DEVELOPMENTAL GENETICS OF THE POSTERIOR LEGS IN DROSOPHILA MELANOGASTER

Abstract

NE of the characteristic morphological features of certain of the species 0 of the Drosophiladae is the sex comb (ctenidium) on the first and in some species also the second tarsal joint of the prothoracic leg of the male but never the female. The males of Drosophila melanogcrster have a single comb of 10-12 morphologically distinct bristles (teeth) on the distal end of the first tarsal segment , but occasionally a single tooth or several teeth occur as a developmental anomaly on the other tarsal segments of the first leg or on the other legs. Five mutants are known in D. melunogmter which induce the formation of typical sex combs on the metatarsi of the posterior legs. If sex comb teeth are developed on the metatarsi of the posterior legs, it may be because the genetic factor causes differentiation of a sex comb only, and this quite independent of differentiation in the rest of the tarsus or the rest of the leg. Conversely an extra sex comb mutant may have a more profound morphogenetic effect. It may redirect the development pattern so that the posterior legs become first legs with the appropriate region of the male metatarsus differentiating a sex comb. The legs of D. melanoguster are admirably suited for morphogenetic studies for the three legs are differentiated on the basis of size, structure and chaetotaxy (HANNAH-ALAVA, 1958). In addition the metatarsus of the first leg is sexually dimorphic in the total chaetotaxal pattern as well as gross anatomy and certain easily distinguishable bristle groups. A preliminary study by HANNAH (STERN 1954b) showed that some of the extra sex comb mutants did indeed induce profound changes in the metatarsi of the posterior legs of which the sexcombs were only the most obvious morphological change. A detailed analysis, presented in this paper, substantiates the hypotheses that the extra sex comb mutants do not induce sex combs per se, but that they are primarily concerned with changing the prepattern in the posterior legs, and after the prepattern is changed development of the legs continues in an orderly fashion but in a new direction. The results also show that the changes in the posterior legs of the female are just as profound as in the male, but always in the direction of the female first leg, as the changes in the posterior legs of the male are in the direction of a male first leg. MATERIALS AND METHODS Morphogenetic studies were made of a hybrid between two wild type strains and various combinations of three extra sex comb mutants: extra sex comb