Gene name - bifid
Synonyms - optomotor-blind (omb) and Quadroon (Qd)
Cytological map position - 4C5-6
Function - T-box transcription factor
Keywords - neural development, T-box transcription factor
Symbol - bi
Genetic map position -
Classification - Brachyury homolog
Cellular location - nuclear
|Recent literature||Wang, D., Li, L., Lu, J., Liu, S. and Shen, J. (2016). Complementary expression of optomotor-blind and the Iroquois complex promotes fold formation to separate wing notum and hinge territories. Dev Biol [Epub ahead of print]. PubMed ID: 27212024
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. This study reports 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. These data help to understand how Dpp signaling is interpreted region-specifically during tissue subdivision.
|Liu, S., Sun, J., Wang, D., Pflugfelder, G. O. and Shen, J. (2016). Fold formation at the compartment boundary of Drosophila wing requires Yki signaling to suppress JNK dependent apoptosis. Sci Rep 6: 38003. PubMed ID: 27897227
Compartment boundaries prevent cell populations of different lineage from intermingling. In many cases, compartment boundaries are associated with morphological folds. However, in the Drosophila wing imaginal disc, fold formation at the anterior/posterior (A/P) compartment boundary is suppressed, probably as a prerequisite for the formation of a flat wing surface. Fold suppression depends on optomotor-blind (omb). Omb mutant animals develop a deep apical fold at the A/P boundary of the larval wing disc and an A/P cleft in the adult wing. A/P fold formation is controlled by different signaling pathways. Jun N-terminal kinase (JNK) and Yorkie (Yki) signaling are activated in cells along the fold and are necessary for the A/P fold to develop. While JNK promotes cell shape changes and cell death, Yki target genes are required to antagonize apoptosis, explaining why both pathways need to be active for the formation of a stable fold.
Bifid, more familiarly known as Optomotor blind, and T-related gene (Brachyenteron) are two Brachyury homologs in the fly. Brachyury has a major role in vertebrates in the differentiation of the notochord and in the formation of the mesoderm. optomotor blind is involved in differentiation of the brain, the CNS, the wing and in patterning of adult abdominal segments. These are all epidermally derived tissues. In the nervous system, omb is found in both neurons and glia.
Optomotor blind plays a prominent role in control of cell fate and polarity in the adult segments of Drosophila. Each abdominal segment produces a large dorsal cuticular plate (the tergite) and a smaller ventral plate (the sternite). Each tergite can be divided into three regions: an acrotergite that contains undecorated sclerotized cuticle, a central region containing an array of microchaetes, and a posterior region that contains a dark pigment band as well as a row of large macrochaetes at the posterior edge of this posterior region. All of the tergite, except the acrotergite, is covered with trichomes. For convenience, the posterior boundary of the tergite is defined to be the posterior edge of the pigment band. The intertergal cuticle is unpigmented and composed of an anterior trichome-bearing region (the posterior hairy zone or PHZ) and a posterior region of naked cuticle (the intersegmental membrane or ISM). All trichomes and bristles in the abdomen are oriented from the anterior to the posterior. The tergite and anterior portion of the PHZ develop from the anterior dorsal histoblast nest; the rest of the PHZ and the ISM develop from the posterior dorsal nest (Kopp , 1997).
Hedgehog protein secreted by posterior compartment cells plays a key role in patterning the posterior portion of the anterior compartment in adult abdominal segments. This patterning function of Hh is mediated by optomotor-blind. omb- mutants mimic the effects of loss-of-function alleles of hh: structures from the posterior of the anterior compartment are lost; often this region develops as a mirror image of the anterior portion. Structures from the anterior part of the posterior compartment are also lost. In the pupa, omb expression in abdominal histoblasts is highest at or near the compartment boundary, and decreases in a shallow gradient toward the anterior. This gradient is due to activation of omb by Hh, secreted by posterior compartment cells. In contrast to imaginal discs, this Hh signaling is not mediated by dpp or wg. Several hh gain-of-function alleles have been described that cause ectopic expression of omb in the anterior of the segment. Most of these cause the anterior region to develop with posterior characteristics without affecting polarity. However, an allele that drives high level ubiquitous expression of omb (QadroondFab) causes the anterior tergite to develop as a mirror-image duplication of the posterior tergite, a pattern just the opposite of that seen in omb- mutants. The Qd Fab allele has a dramatic effect on both polarity and bristle patterning. In Qd Fab hemizygotes and heterozygotes, the anterior tergite and intersegmental membrane (ISM) are deleted and replaced with a mirror-image duplication of the posterior tergite and PHZ. Ectopic macrochaetes are often, but not always, present at the anterior edge of the duplicated tergite structures, and sometimes also in the central tergite. The lines of polarity reversal are not fixed precisely with respect to cuticular pattern. In the most extreme phenotype, polarity is reversed exactly in the middle of the tergite and in the middle of the PHZ. More frequently, the line of polarity reversal is shifted anteriorly in the tergite and posteriorly in the PHZ. The phenotype is stronger in hemizygous males than heterozygous females, and is stronger in more posterior segments. The intertergal region is often compressed, and the dorsal longitudinal muscles underlying the tergites show irregular spacing and attachment sites (Koop, 1997).
omb alleles cause defects that are reciprocal to those of the Qd alleles. Hemizygotes for omb loss-of-function alleles mostly die as late larvae or early pupae; only a small percentage survive to the late pharate adult stage. Among the latter, the loss of structures that lie within the posterior region of the anterior compartment and the anterior region of the posterior compartment have been observed. In many hemisegments, especially those more anterior in the animal, posterior tergite and PHZ are deleted and replaced with a mirror-image duplication of the anterior tergite. This phenotype is exactly reciprocal to the phenotype of Qd Fab (Koop, 1997).
Ubiquitous expression of hh causes double-posterior patterning similar to that of Qd gain of function alleles. omb- alleles suppress this effect of ectopic hh expression and posterior patterning becomes independent of hh in the QdFab mutant. These observations indicate that omb is the primary target of hh signaling in the adult abdomen. However, it is clear that other targets exist. One of these is likely to be Scruffy, a novel gene, which acts in parallel to omb. To explain the effects of omb alleles, it is proposed that both anterior and posterior compartments in the abdomen are polarized by underlying symmetric gradients of unknown origin. It is suggested that omb has two functions: (1) it specifies the development of appropriate structures both anterior and posterior to the compartment boundary and (2) it causes cells to reverse their interpretation of polarity specified by the underlying symmetric gradients (Koop, 1997).
The subdivision of the developing Drosophila wing into anterior (A) and posterior (P) compartments is important for its development. The activities of the selector genes engrailed and invected in posterior cells and the transduction of the Hedgehog signal in anterior cells are required for maintaining the A/P boundary. Based on a previous study, it has been proposed that the signaling molecule Decapentaplegic (Dpp) is also important for this function by signaling from anterior to posterior cells. However, it has not been known whether and in which cells Dpp signal transduction is required for maintaining the A/P boundary. The role of the Dpp signal transduction pathway and the epistatic relationship of Dpp and Hedgehog signaling in maintaining the A/P boundary has been analyzed by clonal analysis. A transcriptional response to Dpp involving the T-box protein Optomotor-blind is required to maintain the A/P boundary. Further, Dpp signal transduction is required in anterior cells, but not in posterior cells, indicating that anterior to posterior signaling by Dpp is not important for maintaining the A/P boundary. Finally, evidence is provided that Dpp signaling acts downstream of or in parallel with Hedgehog signaling to maintain the A/P boundary. It is proposed that Dpp signaling is required for anterior cells to interpret the Hedgehog signal in order to specify segregation properties important for maintaining the A/P boundary (Shen, 2005).
For many years, it was thought that En and Inv regulated the segregation of A and P cells by specifying a P-type cell segregation in a cell-autonomous fashion. Recent work has challenged this view by showing that a unidirectional Hh-mediated signal from P to A cells is required to specify the A-type segregation behavior of A cells and that the role of En and Inv is mainly to control Hh signaling. Based on the findings that A cells signal back to P cells via Dpp and that wings from flies hypomorphic for dpp have a distorted A/P boundary, it has been proposed that A to P signaling by Dpp might also be important to maintain the A/P boundary. However, whether Dpp signal transduction is required for the maintenance of the A/P boundary and in which cells the Dpp signal is required remained unknown. By analyzing clones mutant for tkv, mad, and omb, several independent lines of evidence are provided that Dpp signal transduction is required to maintain the A/P boundary and that it is only required in A cells, but not in P cells. Thus, the results do not support the hypothesis that A to P signaling by Dpp is required to maintain the A/P boundary. Instead, the results suggest that Dpp signaling within Dpp-producing A cells is required to maintain the A/P boundary (Shen, 2005).
Through analysis of mutant clones located at the A/P boundary lacking the activity of the type I Dpp receptor Tkv, evidence is provided that the reception of the Dpp signal in A cells is required to maintain the A/P boundary. When generated in the P compartment, a few tkv−bsk− clones displace the A/P boundary to a small extent: this is attributed to the unusual round shape of these clones. However, the majority of P tkv−bsk− clones do not displace the A/P boundary, suggesting that the reception of the Dpp signal is not required in P cells to maintain the A/P boundary. In contrast, mutant clones generated in the A compartment at the A/P boundary displace the position of the A/P boundary toward P, indicating that the reception of the Dpp signal is required in A cells to maintain the A/P boundary (Shen, 2005).
How does the reception of the Dpp signal control cell segregation at the A/P boundary? Although the molecular basis is unknown, a cell's segregation behavior presumably depends on its cytoskeletal or surface properties (cell affinity). Members of the TGFβ superfamily have been observed in other systems to be able to activate regulators of the actin cytoskeleton independently of Mad/Smad transcription factors, raising the possibility that Dpp reception could control cell segregation by directly altering structural components of the responding cells. Alternatively, Dpp could control the segregation of cells by regulating the transcription of one or several target genes. To distinguish between these possibilities, the role of downstream components of the Dpp signal transduction pathway were analyzed. Three independent lines of evidence is provided that a transcriptional response to the Dpp signal is required to maintain the A/P boundary. (1) The segregation behaviors of mad−bsk− and tkv−bsk− clones are indistinguishable. Like tkv−bsk− clones, A mad−bsk− clones displace the A/P boundary toward P, indicating a role for the transcription factor Mad in A cells to maintain the A/P boundary. (2) mad−brk− clones respect the A/P boundary, indicating that repression of brk transcription by Mad is important for normal A/P cell segregation. (3) A omb− clones displace the A/P boundary toward P. The frequency and extent of the boundary displacement of A omb−, tkv−bsk−, and mad−bsk− clones is comparable, suggesting that the Dpp target gene omb is the main mediator of this aspect of the Dpp signal. In contrast to omb− clones, most A clones mutant for the Dpp target gene sal do not displace the A/P boundary, indicating that sal does not play an important role in maintaining the A/P boundary. Together, these data suggest that the transduction of the Dpp signal controlling the maintenance of the A/P boundary bifurcates at the level of the Dpp target genes (Shen, 2005).
Cells of tkv−bsk−, mad−bsk−, and omb− clones displacing the A/P boundary do not appear to intermingle well with P cells. In fact, within the entire wing disc pouch, these mutant clones have a round shape and smooth borders, suggesting that these mutant cells in general do not intermingle freely with wild-type cells. Similar clone shapes have been reported upon mutation or misexpression of several genes, including mutants in the Dpp target gene sal and misexpression of a constitutively active form of Tkv. The round shapes and smooth borders of clones have been attributed to differences in the affinity of clone cells for their neighbors, suggesting that Tkv, Mad, and the Dpp target genes omb and sal may affect some aspects of wing pouch cell affinity. Therefore, the inability of A tkv−bsk−, mad−bsk−, and omb− clones displacing the A/P boundary to intermingle well with P cells is attributed to this particular role (Shen, 2005).
Taken together, this analysis indicates two roles for Dpp signal transduction: (1) it provides some aspects of the cell affinity of both A and P wing pouch cells; (2) it is required in A cells to specify an A cell affinity important for maintaining the A/P boundary. These two roles of Dpp signal transduction could either be related or distinct. The finding that the Dpp target gene sal is required for the first role, but not the second, provides a first indication that these two roles are implemented by partially distinct molecular mechanisms (Shen, 2005).
How might Omb regulate the segregation behavior of cells at the A/P boundary? Recent work has shown that Omb has at least two roles during the patterning of the Drosophila wing. First, Omb is required for the expression of two Dpp target genes sal and vestigial (vg) (del Alamo Rodriguez, 2004). Since sal mutant clones do respect the A/P boundary, the role of Omb in maintaining the A/P boundary cannot depend on sal induction. Since Vg is required for wing cell proliferation, its role in maintaining the A/P boundary cannot be tested. Second, Omb is involved in shaping the expression pattern of tkv along the A/P axis of the wing disc (del Alamo Rodriguez, 2004). The expression of tkv is reduced in Dpp-producing A cells along the A/P boundary. This reduction of tkv expression is mediated by the transcription factor Master of thickveins (Mtv, also known as Brakeless and Scribbler, which is expressed in these cells in response to the Hh signal. Since both tkv and mtv are upregulated in omb mutant clones, it has been proposed that Omb is required for Mtv to repress tkv (del Alamo Rodriguez, 2004). However, reduction of tkv transcription in A cells does not seem to be important for the segregation of cells at the A/P boundary, because A clones either mutant for mtv, in which tkv levels are increased, or overexpressing tkv, respect the A/P boundary. Thus, neither the role of Omb in repressing tkv nor in activating sal transcription appears to be important for Omb's function in maintaining the A/P boundary. Therefore, other target genes of Omb must exist that mediate Omb's function in maintaining the A/P boundary (Shen, 2005).
Anterior cells at the A/P boundary have been shown to require Hh signal transduction to segregate from P cells. Evidence is provided that A cells in addition need to transduce the Dpp signal for normal segregation. What is the epistatic relationship between Hh and Dpp signaling? The activity of the Hh transduction pathway is not affected in either tkv−bsk− or mad−bsk− clones as monitored by the expression of the Hh target gene ptc, indicating that Hh signal transduction does not require Dpp signal transduction components for its activity. However, the Dpp target gene omb appears to be important for A cells to interpret the Hh signal because the ability of Ci to specify A-type segregation properties depends, in part, on the activity of Omb. Thus, Dpp signaling acts either downstream of or in parallel with Hh signaling in maintaining the A/P boundary (Shen, 2005).
Previously, three transcription factors, a transcriptional activator form of Ci (hereafter referred to as Ci[act]), En, and Inv, have been shown to be required for the segregation of A and P cells. Evidence exists for the involvement of a fourth transcription factor, the T-box protein Omb. Omb is further shown to act downstream of or in parallel with Ci. How could these four transcription factors regulate the segregation of A and P cells? In a simple model, Ci[act], En, Inv, and Omb could regulate the segregation of A and P cells by controlling the transcription of the same set of target genes that may encode cell affinity molecules or regulate the activity of cell affinity molecules. Omb is activated in both A and P cells in a broad domain centered around the A/P boundary by Dpp signaling where Omb may upregulate the expression of this putative target gene(s). The activity of Ci[act] is restricted to Hh-responding A cells along the A/P boundary. In these A cells, the target gene(s) would be further induced. En and Inv expressions are mainly confined to P cells in which they are known to act as repressors of transcription. Thus, En and Inv would repress the putative target gene(s) in P cells. The abrupt difference in the expression of putative target gene(s) would contribute to the segregation of A and P cells. Anterior clones (but not P clones) of cells lacking Omb would displace the A/P boundary because normally the putative target gene would be highly expressed in A cells, but not in P cells, where it would be repressed by En and Inv. Omb may therefore provide a basal affinity to cells in the center of the wing disc that is modified by Ci[act], En, and Inv to create a sharp difference of this affinity in cells on both sides of the A/P boundary. In an alternative model, Omb, Ci[act], En, and Inv would regulate distinct sets of genes. To distinguish among these models, it will be necessary to identify the Ci[act], En, Inv, and Omb target genes mediating cell segregation (Shen, 2005).
The precise position and shape of the Dpp organizer along the A side of the A/P boundary are important for normal growth and patterning of the wing. It has been proposed that the segregation of cells at the A/P boundary contributes to maintain this precise position and shape of the Dpp organizer in the growing wing disc epithelium. It is intriguing to notice that the Dpp-organizing activity itself plays a role in the segregation of A and P cells, suggesting that the Dpp-organizing activity contributes to maintain its own position. It will be interesting to investigate whether other organizers associated with compartment boundaries have similar functions (Shen, 2005).
Optomoter blind is a T-box DNA-binding protein. The T-box family is an ancient group that appears to play a critical role in development in all animal species. These genes were uncovered on the basis of similarity to the DNA binding domain of murine Brachyury (T) gene product, the defining feature of the family. Common features shared by T-box family members are DNA-binding and transcriptional regulatory activity, a role in development and conserved expression patterns, most of the known genes in all species being expressed in mesoderm or mesoderm precursors.
date revised: 3 DEC 97
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