opa transcript first appears at the beginning of cellularization (stage 5) in a single stripe located 80% of the distance to the anterior end of the embryo. With time its expression extends to form a large area reaching to 20% from the posterior end. Only at stage 7, once germ band expression begins, does the transcript begin to fade, generating 14 weak stripes over a low background level of expression (Benedyk, 1994).

As development proceeds, opa expression ceases briefly both in the ectoderm and in the underlying mesodermal cells that later become the visceral mesoderm (Cimbora, 1995).

Effects of Mutation or Deletion

Only half the number of denticle belts are present in opa mutants as a result of deletion of alternate segments (Benedyk, 1994).

odd-paired is required for normal midgut development. opa function is required for formation of the three characteristic midgut constrictions. In the cellular blastoderm, opa is expressed ubiquitously in the ectoderm and mesoderm precursors throughout the presumptive segmented region. As development proceeds, opa expression ceases briefly both in the ectoderm and in the underlying mesodermal cells that later become the visceral mesoderm. In opa mutants the visceral mesoderm is interrupted, evidently due to abnormal expression of bagpipe, a homeodomain gene required for the formation of the visceral mesoderm. At early stages of development in opa mutants, interruptions in the visceral mesoderm are observed at many positions along the anterior-posterior axis. As development proceeds, interruptions are less frequently observed; however, one interruption is coincident with the Antennapedia expression domain and variability of Ultrabithorax expression in opa mutants. From these observations, it is inferred that the loss of at least the first and second midgut constrictions in opa mutants is the result of defects first evident in the early stages of visceral mesoderm development (Cimbora, 1995).

During later stages in development, opa expression reinitiates in spatially restricted domains of the visceral mesoderm, coinciding with the locations at which the first and third constrictions will form. At the locations of the first and third constrictions, opa is positively regulated by Antennapedia and abdominal-A, respectively, while between these domains opa is negatively regulated by Ultrabithorax and dpp. The coincidence of opa expression with the locations of the first and third midgut constrictions and the complex regulation of opa expression leads to a speculation that in addition to contributing to visceral msoderm development, opa, acting in the mesoderm, may mediate homeotic gene function during midgut constriction formation (Cimbora, 1995).

In addition to its requirement for visceral mesoderm development, opa is required for proper gut expression in the endodermal component of the midgut. Repression of the POU-domain gene pdm-1 in a specific region of the endoderm is dependent on opa function. pdm-1 is expressed in much of the endoderm at stage 13, but is repressed in a domain of the endoderm underlying the Ubx and dpp expression domain in the visceral mesoderm. Repression of pdm-1 in the endoderm is dependent of Ubx and dpp, and the accumulation of DPP protein in this region of the endoderm suggests that it may be the secreted factor responsible for regulating pdm-1 across germ layers. In opa mutants, pdm-1 repression does not occur. opa does not appear to contribute to pdm-1 repression through Ubx or dpp, as these genes are expressed in opa mutants, and the activation of the homeotic gene labial in the endoderm (which is dependent on Ubx/dpp signaling) occurs normally. These observations suggest that repression of pdm-1 occurs by a novel signaling pathway, independent of dpp. Since opa is not transcribed in the visceral mesoderm near the pdm-1 expression domain, interruptions in the endoderm may reflect the inability of the endodermal primordia to migrate properly along the defective visceral mesoderm, an interpretation consistent with the proposal that the visceral mesoderm is required as a substrate for endodermal migration (Affolter, 1993 and Cimbora, 1995).


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odd-paired: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation

date revised: 2 March 2020

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