CONTROL OF SEGMENTAL ASYMMETRY IN DROSOPHILA EMBRYOS

During Drosophila development. an important aspect of body patterning is the division of the embryo into repeating morphological units refer red to as parasegments. The parasegmental domains are first defined at the blastoderm stage by alternating stripes of transcripts encoded by the pair-rule genes fushi tarazu (ftz) and even-skipped (eve) and lat er by stripes encoded by the segment polarity genes engrailed (en) and wingless. Here, we show that the runt gene (run) is required to gener ate asymmetries within these parasegmental domains. Using a heat-shock -inducible run transgene, we found that ectopic run expression leads t o rapid repression of eve stripes and a somewhat delayed expansion of ftz stripes. Unexpectedly, we also found that ectopic run was a rapid and potent repressor of odd-numbered en stripes. Two remarkably differ ent segmental phenotypes were generated as a consequence of these effe cts. In solving the mechanisms underlying these phenotypes, we discove red that the positioning of en stripes is largely determined by the ac tions of negative regulators. Our data indicate that run is required t o limit the domains of en expression in the odd-numbered parasegments, while the odd-skipped gene is required to limit the domains of en exp ression in the even-numbered parasegments. Activation of en at the ant erior margins of both sets of parasegments requires the repression of run and odd by the product of the eve gene. The spatial restriction of gene expression via negative and double negative pathways such as the se is likely to be a common theme during development.