The developmental autonomy of extra sex combs in Drosophila melanogaster

Abstract

In males of Drosophila melanogaster the first legs possess a sex comb, certain tibial transverse rows of bristles, and certain basitarsal transverse rows. These structures are absent on the second and third legs. The recessive autosomal mutant esc (extra sex comb) transforms certain features of the second and third into those of first legs. In different legs the transformation may be incomplete. The different parts may be transformed independently of each other, resulting in an array of partially transformed legs which are epigenetic mosaics of changed and unchanged areas. Genetic mosaics consisting of esc esc patches on esc + legs were obtained in consequence of X-ray induced somatic crossing over. By means of a translocation the esc esc areas were marked by genetically yellow pigmentation on the background of wild-type esc + . In the genetic mosaics esc acts autonomously in causing the appearance of sex combs and both types of transverse rows even in very small esc esc areas. Wild-type colored sex comb teeth on second and third legs were induced by X-ray irradiation of esc + males. The second legs of males with the combination esc D en +en of esc d , a dominant allele of esc, and en (engrailed) had a typical (primary) sex comb as well as the secondary comb normally placed by en on the first leg only. The gene esc as compared to esc + controls the terminal pattern of leg differentiation by changing the competence of imaginal disk cells to respond to a prepattern that is alike for both alleles. The changed response consists in a tendency in certain areas toward growth in a transverse direction coupled with differentiation in a closely packed row, of bristles or teeth.