the thoracic MM-CBG pattern. However, derepressed BX-C gene products caused by PcG mutations do not affect MM-CBG pattern. Ubx, abd-A and Abd-B genes begin to be weakly misexpressed from stage 11 and shows strong ectopic expression at stage 13 in Pc and esc mutant embryos. In wild type embryos MM-CBG appears to proliferate once between stage 11 and 12, and become four cells per segment in the thorax, while there is no cell division of MM-CBG in the abdomen because Abd-A and Abd-B proteins repress CycE expression. So PcG mutants seems to cause the ectopic expression of the BX-C genes after MMCBG are already determined to be prolifered in the thorax. Early segment-specific commitment of NB6-4 progeny cells also supports this conclusion. When BX-C genes are overexpressed from stage 10 using eg-GAL4, thoracic MM-CBG pattern was not changed. Taken together, temporal and spatial expression of the homeotic genes is important to determine segmental-specificity of MM-CBG along the anterior-posterior (A/P) axis (Kang, 2006).

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