CONTROL OF CELL FATE AND POLARITY IN THE ADULT ABDOMINAL SEGMENTS OF DROSOPHILA BY OPTOMOTOR-BLIND

In an accompanying report (Kopp, A., Muskavitch, M, A, T. and Duncan, I, (1997) Development 124, 3703-3714), we show that Hh protein secrete d by posterior compartment cells patterns the posterior portion of the anterior compartment in adult abdominal segments, Here nle show that this function of hh is mediated by optomotor-blind (omb). omb(-) mutan ts mimic the effects of loss-of-function alleles of hh: structures fro m the posterior of the anterior compartment are lost, and often this r egion develops as a mirror image of the anterior portion, Structures f rom the anterior part of the posterior compartment are also lost, In t he 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 H h signaling is not mediated by dpp or wg. We describe several gain-of- function alleles that cause ectopic expression of omb in the anterior of the segment, Most of these cause the anterior region to develop wit h posterior characteristics without affecting polarity. However an all ele that drives high level ubiquitous expression of omb (Qd(Fab)) caus es the anterior tergite to develop as a mirror-image duplication of th e posterior tergite, a pattern opposite to that seen in omb-mutants, U biquitous expression of hh causes similar double-posterior patterning, We find that omb(-) alleles suppress this effect of ectopic hh expres sion and that posterior patterning becomes independent of hh in the Qd (Fab) mutant, These observations indicate that omb is the primary targ et of hh signaling in the adult abdomen, However, it is clear that oth er targets exist. One of these is likely Scruffy, a novel gene that we describe, which acts in parallel to omb. To explain the effects of om b alleles, we propose that both anterior and posterior compartments in the abdomen are polarized by underlying symmetric gradients of unknow n origin, We suggest that omb has two functions. First, it specifies t he development of appropriate structures both anterior and posterior t o the compartment boundary, Second, it causes cells to reverse their i nterpretation of polarity specified by the underlying symmetric gradie nts.