The transcription factor optomotor-blind restricts apterous expression through TrxG and PcG genes.

The establishment of body pattern is a fundamental process in developmental biology. In Drosophila, the wing disc is subdivided into dorsal (D) and ventral (V) compartments by the D/V boundary. The dorsal fate is adopted by expressing the selector gene apterous (ap). ap expression is regulated by three combinational cis-regulatory modules which are activated by EGFR pathway, Ap-Vg auto-regulatory and epigenetic mechanisms. Here, we found that the Tbx family transcription factor Optomotor-blind (Omb) restricted ap expression in the ventral compartment. Loss of omb induced autonomous initiation of ap expression in the middle third instar larvae in the ventral compartment. Oppositely, over-activation of omb inhibited ap in the medial pouch. All three enhancers apE, apDV and apP were upregulated in omb null mutants, indicating a combinational regulation of ap modulators. However, Omb affected ap expression neither by directly regulating EGFR signaling, nor via Vg regulation. Therefore, a genetic screen of epigenetic regulators, including the Trithorax group (TrxG) and Polycomb group (PcG) genes was performed. We found that knocking down the TrxG gene kohtalo (kto), domino (dom) or expressing the PcG gene grainy head (grh), the ectopic ap in omb mutants was repressed. The inhibition of apDV by kto knockdown and grh activation could contribute to ap repression. Moreover, Omb and the EGFR pathway are genetically parallel in ap regulation in the ventral compartment. Collectively, Omb is a repressive signal for ap expression in the ventral compartment, which requires TrxG and PcG genes.

The T-box gene optomotor-blind organizes proximodistal leg patterning in the beetle Tribolium castaneum by repressing dorsal Dpp pathway activity.

Leg axis formation in Drosophila is organized by Wingless (Wg) and Decapentaplegic (Dpp) that control a number of downstream factors to pattern the dorsoventral (DV) and proximodistal (PD) axis. The T-box genes are important downstream factors mainly involved in dorsoventral leg axis formation. The ventral side is specified by H15 and midline, whereas optomotor-blind (omb) and Dorsocross (Doc1) are factors to specify dorsal cell fates. We show here that omb also organizes PD leg axis patterning in the beetle Tribolium castaneum. In the legs, Tc-omb is expressed along the dorsal side and represses ventral factors like wg and H15. Intriguingly, removing Tc-omb function leads to the activation of the Dpp pathway along the dorsal side of the legs, thus mimicking normal dpp expression in Drosophila. Dpp activity along the dorsal side leads to altered expression of proximal-distal patterning genes such as Distal-less (Dll) and dachshund (dac). Our results indicate a cell-autonomous activation of Dll and repression of dac by dpp. These findings are compatible with the cross-regulatory "cascade model" of proximal-distal leg imaginal disc patterning of Drosophila.

T-box transcription factors Dorsocross and optomotor-blind control Drosophila leg patterning in a functionally redundant manner.

The T-box genes are essential transcription factors during limb development. In Drosophila, Dorsocross (Doc) and optomotor-blind (omb), members of the Tbx2 and Tbx6 families, are best studied in the Drosophila wing development. Despite prominently expressed in leg discs, the specific function of these genes in leg growth is still not revealed. Here we demonstrated that Doc and omb regulated the morphogenesis of leg intermediate regions in a functionally redundant manner. Loss of Doc or omb individually did not result in any developmental defects of the legs, but loss of both genes induced significant defects in femur and proximal tibia of the adult legs. These genes located in the dorsal domain, where the Doc region expanded and cross-overlapped with the omb region corresponding to the presumptive leg intermediate region. We detected that the normal epithelial folds in the leg discs were disrupted along with dorsal repression of cell proliferation and activation of cell apoptosis when Doc and omb were both reduced. Furthermore, the dorsal expression of dachshund (dac), a canonical leg developmental gene specifying the leg intermediate region, was maintained by Doc and omb. Meanwhile, the Notch pathway was compromised in the dorsal domain when these genes were reduced, which might contribute to the joint defect of the adult leg intermediate regions. Our study provides cytological and genetic evidence for understanding the redundant function of Doc and omb in leg morphogenesis.

Co-evolving wing spots and mating displays are genetically separable traits in Drosophila.

The evolution of sexual traits often involves correlated changes in morphology and behavior. For example, in Drosophila, divergent mating displays are often accompanied by divergent pigment patterns. To better understand how such traits co-evolve, we investigated the genetic basis of correlated divergence in wing pigmentation and mating display between the sibling species Drosophila elegans and Drosophila gunungcola. Drosophila elegans males have an area of black pigment on their wings known as a wing spot and appear to display this spot to females by extending their wings laterally during courtship. By contrast, D. gunungcola lost both of these traits. Using Multiplexed Shotgun Genotyping (MSG), we identified a ∼440 kb region on the X chromosome that behaves like a genetic switch controlling the presence or absence of male-specific wing spots. This region includes the candidate gene optomotor-blind (omb), which plays a critical role in patterning the Drosophila wing. The genetic basis of divergent wing display is more complex, with at least two loci on the X chromosome and two loci on autosomes contributing to its evolution. Introgressing the X-linked region affecting wing spot development from D. gunungcola into D. elegans reduced pigmentation in the wing spots but did not affect the wing display, indicating that these are genetically separable traits. Consistent with this observation, broader sampling of wild D. gunungcola populations confirmed that the wing spot and wing display are evolving independently: some D. gunungcola males performed wing displays similar to D. elegans despite lacking wing spots. These data suggest that correlated selection pressures rather than physical linkage or pleiotropy are responsible for the coevolution of these morphological and behavioral traits. They also suggest that the change in morphology evolved prior to the change in behavior.

T-Box Genes in Drosophila Limb Development.

T-box genes are essential for limb development in vertebrates and arthropods. The Drosophila genome encodes eight T-box genes, six of which are expressed in limb ontogenesis. The Tbx20-related gene pair midline and H15 is essential for dorso-ventral patterning of the Drosophila legs. The three Tbx6-related Dorsocross genes are required for epithelial remodeling during wing development. The Drosophila gene optomotor-blind (omb) is the only member of the Tbx2 subfamily in the fly and is predominantly involved in wing development. Omb is essential for wing development and is sufficient to promote the development of a second wing pair. Targeted manipulations of omb expression have shown that the bulk omb requirement for wing development can be deconstructed into a number of individual functions. Even though omb expression in the wing disc is symmetrical with regard to the anterior/posterior (A/P) compartment boundary, anterior and posterior knockdowns have distinct consequences: Anterior Omb is required for the maintenance of a straight A/P lineage restriction boundary. Posterior Omb suppresses formation of an apical epithelial fold along the A/P boundary. Drosophila T-box gene expression is not confined to the ectoderm-derived epithelia of the imaginal discs. Both Doc and Omb are prominently expressed in leg disc muscle precursor cells. Omb is also strongly expressed in a tracheal branch that invades the extracellular matrix of the wing disc. The function of Doc and Omb in the latter tissues is not known, indicative of the many questions still open in the field.

Complementary expression of optomotor-blind and the Iroquois complex promotes fold formation to separate wing notum and hinge territories.

Animal morphogenesis requires folds or clefts to separate populations of cells which are often associated with different cell affinities. In the Drosophila wing imaginal disc, the regional expression of the Iroquois complex (Iro-C) in the notum leads to the formation of the hinge/notum (H/N) fold that separates the wing hinge and notum territories. Although Decapentaplegic (Dpp) signaling has been revealed as essential for the hinge/notum subdivision through the restriction of Iro-C toward the notum region, the mechanism by which the H/N border develops into a fold is unknown. Here, we report that a Dpp target gene, optomotor-blind (omb), mediates the role of Dpp signaling in Iro-C inhibition. omb is complementarily expressed on the dorsal hinge side, abutting the Iro-C domain along the H/N border. Ectopic omb expression inhibits Iro-C in the notum territory, independent of known Iro-C regulators Msh and Stat92E. Uniform manipulation of either omb or Iro-C genes spanning the presumptive H/N border significantly suppresses H/N fold formation via inhibition of the apical microtubule enrichment. Ectopically sharp border or discontinuity in level of Iro-C or Omb is enough to generate ectopic fold formation. These results reveal that omb and Iro-C not only are complementarily expressed but also cooperate to promote H/N fold formation. Our data help to understand how Dpp signaling is interpreted region-specifically during tissue subdivision.

Dpp/BMP signaling in flies: from molecules to biology.

Decapentaplegic (Dpp), the fly homolog of the secreted mammalian BMP2/4 signaling molecules, is involved in almost all aspects of fly development. Dpp has critical functions at all developmental stages, from patterning of the eggshell to the determination of adult intestinal stem cell identity. Here, we focus on recent findings regarding the transcriptional regulatory logic of the pathway, on a new feedback regulator, Pentagone, and on Dpp's roles in scaling and growth of the Drosophila wing.

Dpp signaling inhibits proliferation in the Drosophila wing by Omb-dependent regional control of bantam.

The control of organ growth is a fundamental aspect of animal development but remains poorly understood. The morphogen Dpp has long been considered as a general promoter of cell proliferation during Drosophila wing development. It is an ongoing debate whether the Dpp gradient is required for the uniform cell proliferation observed in the wing imaginal disc. Here, we investigated how the Dpp signaling pathway regulates proliferation during wing development. By systematic manipulation of Dpp signaling we observed that it controls proliferation in a region-specific manner: Dpp, via omb, promoted proliferation in the lateral and repressed proliferation in the medial wing disc. Omb controlled the regional proliferation rate by oppositely regulating transcription of the microRNA gene bantam in medial versus lateral wing disc. However, neither the Dpp nor Omb gradient was essential for uniform proliferation along the anteroposterior axis.

Enhancer trap infidelity in Drosophila optomotor-blind.

Reporter gene activity in enhancer trap lines is often implicitly assumed to mirror quite faithfully the endogenous expression of the "trapped" gene, even though there are numerous examples of enhancer trap infidelity. optomotor-blind (omb) is a 160 kb gene in which 16 independent P-element enhancer trap insertions of three different types have been mapped in a range of more than 60 kb. We have determined the expression pattern of these elements in wing, eye-antennal and leg imaginal discs as well as in the pupal tergites. We noted that one pGawB insertion (omb (P4) ) selectively failed to report parts of the omb pattern even though the missing pattern elements were apparent in all other 15 lines. We ruled out that omb (P4) was defective in the Gal4 promoter region or had inactivated genomic enhancers in the integration process. We propose that the Gal4 reporter gene in pGawB may be sensitive to orientation or promoter proximity effects.