teashirt

DEVELOPMENTAL BIOLOGY

Embryonic

See the embryonic expression pattern of tsh at the Berkeley Drosophila Genome Project Patterns of Gene Expression Site.

Transcripts are initially detected at the beginning of the 14th nuclear division in a central ring of cells spanning 45-60% egg length (where 0% is the posterior pole). Three segments-worth of cells are present with three bands resolving in the abdominal region. By the extended germband stage [Images], cells specific to parasegments 3-13 express tsh, forming a striped pattern. At the posterior tip of older embryos, a small group of cells exhibit tsh expression, corresponding to the future anal opening. Expression of tsh can be detected in internal tissues, the trunk region of the CNS, part of the visceral mesoderm and the somatic musculature (Fasano, 1991 and Mathies, 1994).

The teashirt gene has an homeotic function which, in combination with HOM-C genes, determines thoracic and abdominal (trunk) identities. Analysis of Tsh protein distribution during embryogenesis using a specific polyclonal antibody shows that it is nuclear. The protein is present with regional modulation in several tissues within the trunk, suggesting additional tsh functions to those already studied. During gastrulation, Tsh protein is first detected at a low level in a domain located in the central part of the embryo. A transient pattern of five stripes with variable width and spacing is observed. During early germ band elongation (stage 7), the striped pattern disappears and the whole trunk region of the elongateing germ band becomes stained, although more protein is detected in the thoracic than in the abdominal segments. After germ band shortening is completed, one row of cells is more intensively stained in the anterior part of every thoracic and abdominal segment. At this stage, a longitudinal line of labelled cells can be detected at the junction between the amnioserosa and the segmented ectoderm of the trunk. During late stages 14 and 15, these labelled cells provide a good marker to follow dorsal closure and heart morphogenesis. At these stages and later, labelling is also detected in a small group of cells corresponding to the anal tuft, and in three contiguous rows of cells in the eighth abdominal segment (Alexandre, 1996).

The Drosophila visceral mesoderm (VM) is a favorite system for studying the regulation of target genes by Hox proteins. The VM is formed by cells from only the anterior subdivision of each mesodermal parasegment (PS). The VM itself acquires modular anterior-posterior subdivisions similar to those found in the ectoderm. Mesodemal cells located just under the engrailed-expressing cells in the posterior ectodermal compartment have been called the mesodermal "P domain." The dorsal-most cells of the mesodermal P domain in each PS express the homeobox gene bagpipe (bap); they detach from the mesodermal fold and move inward toward the center of the embryo. These bap-expressing cells form the VM progenitor groups. The VM subdivisions, and the metameric expression of Connectin, form in response to ectodermal production of secreted signals encoded by the segment polarity genes hedgehog and wingless and are independent of Hox gene activity. A cascade of induction from ectoderm to mesoderm to endoderm thus subdivides the gut tissues along the A-P axis. Induction of VM subdivisions may converge with Hox-mediated information to refine spatial patterning in the VM. Con patches align with ectodermal engrailed stripes, so the VM subdivisions correspond to PS 2-12 boundaries in the VM. The PS boundaries demarcated by Connectin in the VM can be used to map expression domains of Hox genes and their targets with high resolution. The resultant map suggests a model for the origins of VM-specific Hox expression in which Hox domains clonally inherited from blastoderm ancestors are modified by diffusible signals acting on VM-specific enhancers (Bilder, 1998).

Since Con expression marks the imprint of ectodermal PS boundaries on the VM, Con patches can be used to precisely map the domains of Hox gene transcription in relation to Con patches. teashirt is expressed in two domains. The anterior midgut domain extends from visceral mesoderm segment (VS) 4 to mid-VS 6, where it shares a posterior boundary with Antennapedia; the central midgut domain extends several cells to either side of the VS 8 boundary. dpp is also expressed in two domains: at the gastric caeca, it is found in the A domain of VS2 and the P domain of VS 3, while in the central midgut it extends from the A domain of VS 6 to terminate just anterior to the VS 8 boundary. wg is expressed just anterior to the VS 8 boundary, with some cells after stage 12 lying in VS 8. pnt is expressed throughout VS 8, although expression is not seen until early stage 13. At stage 13, the two domains of odd paired (opa) expression extend from the P domain of VS 4 to the VS 6 boundary and from VS 9 through VS 11 (Bilder, 1998).

In the ventral cord, the protein is abundantly expressed, with higher levels in the three thoracic segments. In the gut of stage 13 embryos, the visceral mesoderm surrounding the midgut in posterior parasegments 4, 5 and 8 is strongly stained, as well as that for parasegment 6 (with weaker intensity). At stage 16, the protein is present in the first and second midgut constrictions and endodermal labelling is clearly visible. The Tsh protein appears to be differentially located in the primordia of leg imaginal discs, where it seems to be present in the proximal, but not in the distal part of the disc primordium (Alexandre, 1996).

Proximodistal subdivision of Drosophila legs and wings: The elbow-no ocelli gene complex functions upstream of Hth and Tsh in the formation of the leg primordium

Appendages are thought to have arisen during evolution as outgrowths from the body wall of primitive bilateria. In Drosophila, subsets of body wall cells are set aside as appendage precursors through the action of secreted signaling proteins that direct localized expression of transcription factors. The Drosophila homeodomain protein Distal-less is expressed in the leg primordia and required for formation of legs, but not wings. The homeodomain protein Nubbin is expressed in the wing primordia and required for formation of wings, but not legs. Given that insect legs and wings have a common developmental and evolutionary origin, attempts were made to identify genes that underlie the specification of all appendage primordia. Evidence is presented that the zinc-finger proteins encoded by the elbow and no ocelli genes act in leg and wing primordia to repress body wall-specifying genes and thereby direct appendage formation (Weihe, 2004).

Evidence suggests that the el and noc genes serve as mediators of the function of the Wg and Dpp signaling systems in specification of the appendage field within the imaginal discs. El and Noc are induced by Wg and Dpp and are required to repress the proximally expressed proteins Hth and Tsh. Previous work had identified Dll as a gene required for appendage formation in leg and antenna, and nub as a gene required for wing. This report identifies El and Noc as a pair of zinc-finger proteins that function in both ventral and dorsal appendages. However, there are interesting differences in the way that they do so, when examined in detail (Weihe, 2004).

Dll expression is required for the formation of all leg and antenna elements in the ventral (leg) discs, and until this work Dll was the earliest known marker for the distal region leg disc. Previous work has shown that repression of Hth and Tsh by Dpp and Wg was not required for expression of Dll in the leg, nor could Dll repress Hth and Tsh. Thus an essential mediator of the effects of Wg and Dpp was missing. The current results present evidence that El and Noc serve this function, since their removal leads to ectopic expression of Hth and Tsh. Removal of El and Noc does not cause loss of Dll expression, so it is concluded that Wg and Dpp act independently to induce El and Noc expression and Dll to define the distal region of the leg disc (Weihe, 2004).

The situation differs slightly in the wing. Repression of Tsh is the earliest marker for specification of the distal wing region, preceding the onset of Hth repression or of Nub induction. Loss of Tsh and Hth are required to allow Nub expression. Ectopic expression of Hth and Tsh and loss of Nub is observed in clones lacking El and Noc activity. Thus in the wing, expression of the distal marker Nub cannot be demonstrated to be independent of El and Noc (because ectopic Hth can repress Nub, but not Dll). The vestigial gene is also important for wing development and has been proposed to be a wing specifying gene. However, Vestigial is expressed all along the DV boundary of the wing, both in the wing primordium and in the body wall. This led to the suggestion that while Vestigial is essential for wing development, its expression cannot be taken as a molecular marker for wing identity per se, particularly at early stages. For this reason analysis of the relationship between El, Noc and Vestigial was not performed in this study (Weihe, 2004).

Is the repression of trunk genes needed to specify appendage, as opposed to body wall, in wing and leg discs? In the wing disc the answer appears to be yes; repression of 'trunk genes' like hth is necessary to make the remaining part of the disc competent to form the appendage. However, in the leg the situation is more complex. Coexpression of Dll and Hth does not disrupt proximal-distal axis formation, but leads to homeotic transformation of leg tissue into antennal tissue. Hth is not repressed and limited to proximal areas in the antenna. However, loss of el and noc activities in the leg disc leads to loss of distal leg tissue without any evident transformation into antennal tissue. Thus, El and Noc may regulate the expression of other 'trunk genes', whose restricted expression is required to make the remaining leg and antenna disc competent to form the appendage (Weihe, 2004).

The regional requirements for El and Noc highlight another interesting difference between leg and wing disc development. el noc double mutant cells are excluded from contributing to the tarsal region of the leg but not from contributing to the femur and tibia. Lineage tracing has shown a considerable net flux of cells from the proximal (Tsh-expressing domain) into femur and tibia. While there is no boundary of lineage restriction separating these domains, cells must be able to change from expressing the proximal marker Hth to expressing the distal marker Dll in order to move from one territory to the other. The wing in contrast does not appear to normally exhibit this large net flux of cells from proximal to distal and the el noc double mutant cells are excluded from contributing to the entire wing region. Clonal analysis has suggested that el noc double mutant cells attempt to sort out toward proximal territory, or if that fails, they can be lost from the disc, apparently by sorting out perpendicular to the epithelium. These observations suggest that El and Noc activity may contribute to the production of proximal-distal differences in cell affinities and thereby may help to maintain segregation of these cell populations during development (Weihe, 2004).

Larval

Patterning of the developing limbs by the secreted signaling proteins Wingless, Hedgehog and Dpp takes place while the imaginal discs are growing rapidly. Cells born in regions of high ligand concentration may be displaced through growth to regions of lower ligand concentration. A novel lineage-tagging method was used to address the reversibility of cell fate specification by morphogen gradients. Responses to Hedgehog and Dpp in the wing disc are readily reversible. In the leg, cells readily adopt more distal fates, but do not normally shift from distal to proximal fate. However, they can do so if given a growth advantage. These results indicate that cell fate specification by morphogen gradients remains largely reversible so long as the imaginal discs are growing. In other systems, where growth and patterning are uncoupled, nonreversible specification events or ëratchetí effects may be of functional significance (Weigmann, 1999).

In the developing leg disc, some responses to Wg and Dpp are readily reversible, while others are not. Lineage tracing of cells born in the TshGAL4 domain (proximal) suggests that cells readily lose Tsh (and Hth) expression and instead express Dll and Dac (distal markers). In young discs, a small proportion of cells expressing low levels of both Dll and Tsh are found at the edge between these domains. It is possible that these are cells in transition between the domains. These results suggest that cells born in the presumptive body wall readily contribute to formation of more distal leg regions. Under normal circumstances cells born in the Dll-expressing distal domain of the leg do not contribute to the body wall. However, they are not prohibited from doing so when given a strong growth advantage. The progeny of Dll-expressing cells in second instar are mostly fated to give rise to the tarsus and do not contribute to femur. In contrast, femur, tibia and tarsal segments (the distal segments) derive from cells that have expressed Dll in early third instar. The difference between these stages suggests that new cells must be induced to turn on Dll in order to provide the population of cells that contribute to the femur (the most proximal of the distal segments). These cells must derive from the Tsh domain in second instar and acquire Dll and Dac expression. At later stages, Dll is expressed at very low levels in the femur, where it may be repressed by Dac. Downregulation of Dll expression in the femur is unlikely to be a direct response to a lowering of Wg and Dpp signaling, because clones of cells unable to respond to these signals do not show abnormal Dll or Dac expression. This contrasts with the situation in the wing where removal of Dpp signaling leads to loss of Spalt expression. The low level of Dll expression in the femur is in part due to Dac activity, since dac mutant clones show elevated levels of Dll. The transient induction of Dll in the precursors of the femur is consistent with genetic analyses showing that formation of all leg segments except coxa depends on Dll activity in early development, whereas the low level of Dll expressed later is apparently not required for normal femur development (Weigmann, 1999).

When the distal part of a leg imaginal disc, or of an amphibian or a cockroach leg is removed, distal structures will regenerate from the cut edge. If the distal part of the leg disc is cultured in isolation, distal structures will regenerate from the cut edge, leading to a duplication of the fragment. The fact that distal structures regenerate but proximal structures do not has been termed distal transformation. These classical experiments have shown that cells have a general tendency to distalize, whereas their capability to proximalize is restricted. The current experiments show that distal transformation happens during normal development. Some proximal cells switch their pattern of gene expression as the disc grows and they acquire distal fate. Distal cells do not normally switch to proximal fate, but can do so if forced during early development. The classical regeneration studies suggest that the ability to shift from distal to proximal fate may be lost as development proceeds (Weigmann, 1999).

The teashirt (tsh) gene has dorso-ventral (DV) asymmetric functions in Drosophila eye development: promoting eye development in dorsal and suppressing eye development in ventral regions by Wingless mediated Homothorax (HTH) induction. A search was carried out for DV spatial cues required by tsh for its asymmetric functions. The dorsal Iroquois-Complex (Iro-C) genes and Delta (Dl) are required and sufficient for the tsh dorsal functions. The ventral Serrate (Ser), but not fringe (fng) or Lobe (L), is required and sufficient for the tsh ventral function. It is proposed that DV asymmetric function of tsh represents a novel tier of DV pattern regulation, which takes place after the spatial expression patterns of early DV patterning genes are established in the eye (Singh, 2004).

The three genes of the Iro-C (ara, caup and mirr) are expressed in the dorsal domain of the eye disc and are functionally redundant. Misexpression of ara (driven by bi-GAL4 and abbreviated as bi>ara) results in eye suppression on both dorsal and ventral margins. However, coexpression of tsh and ara in bi>tsh+ara results in overall enlargement of the eye. Clonal induction of ara (abbreviated as Act>ara) and coexpression of tsh and ara (Act>tsh+ara) gives the same results. Thus, ara provides the dorsal cue for tsh to induce eye enlargement on both margins (Singh, 2004).

The requirement for ara was further confirmed by misexpressing tsh (bi>tsh) in Df(3L) iro^DFM3/+ background. This deficiency uncovers ara, caup and the promoter region of mirr. In this background, bi>tsh suppresses eye development in both ventral and dorsal. Thus, when the Iro-C dosage is reduced, the dorsal function of tsh can be reversed to its ventral function. These results suggest that the dorsal function of tsh is dependent on the Iro-C genes (Singh, 2004).

Dl is expressed preferentially in the dorsal eye. Misexpression of Dl anterior to morphogenetic furrow in the hairy domain (hairy>Dl) accelerates photoreceptor differentiation but does not result in eye enlargement. bi>Dl (bifid-Gal4, UAS Delta) does not affect eye size. However, coexpression of tsh with Dl (bi>tsh+Dl) results in eye enlargements on both dorsal and ventral margins. Act>tsh+Dl clones in both dorsal- and ventral-eye also causes enlargements. These results suggest that Dl can provide the dorsal cue for tsh function (Singh, 2004).

Dl function was blocked by a dominant-negative form of DL, DL^DN. bi>Dl^DN causes reduction of eye on both dorsal and ventral margins whereas coexpression of tsh and Dl^DN (bi>tsh+Dl^DN) further enhances this phenotype. A dorsal Act>tsh+Dl^DN clone suppresses eye development. Act>tsh+Dl^DN clones also non-autonomously suppress eye development, a phenotype also seen in Act>Dl^DN clones. These phenotypes suggest that Dl is also required for the dorsal function of tsh in eye. In the absence of Dl, tsh exerts its ventral function in dorsal eye (Singh, 2004).

Ser is preferentially enriched in the ventral eye until late second instar of larval development. Misexpression of Ser (bi>Ser) does not suppress eye development whereas coexpression of tsh+Ser(bi>tsh+Ser) suppresses eye development on both dorsal and ventral margins. Despite the suppression of photoreceptor differentiation, bi>tsh+Ser eye disc shows overall enlargement. The adult eyes were also enlarged and folded despite the suppression of photoreceptors differentiation on the dorsal and ventral margins. These results suggest that Ser can provide the ventral cue for the eye suppression function of tsh but does not affect its early function in promoting growth (Singh, 2004).

The dominant-negative form of Ser, Ser^DN was used to block Ser function. In bi>Ser^DN, the eyes are suppressed on both dorsal and ventral margins. This phenotype is partially blocked in bi>tsh+Ser^DN eye. Similar results were observed in Act>tsh+Ser^DN clones. Thus, tsh requires Ser for its ventral function (Singh, 2004).

These results show that tsh requires several early DV eye patterning genes for its dorsal and ventral specific functions in the eye. The requirement for these DV patterning genes is specific, because not all the DV patterning genes have similar effects. Eye suppression by tsh is prevented in the dorsal eye region. This function requires the normal dosage of both Iro-C and Dl genes, because the reduction of either Iro-C or Dl allows tsh to suppress eye development even in the dorsal eye. However, when ectopically expressed in the ventral eye, either Iro-C genes or Dl can block the ventral function of tsh, suggesting that the two genes may play similar roles (Singh, 2004).

The genes involved in early DV eye patterning can be categorized in two classes: (1) genes that are preferentially expressed in dorsal (e.g., Iro-C, Dl) or ventral (e.g. Ser, fng) and (2) genes that are uniformly expressed but function only in one domain (e.g., L). It is proposed that tsh comprises a new class of genes, which is expressed symmetrically but perform asymmetric functions in dorsal and ventral eye (Singh, 2004).

Although tsh is expressed ubiquitously in the early eye disc, the DV asymmetric functions of tsh can be uncovered only after the expression of early DV patterning genes is established. These results suggest that early expression of tsh may be responsible for its growth function only, whereas for the DV asymmetric functions the expression of early DV patterning genes is a prerequisite. Therefore, TSH function in eye represents a new tier of DV pattern regulation, which functions in interpreting the DV spatial cues in eyes. It would thus be interesting to identify other members of this class. Interestingly, two orthologs of tsh have been identified in mouse but their function in eye is not yet known. Since there is evolutionary conservation in patterns of gene expression and functions, it would be interesting to look for the role of tsh during eye development in higher organisms (Singh, 2004).

Functional analyses of tiptop and Antennapedia in the embryonic development of Oncopeltus fasciatus suggests an evolutionary pathway from ground state to insect legs

In insects, selector genes are thought to modify the development of a default, or 'ground state', appendage into a tagma-specific appendage such as a mouthpart, antenna or leg. In the best described example, Drosophila melanogaster, the primary determination of leg identity is thought to result from regulatory interactions between the Hox genes and the antennal-specifying gene homothorax. Based on RNA-interference, a functional analysis of the Teashirt family selector gene tiptop (see Drosophila tiptop) and the Hox gene Antennapedia in Oncopeltus fasciatus embryogenesis is presented. It is shown that, in O. fasciatus, tiptop is required for the segmentation of distal leg segments and is required to specify the identity of the leg. The distal portions of legs with reduced tiptop develop like antennae. Thus, tiptop can act as a regulatory switch that chooses between antennal and leg identity. By contrast, Antennapedia does not act as a switch between leg and antennal identity. This observation suggests a significant difference in the mechanism of leg specification between O. fasciatus and D. melanogaster. These observations also suggest a significant plasticity in the mechanism of leg specification during insect evolution that is greater than would have been expected based on strictly morphological or molecular comparisons. Finally, it is proposed that a tiptop-like activity is a likely component of an ancestral leg specification mechanism. Incorporating a tiptop-like activity into a model of the leg-specification mechanism explains several mutant phenotypes, previously described in D. melanogaster, and suggests a mechanism for the evolution of legs from a ground state (Herke, 2005).

To investigate the variation and evolution of mechanisms of insect appendage formation, the role of selector genes in the formation of embryonic appendages of the milkweed bug O. fasciatus was examined. There are several advantages in using this species to study leg development and evolution. (1) As a hemimetabolous insect, O. fasciatus first instars have fully formed legs (i.e. all segments are present). Thus, the entire process of leg formation is readily apparent in the embryo and is not stretched out over the process of imaginal disc formation and metamorphosis as it is in holometabolous insects such as D. melanogaster. (2) O. fasciatus is positioned more basally on the insect phylogeny than any other insect for which the RNA-I technique has been successful in assaying gene function. (3) Because O. fasciatus is more distantly related to D. melanogaster than the more common genetically tractable model insects (e.g., the holometabolous insects Tribolium castaneum and Bombyx mori), there is greater potential for uncovering regulatory variation and perhaps gaining greater insight into the evolution of the leg specification mechanism. (4) Because O. fasciatus is generally less derived and has an ancestral leg composition, it may also have conserved the ancestral mechanisms of leg development allowing the direction of evolutionary change to be inferred (Herke, 2005).

From O. fasciatus embryos, partial cDNAs were cloned that represent a homolog of the D. melanogaster gene tiptop. tiptop is a member of the tsh-family that is typified by zinc-finger motifs and is presumed to be a transcription factor. Also, while present in the D. melanogaster genome, this gene was identified solely on the basis of molecular data. No mutations have been reported in tiptop, and its developmental function has not been previously reported for any arthropod (Herke, 2005).

tsh-family genes have been cloned by different methods from at least five insect species and, with the exception of D. melanogaster tsh, all appear to have greater similarity to tiptop. Through extensive PCR on genomic DNA and cDNA, both tiptop and tsh were recovered from D. melanogaster, but only a single gene was recovered from O. fasciatus or other insects. Also, a gene tree constructed using PAUP^* for the tsh-family genes shows that the two D. melanogaster genes cluster together. The gene tree and the absence of a tsh gene in the other insects surveyed suggest that the two tsh-family genes in D. melanogaster result from a recent duplication of an ancestral gene. This ancestral gene has been called tiptop because of its greater similarity to that gene and not to imply a closer evolutionary relationship of the ancestral gene to either the D. melanogaster tsh or tiptop (Herke, 2005).

tsh has a variety of developmental functions in the cuticle of the Drosophila larva and adult. Specifically, tsh is thought to specify trunk identity in the larva through interactions with Hox genes, is required for the formation of proximal regions of appendages in adults, and plays a role in restricting the development of the adult eye. There is little similarity between any of these activities of the Drosophila tsh gene and O. fasciatus tiptop. Thus, these activities appear to have been acquired by the tsh-family relatively recently. Significant to the discussion here is that the function tsh has in the formation of the proximal region of the leg in D. melanogaster cannot be provided by tsh in O. fasciatus because the gene is not present. These roles may be provided by other proximally expressed genes such as hth and exd (Herke, 2005).

A model of the leg specification mechanisms in D. melanogaster and the proposed differences from O. fasciatus is presented. In contrast to D. melanogaster, where loss of Hox (Antp, Scr, Ubx) function produces dramatic transformations of leg to antenna, no transformation toward antennae of Antp-phenocopy legs is detected that could be interpreted as an expansion of hth activity. Thus, although it is possible that some residual Antp function remains in these animals, it is suggested that it is tiptop and not Antp that represses the activity of hth (or other antennal specifier) in the O. fasciatus leg. A role for Antp in the segmentation of the distal region is absent in D. melanogaster while its role in medial segmentation is conserved between the two insects. Also, given that neither tiptop nor the Hox genes act as specifiers of proximal identity (leg vs. antenna) in the leg specification mechanism of milkweed bugs, additional undetermined genes are implicated. This further distances the mechanism of appendage specification in milkweed bugs from the relatively simple two-gene (Hox, hth) system evident in D. melanogaster (Herke, 2005).

A tiptop-like activity is also evident in D. melanogaster. This is illustrated most convincingly by the persistent pretarsi formed on legs that are otherwise transformed to antennae in the absence of Antp activity. Also, the leg-like appendage (composed primarily of tarsi and pretarsi) produced by hth Antp null clones in D. melanogaster is what might be predicted if an independent tiptop-like activity for distal segmentation and specification remained active in these appendages. Genetic analysis of Drosophila tiptop has not revealed a role in distal specification or segmentation of the adult leg (Laurent Fasano, personal communication to Herke, 2005). However, due to the technical difficulties of determining the role embryonic gene activities have in adult structures in D. melanogaster, it has not been possible to rule out that embryonic activities of either tiptop or tsh affect the adult leg. Thus, it remains a possibility that a tiptop-like activity could be provided by tiptop or tsh, as well as by other genes in D. melanogaster (Herke, 2005).

Interestingly, the defects induced by reduced Antp activity in O. fasciatus are in striking contrast to the transformations of mouthparts to antennae seen when Scr and Dfd activity are reduced. These latter transformations have been used as evidence for a universal mechanism of Hox specification of insect appendages. However, in O. fasciatus, Scr and Dfd apparently repress the activity of antennal specification in gnathal appendages while Antp does not repress this activity in thoracic appendages. Thus, in O. fasciatus, two mechanisms (one Hox-dependent and one Hox-independent) exist for specifying the identity of appendages. Additional factors (including tiptop) must mediate the differences in the active mechanisms in these tagma (Herke, 2005).

It is possible to describe the genetic changes required for the O. fasciatus mechanism of leg development to evolve into that of D. melanogaster. (1) Antp acquired the ability to repress the antennal specifier (hth) in the distal leg and lost its role in distal segmentation. These changes might have been relatively simple. A mechanism for Hox genes (e.g. Scr and Dfd) to repress the antennal specifier already existed and the segmentation functions of Antp might be partially provided by tiptop. (2) The change in Antp function relaxed the constraints on tiptop, thereby allowing its function (including hth repression) to diverge. (3) Duplication and further divergence of the ancestral tiptop gene produced the tsh and tiptop genes of D. melanogaster (Herke, 2005).

Effects of Mutation or Deletion

Revertants of embryonic lethal mutants show normal head and tail regions but disrupted trunk regions. Monoclonal antibody to teashirt ventral neuronal clusters in the trunk are disrupted and axons are misrouted. Dorsal parts of the embryos, with the exception of the prothorax, are morphologically normal (Fasano, 1991).

teashirt was initially identified as a gene required for the specification of the trunk segments in Drosophila embryogenesis. Targeted expression of teashirt in imaginal discs is sufficient to induce ectopic eye formation in non-eye tissues, a phenotype similar to that produced from targeted expression of eyeless, dachshund, and eyes absent. The expression of so and dac are induced in the antennal disc by the ectopic expression of tsh, suggesting that tsh may act upstream of these genes in eye development. Furthermore, teashirt and eyeless induce the expression of one another, suggesting that teashirt is part of the gene network that functions to specify eye identity (Pan, 1998).

However, these results do not prove that tsh does play a role in specifying the eye identity during normal development. To address this issue, an examination was carried out to see if tsh is expressed at the right time and the right place to have a role in specifying the eye identity. Indeed, TSH mRNA is expressed in the eye disc, with the strongest expression anterior to the morphogenetic furrow. This pattern of expression is similar to that of ey, a gene that is known to play an essential role in specifying eye identity. An examination was carried out to see if loss-of-function mutations of tsh affect eye development. Several weak loss-of-function tsh alleles were examined and no eye defects were found. X-ray-induced mitotic recombination was used to generate mutant clones of a null tsh allele. tsh mutant clones were recovered at a frequency similar to the wild-type control, and sections through the mutant clones revealed a normal ommatidial organization. These data suggest that tsh may play a redundant role during normal eye development, and the requirement for tsh may be masked by other factor(s) that play a role similar to tsh (Pan, 1998).

Localized transcription factors specify the identity of developmental domains. The function of the Teashirt zinc finger protein, which is expressed in the proximal domain of the Drosophila leg, has been analyzed. At stage 10 of embryogenesis, Distal-less is detected in the putative distal part of the primordia of the leg in each of the thoracic hemisegments of the embryo. Tsh is coexpressed with Dll at this stage. By stage 15 the cells of the presumptive leg imaginal discs have invaginated inside the embryo and Tsh is not detected in the most distal part of the leg primordium, where Dll is expressed alone. However, Tsh is still coexpressed with Dll in a ring of cells at the periphery of the Dll domain. At the beginning of the third instar, Dll occupies a distinct distal ring of cells in the disc; Tsh is expressed in a proximal ring. These territories are separated by 2 or 3 cells in ventral and lateral regions and up to 10 cells in dorsal parts. The Dachshund transcription factor is expressed in the intermediate ring of cells overlapping the Dll expression domain by, at the most, 1 or 2 cells. By mid-third instar Dll is expressed in a new 4-cell-wide proximal ring that is destined to make the proximal femur and possibly the distal edge of the trochanter. Tsh overlaps with this new Dll domain at the proximal edge, which persists until the late third instar stage. By ectopic expression of a teashirt transgene it has been shown that Teashirt contributes to the differences in cell-cell adhesion between proximal and distal leg cells. Whereas clones of cells expressing the teashirt transgene survive in the endogenous Teashirt domain, most cells expressing Teashirt in an ectopic distal position are lost from the epithelium. In clones that were recovered in the distal domain, different effects were seen, dependent on position with respect to the dorsal-ventral axis. In the ventral region, where Wingless is signaling, surviving clones express Teashirt and cause abnormalities in the adult leg. In contrast, lateral and dorsal clones generally do not accumulate Teashirt and have no effect on patterning. One exception to the differential dorsal-ventral effects occurs at the boundary between Teashirt-expressing and -nonexpressing cells. Both ectopic and hypomorphic loss of teashirt affects patterning at all positions at the boundary, suggesting that Teashirt plays a crucial role in boundary formation. The results are discussed with respect to the roles of transcriptional and posttranscriptional mechanisms in proximal-distal axis patterning of the Drosophila legs. It is suggested that because Arm binds to Tsh, this binding stablizes Tsh in cells within the Wg signaling domain (Erkner, 1999).

The proximal distal axis of the Drosophila leg is patterned by expression of a number of transcription factors in discrete domains along the axis. The homeodomain protein Homothorax and the zinc-finger protein Teashirt are broadly coexpressed in the presumptive body wall and proximal leg segments. Homothorax has been implicated in forming a boundary between proximal and distal segments of the leg. Evidence is presented that Teashirt is required for the formation of proximal leg segments, but Tsh has no role in boundary formation (Wu, 2000).

The leg disc consists of a single epithelial sheet in which the presumptive distal segments are specified in the center and the presumptive proximal segments are specified in the periphery. Cross-sections show that proximal segments, which express Hth and Tsh, fold back over the distal segments, which express Dll and Dac. Hth and Tsh expression is limited to the proximal region of the disc through repression by the combined activities of Wg and Dpp. Although the Hth and Tsh expression domains overlap through much of the proximal region, Hth expression extends more distally than Tsh. This is visible as a band of Hth expression that does not overlap Tsh in a basal optical section. This band coincides with the outer ring of Dll expression. The Tsh domain overlaps the proximal edge of the Dll ring by one or two cells. Tsh expressing cells are also found beneath the disc epithelium. Their location suggests that these may be adepithelial cells. Hth functions as a repressor to modulate Tsh expression. More distally located hth mutant clones lose Tsh expression. Loss of Tsh expression in hth correlates with ectopic expression of Dachshund. hth mutant clones cause ectopic expression of Dac close to the endogenous Dac domain, but do not do so in more proximal regions. The differential effect on Dac expression of hth clones located at different positions along the PD axis has been attributed to a role of Hth as a repressor of Wg and Dpp signaling. Thus the paradoxical loss of Tsh in more distal hth clones can be explained as an indirect effect of Hth on Dac expression. Dac can repress both Tsh and Hth when overexpressed. Thus the different distal limits of the Hth and Tsh expression domains presumably reflect a difference in their sensitivity to repression by Dac. The observation that Tsh levels increase in proximal hth clones suggests that Hth serves as a repressor of Tsh. Thus Hth modulates Tsh expression levels in the proximal leg in two ways. Hth may act directly to reduce Tsh expression levels in the proximal leg, and indirectly via repression of Dac to define the distal limit of Tsh expression (Wu, 2000).

To assess the role of Tsh in development of the proximal leg the phenotypes of adult viable mutant alleles of tsh were examined. tsh^GAL4 is a weak allele caused by insertion of the GAL4 enhancer-trap P-element. The trochanter is strongly reduced in legs of flies homozygous for tsh^GAL4. The coxa is reduced and lacks most of the bristles and sense organs found in wild-type. The femur is short, but contains the normal complement of proximal sense organs (including the sc11 group of campaniform sensillae that is normally located at the joint between femur and trochanter. Although the trochanter is reduced, joints can still be seen between coxa, trochanter and femur segments. One or two sensilla trichodea are generally found at the articulation between the reduced trochanter and coxa segments (there are normally two groups of 5-7 sensilla trichodea at this position in wild-type). The tibia and tarsal segments appear to be normal in tsh mutants. In a stronger mutant combination, tsh^GAL4/tsh^HD1 the trochanter is no longer detectable as a discrete segment and the coxa appears to articulate directly with the femur. Both coxa and femur are reduced in size. No sense organs can be recognized on the coxa and trochanter rudiment, but the sc11 group of campaniform sensillae was reliably found on the proximal femur where it articulates with the coxa. The shortening of the femur in the strong tsh mutant combination suggests that defects in the trochanter and coxa may have non-autonomous effects on femur development. This may reflect the finding that many of the cells that contribute to femur development originate in the Tsh-expression domain at earlier stages of development and are displaced distally as the disc grows (Wu, 2000).

Reducing Tsh activity produces a phenotype quite distinct from that of removing Hth activity. Tsh is required for the development of trochanter and coxa but does not appear to have a role in segment boundary formation. Hth and its partner Extradenticle are required to prevent fusion of coxa and trochanter with the femur. To better understand the basis for the defects in tsh mutant legs, Hth, Dll and Dac expression were studied in tsh^GAL4/tsh^HD1 and tsh^GAL4/tsh^GAL4 mutant discs. In wild-type discs Hth and Dac expression overlap in the proximal ring of Dll expression. Hth function in this ring is required for the affinity boundary between proximal and distal regions of the leg. This basic relationship holds in the tsh^GAL4/tsh^HD1leg disc. Hth, Dll and Dac expression overlap, and the affinity boundary between proximal and distal leg segments appears to be intact. The principal difference in these discs is expansion of the Dll domain into the proximal, Hth-expressing region. The spatial relationship between Hth and Dac is normal. Ectopic Dll expression is not sufficient to repress Hth but does appear to reduce the size of the coxa and trochanter and to cause problems that result in loss of pattern elements from the remaining portions of these segments. Even slight reductions in Tsh activity causes loss of sensory bristles from the coxa. In contrast, small clones of hth mutant cells are capable of differentiating bristles (Wu, 2000).

Taken together, these observations suggest that Tsh and Hth have distinct functions in the proximal leg. Hth limits the proximal extent of Dac expression, and is required for the affinity boundary between trochanter and femur. Tsh limits the proximal extent of Dll expression and is required for proper growth and differentiation of proximal segments, but does not appear to have a role in PD boundary formation (Wu, 2000).

Split ends (Spen) is a protein that acts in parallel with Hox proteins to regulate different segmental morphologies. spen plays two important segment identity roles. One is to promote sclerite development in the head region, in parallel with Hox genes; the other is to cooperate with Antennapedia and teashirt to suppress head-like sclerite development in the thorax. Without spen and teashirt functions, Antennapedia loses its ability to specify thoracic identity in the epidermis. Spen is the only known homeotic protein with RNP binding motifs: this indicates that splicing, transport, or other RNA regulatory steps are involved in the diversification of segmental morphology. Other studies have identified spen as a gene that acts downstream of Raf to suppress Raf signaling in a manner similar to the ETS transcription factor Aop/Yan. This raises the intriguing possibility that the Spen RNP protein might integrate signals from both the Raf and Hox pathways (Wiellette, 1999).

A Drosophila gene involved in distinguishing head from body is teashirt (tsh). Tsh protein is expressed only in the labial segment and trunk region of embryos, where it is required to repress head identity and to promote thoracic and abdominal segment identities. tsh transcription levels in the thorax are maintained by Antp, but a variety of genetic interaction tests have shown that Antp and Tsh have independent functions in repressing head development. Is spen integrated into the Antp;tsh pathways by regulation of the tsh or Antp expression patterns? Experiments show that (1) expression pattern of Tsh protein is unchanged in spen mutant embryos; (2) protein expression patterns of Scr or Antp in spen^-;tsh^- double mutant embryos are unchanged from the pattern seen in tsh mutants alone, and (3) the spen transcript pattern is normal in tsh mutants. Therefore, Spen suppression of head-like sclerites is not exerted by a regulatory effect on the Tsh protein expression pattern, nor by Tsh effects on the spen transcript pattern, nor through combinatorial effects of spen and tsh on Scr or Antp protein abundance (Wiellette, 1999).

The phenotype of tsh ^- , spen^- mutant embryos suggests that the two genes act to promote thoracic development. In tsh mutant embryos, the T1 denticle belt is absent and although the remaining denticle belts appear to have the appropriate segmental identities, the denticles themselves are disorganized and smaller than in wild type. In contrast, tsh^-;spen^- double mutants completely lack denticle belts in the thorax. This may be due to the the death of cells in the denticle field in the thorax of the double mutants, or to the inability of Antp protein, still expressed in the remaining cells, to promote the development of thorax-specific structures (Wiellette, 1999).

As to whether tsh and spen collaborate in repressing head-like sclerites, it was found that the tsh^-, spen^- double mutants still have bits of sclerite in the 'thorax' of the double mutants, so this phenotype is not enhanced. However, the effects of Tsh overexpression on the ectopic head-like sclerites was also examined in spen mutants. In wild-type embryos, overexpression of Tsh protein throughout the embryo results in transformation of head regions toward thoracic identity, as well as poorly differentiated denticle belts, especially in the thorax. In the thorax of spen mutant embryos that also overexpress Tsh, the ectopic ventral head-like sclerites are strongly suppressed. Taken together, these results suggest that spen, tsh and Antp function in a combinatorial manner to repress the development of head-like sclerites and promote the development of thoracic identity (Wiellette, 1999).

Teashirt and Disconnected contribute to regionalization of the Drosophila embryo and establishes the domains of HOM-C protein function

During animal development, the HOM-C/HOX proteins direct axial patterning by regulating region-specific expression of downstream target genes. Though much is known about these pathways, significant questions remain regarding the mechanisms of specific target gene recognition and regulation, and the role of co-factors. From studies of the gnathal and trunk-specification proteins Disconnected (Disco) and Teashirt (Tsh), respectively, evidence is presented for a network of zinc-finger transcription factors that regionalize the Drosophila embryo. Not only do these proteins establish specific regions within the embryo, but their distribution also establishes where specific HOM-C proteins can function. In this manner, these factors function in parallel to the HOM-C proteins during axial specification. In tsh mutants, disco is expressed in the trunk segments, probably explaining the partial trunk to head transformation reported in these mutants, but more importantly demonstrating interactions between members of this regionalization network. It is concluded that a combination of regionalizing factors, in concert with the HOM-C proteins, promotes the specification of individual segment identity (Robertson, 2004).

disco was initially identified in a screen for mutations affecting neural development. It was not until the discovery of disco-related (disco-r) that a patterning role was uncovered (Mahaffey, 2001). The phenotype of terminal embryos lacking disco and disco-r is similar to those lacking the gnathal HOM-C genes Dfd and Scr; that is, structures from the gnathal segments (mandibular, maxillary and labial) are missing. This phenotype is due to reduced expression of Dfd and Scr target genes. Since HOM-C protein distribution is normal in disco, disco-r null embryos, and vice versa, these factors appear to act in parallel pathways (Robertson, 2004).

These studies have been extended and it is shown that: (1) Dfd can only direct maxillary developmental when Disco and/or Disco-R are present; (2) Tsh represses disco (and disco-r), helping to distinguish between trunk and gnathal segment types, and thereby establishing domains for appropriate HOM-C protein function, and (3) when ectopically expressed in the trunk, Disco represses trunk development and may transform these segments towards a gnathal segment type (Robertson, 2004).

Though HOM-C genes have a clear role in establishing segment identities, ectopic expression often has only a limited effect. The data indicate that, for Dfd, this restriction arises because of the limited distribution of Disco in the trunk segments. There are two important conclusions from these observations: (1) the spatial distribution of Disco establishes where cells can respond to Dfd, and this is probably true for Scr as well. Cells expressing disco develop a maxillary identity when provided with Dfd, even though this may not have been their original HOM-C-specified fate. This highlights (2) -- the combination of Disco and Dfd overrides normal trunk patterning, without altering expression of tsh and trunk HOM-C genes. As with the maxillary segment, identity is lost in the mandibular and labial segments when embryos lack disco and disco-r. This indicates that Disco and Disco-R may have similar roles in all gnathal segments. That co-expression of Disco and Scr in the trunk activates the Scr gnathal target gene pb strengthens this conclusion. Therefore, it is proposed that Disco defines the gnathal region, and establishes where the gnathal HOM-C proteins Dfd and Scr can function (Robertson, 2004).

Alone, ectopic Disco significantly alters development, indicating that Disco has a morphogenetic ability, separate from gnathal HOM-C input. Since Disco is required for normal gnathal development, it is suspected that disco specifies a general gnathal segment type. Definitive identification is difficult because of the lack of morphological or molecular markers that denote a general gnathal segment type. Yet, there is support for the conclusion that disco expression establishes a gnathal segment type. Ectopic Disco can, to some extent, override the trunk specification system and repress trunk development (repressing denticles, oenocytes and trachea). Furthermore, ectopic Disco blocks dorsal closure, which is similar to the role of endogenous Disco in the gnathal segments (Robertson, 2004).

Perhaps the most compelling evidence that Disco specifies a gnathal segment type comes from the observation that disco is activated in the trunk segments when embryos lack Tsh. The identity of the trunk segments in tsh mutant embryos is somewhat uncertain. It has been suggested that some aspects of the tsh phenotype indicate the trunk segments acquire gnathal characteristics; for example, the ventral neural clusters appear to be transformed to a gnathal-like identity. Mutations in the tsh gene can therefore be interpreted in two ways -- either they partially transform the trunk segments into a gnathal-like identity, and in particular the prothoracic segment into a labial one, or they cause a non-specific change in segmental identity perhaps due to cell death. However, the loss of tsh and the trunk HOM-C genes may transform the trunk cuticle toward anterior head cuticle. Again, the difficulty in assigning an identity is due to the lack of a readily discernable gnathal morphological or molecular marker. Evidence is presented that disco and disco-r are reliable molecular markers for gnathal identity, and disco mRNA is shown to be present in the ventral and lateral regions of the trunk segments in tsh mutant embryos. This expression of disco coincides, spatially, with the region of the trunk that is transformed in tsh mutant embryos. UAS-driven disco does mimic some aspects of tsh mutants, denticles are reduced and the ventral chordotonal neurons do not develop, but since Tsh is still present, the transformation caused by ectopic disco may be incomplete. Finally, Dfd cannot induce maxillary structures, even in tsh mutants, when disco and disco-r are absent. This reinforces the role for Disco in establishing gnathal identity, and indicates that the ectopic Disco present in embryos lacking Tsh is functional. Therefore, considering these arguments, it is proposed that Disco and Disco-R establish the gnathal region of the Drosophila embryo, and in this regard, they function similarly to Tsh, which specifies the trunk region (Robertson, 2004).

There are other parallels between Disco/Disco-R and Tsh. They are regionally expressed zinc-finger transcription factors, and they are required in parallel with the HOM-C proteins for proper segment identity. Furthermore, the distribution of these proteins establishes domains in which specific HOM-C proteins can properly direct embryonic development. The data reveal a regulatory relationship between Tsh and disco (and disco-r), indicating they are part of an interacting network that helps regionalize the Drosophila embryo. The HOM-C proteins then establish specific segmental identities in the appropriate region. In the trunk segments, Tsh, along with the trunk HOM-C proteins, specifies the trunk segment characteristics, in part by repressing disco and, thereby, preventing gnathal characteristics from arising in the trunk segments. The presented model requires that tsh expression be limited to the trunk segments, and it is proposed this is accomplished by another C2H2 zinc-finger protein, Salm. tsh expression has been shown to expand into the posterior gnathal and posterior abdominal segments in embryos lacking Salm. Therefore, Salm establishes the boundary between the Tsh and Disco domains. It is stressed that, at this time, it is not known what parts of this regulation are direct. Interestingly, other zinc-finger transcription factors are responsible for positioning salm expression, so that a more extensive hierarchy of zinc-finger transcription factors leads to regionalization, eventually establishing the domains of HOM-C protein function. It is also noted that Tsh has other roles than just repressing disco. Tsh actively establishes the trunk region, just as Disco does the gnathal. It is also noteworthy that ectopic Tsh activates disco in the labial sense organ primordia, leading to a Keilin's Organs fate, as occurs in the thoracic segments. Therefore, for unknown reasons, Tsh changes from a repressor of disco to an activator in these cells. This observation highlights the complex interplay between factors like Tsh and Disco, and it will be interesting to determine what causes these opposing roles (Robertson, 2004).

Many other questions remain. For example, how are the expression domains for these factors established? It is clear that Salm could form a boundary separating gnathal from trunk, but in salm mutants, tsh is only ectopically activated in the posterior labial segment, not in every gnathal segment. This implies that Salm forms a boundary, not by repressing tsh throughout the head, but by, in a sense, drawing a line between the head and trunk regions. What then prevents tsh expression from crossing that line and extending further into the gnathal segments in salm mutants? Is there an activator of tsh that is limiting, another gnathal repressor, or is something else involved? Likewise, what activates tsh and disco? It is unlikely that lack of Tsh is the only requirement for disco expression. More likely, this relies on the prior segmentation pathway. With regard to the HOM-C specification of segment identity, questions remain as to how the zinc-finger proteins establish where specific HOM-C proteins can function. Are the zinc-finger proteins co-factors or simply a parallel pathway? Furthermore, if they are co-factors for the HOM-C proteins, how can different HOM-C proteins establish different segment identities with the same co-factor (for example, Dfd and Scr with Disco), or how can different co-factors alter the role of a HOM-C protein (Scr with Disco or Tsh) (Robertson, 2004)?

Finally, the question remains of whether or not factors such as Disco and Tsh establish head/trunk domains and delimit HOM-C protein function only in the Drosophila embryo, in all stages of Drosophila or in other animals as well. Though this remains to be tested experimentally, there are indications that this may be a general mechanism. homologs of these zinc-finger genes are found in vertebrates and in other invertebrates, and, although only limited data are currently available, expression data indicate that these genes may have similar roles to their Drosophila counterparts during embryonic patterning. In an informative experiment the Tribolium Dfd homolog, Tc-Dfd, has been expressed in Drosophila embryos lacking the endogenous Dfd gene; persistent expression of Tc-Dfd rescues maxillary development. Though at present, it is not known whether or not a direct interaction is required between Disco and Dfd, this result would indicate that the Tribolium Dfd protein can fulfill the same roles as the Drosophila protein, and, therefore, it must be able to function with the Drosophila regionalization system. In any case, it will be important to investigate and interpret the role of the regionalizing genes as they relate to development and evolution of body pattern in other animals, and to ask whether a similar network is involved in patterning all animals (Robertson, 2004).

Restricted teashirt expression confers eye-specific responsiveness to Dpp and Wg signals during eye specification

In Drosophila, the eye primordium is specified as a subdomain of the larval eye disc. The Zn-finger transcription factor teashirt (tsh) marks the region of the early eye disc where the eye primordium will form. Moreover, tsh misexpression directs eye primordium formation in disc regions normally destined to form head capsule, something the eye selector genes eyeless (ey) and twin of eyeless (toy) are unable to do on their own. Evidence suggests that tsh induces eye specification, at least in part, by allowing the activation of eye specification genes by the wingless (wg) and decapentaplegic (dpp) signaling pathways. Under these conditions, though, terminal eye differentiation proceeds only if tsh expression is transient (Bessa, 2005).

The specification of the eye primordium within the main epithelium (ME) of L2 eye discs correlates with tsh expression, suggesting that tsh might be involved in this specification. If this is the case, it would be expected that ectopic tsh expression will transform the overlaying squamous layer, the peripodial epithelium (PE) cells into an eye primordium, characterized by: (1) columnar morphology of the epithelial cells; (2) eye-specific gene expression, and (3) eye-specific response to key signaling pathways. Each of these points has been analyzed in turn by inducing the expression of tsh in marked clones of cells in the PE (Bessa, 2005).

Cells expressing tsh in the margin of the disc or in the PE overproliferate, adopt a columnar shape, with elongated nuclei, and are more densely packed than non-expressing cells. Some of these clones further show a sorting behavior, by which the tsh-expressing cells arrange themselves as hollow sacs with their apical sides pointing inwards, as monitored by expression of armadillo/ß-catenin, which localizes to adherens junctions. Such a sorting behavior is usually considered to be the consequence of the cells adopting a new identity (Bessa, 2005).

In order to test if tsh is sufficient to induce eye primordium identity in PE cells, the expression of the eye selector gene ey, as well as that of the early retinal genes eya and Dac, was examined in tsh-expressing clones. tsh-positive cells show increased Ey expression. In addition, PE tsh-expressing clones that lie close to the posterior margin activate eya and the eya target Dac, indicating that these cells adopt an eye primordium-like fate. PE clones overexpressing ey are not able to induce eya, neither are similar toy-expressing clones, in which ey expression is upregulated. In these PE clones, tsh expression is not induced. Therefore, it is concluded that neither ey upregulation nor the joint overexpression of toy and ey are able to re-specify the peripodial epithelium. In addition, overexpression of eya in PE clones do not turn Dac on either, which reinforces the idea that PE re-specification as eye primordium occurs only if tsh is expressed (Bessa, 2005).

Expression of tsh activates eya expression mostly in the center and posterior half of the PE, but not in the anterior half. Clones in this anterior region retain the expression of hth, which is normally expressed in all PE cells. Since dpp and wg are expressed in the domains of the posterior and anterior discs, respectively, it was reasoned that these differences in the response of tsh-expressing cells could be the result of these signaling pathways acting differently in anterior and posterior domains of the PE (Bessa, 2005).

To test this hypothesis, the response of normal PE cells to variations in both wg and dpp pathways was tested. Clones where the dpp pathway was hyperactivated through the expression of a constitutively active dpp-receptor, thick veins (tkvQD), or blocked by removing the signal transducer Mothers against dpp (Mad), showed no induction of eya expression or cell morphology changes. Neither did anterior clones expressing Axin, a negative regulator of the wg pathway or overexpressing wg. Nevertheless, when alterations in the dpp and wg pathways were performed in the presence of ectopic tsh, PE cells showed gene expression responses characteristic of the ME. Thus, whereas posterior tsh-expressing PE cells induce eya expression, tsh-expressing cells in which the dpp pathway has been blocked by removing Mad no longer express eya. Again, this is the behavior exhibited by tsh+ ME cells deprived of dpp signaling. Similarly, while anterior tsh-expressing PE cells retain hth expression, most clones expressing both tsh and Axin lose hth expression, as they do if Axin is expressed in the ME within the tsh domain. PE tsh+ tkv+ clones still fail to activate eya in anterior dorsal and anterior ventral regions, suggesting that even in these clones wg signaling can prevent PE re-specification. Clones of PE cells expressing tsh, tkvQD and Axin now activate eya anywhere in the disc, indicating that, in the presence of tsh, wg and dpp antagonize each other to regulate eya expression. It is noted, however, that the squamous to columnar cell shape change induced by tsh is independent of the activity of the wg and dpp pathways. These results suggest that tsh, when expressed in the PE, can reprogram this epithelial layer to respond to wg and dpp signals such that it develops in an eye primordium-specific manner (Bessa, 2005).

During the development of the eye disc, only cells of the ME will be specified as eye primordium. Although Wg and Dpp signals play essential roles during eye development, PE cells are relatively insensitive to these signaling pathways, as measured by cell survival, morphology, proliferation or gene expression changes. tsh starts being expressed in the ME around the time when the eye primordium is specified, and tsh has the potential to redirect eye disc PE cells towards eye development, an ability the eye selector genes toy and ey do not have on their own. These results indicate that the PE can be re-specified by tsh throughout most of the life of the larva. Thus, tsh-expressing clones induced during L1 and L2 induce eya and Dac expression. The transient expression of tsh during L2, or its induction by Gal4 drivers active during late-L2/L3, results in ectopic PE eyes (Bessa, 2005).

It is proposed that one way in which tsh might be involved in eye fate specification is by altering the response of eye disc cells to Dpp and Wg signals. The molecular mechanisms by which tsh might achieve this during eye development remain to be further investigated, but they might be similar to those already described during embryogenesis, where Tsh modulates wg and dpp pathways directly interacting with Armadillo, the wg signaling transducer, and with Brinker, a transcriptional repressor of the dpp pathway (Bessa, 2005).

In the eye disc, the cells specific response to wg and dpp enabled by tsh is superimposed onto the expression of eye-selector genes. Such combination of factors in turn would specify the eye primordium. The fact that Tsh and Ey have the potential to interact directly makes it possible for Ey to tether Tsh-containing transcriptional complexes to eye-specific targets genes (Bessa, 2005).

It is also observed that ato expression is induced in some of the tsh-overexpressing eye-disc cells. Therefore, tsh has the potential not only to sensitize eye disc cells to wg and dpp signals, but also to make them prone to neural differentiation. dpp and wg have been shown to regulate the spatial activation of ato to position several adult sensory organs, including the eye, within the corresponding imaginal discs. This mechanism for positioning ato would define a sensory organ prototype upon which selector genes, such as ey, would specify the final sensory type. Interestingly, the ectopic ato expression induced by tsh is not disc specific and, if tsh induction is transient, results in ectopic neurons. This ato induction might be mediated by tsh enabling cells to respond to dpp and wg (Bessa, 2005).

These results underlie the importance of the precise and dynamic spatiotemporal pattern of expression of tsh: although tsh expression must be confined to the ME layer of the eye disc, in order for eye development to proceed, tsh has to be first expressed in undifferentiated cells to be later turned off to allow retinal differentiation. The earlier paradox of tsh acting both as eye repressor and inductor, depending on the Gal4 promoters used, can now be explained as follows: Gal4 promoters that are not repressible by the gene expression changes induced upon tsh overexpression, such as ey-GAL4, will lead to sustained expression of tsh and, therefore, to a blockage of eye development. Other drivers that are turned off after tsh expression (i.e., MS1096, MD705) will mimic the situation found in the ME (that is, on/off), and in these cases, eye development will proceed. It is noted that in experiments where ey is ectopically expressed, eyes tend to develop in the proximal parts of appendages which derive from tsh-expressing domains in their respective imaginal discs. This correlation reinforces the idea of tsh as a potential eye-competence factor (Bessa, 2005).

At least three roles for tsh during eye development have been uncovered: promoting proliferation, acting as an eye repressor and acting as an eye inducer. The first two roles (proliferation and eye repression) are linked to the function of the transcription factor Hth. Thus, Tsh and Hth (together with Ey) maintain the eye disc cells in a proliferative, undifferentiated state, which is incompatible with eye differentiation. This state is kept as long as cells express hth, which is positively regulated by wg and repressed by dpp. Since tsh keeps hth on, sustaining tsh expression artificially in the disc blocks further eye differentiation. Once hth is repressed by Dpp signaling close to the MF, cells enter a preproneural state, that still maintains tsh expression, in which dpp activates the expression of retinal genes such as eya. The results suggest that tsh is required for the eye-specific interpretation of Wg and Dpp signals, and therefore for both the maintenance of proliferation and the specification of the retina. This model thus predicts that removal of the earliest tsh function (which corresponds to the most anterior regions in older discs) should result in eye loss due to either lack of proliferation or to the incorrect specification of the primordium; removal of later tsh function (which corresponds to more posterior regions of older discs) should cause a premature derepression of the eye differentiation program and excess of eye. In fact, both phenotypes have been described in tsh loss-of-function clones: eye loss and eye overgrowth. The current experiments, in which tsh function is reduced uniformly from early stages of eye development, agrees with an early role of tsh in eye specification and/or proliferation. This model of tsh function is further complicated by the fact that the dorsoventral genes also impinge on tsh function. Still, some tsh-clones show no phenotype. This might be explained by perdurance of the Tsh product, local differences in the requirement of tsh within the eye disc or the existence of compensatory functions (Bessa, 2005).

toy and ey lay atop the eye specification genetic network in Drosophila. However, neither Toy nor Ey is able to activate the expression of tsh in the PE, and tsh expression in maintained in ey mutant discs. The reverse is also true; tsh upregulates ey expression in the eye disc, but is unable to activate its expression de novo in any other disc. This indicates that tsh expression is regulated independently of the Pax6 genes in the eye disc. This situation is analogous to that of Optix, a Six3 homolog, which is expressed in the eye disc independently of ey with a pattern reminiscent of that of tsh. Nevertheless, Optix does not seem to regulate tsh; ectopic expression of Optix in the eye disc does not trigger tsh expression. Taking into account all these results, it is proposed that tsh functions in parallel to ey (and probably to toy) as an eye competence factor (Bessa, 2005).

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