Cell lineage and differentiation on the male foreleg of Drosophila melanogaster

Abstract

Cell lineage was studied in the epidermis of the basitarsus of the foreleg of male Drosophila melanogaster. This was accomplished by “marking” single cells, and their descendants, with genotypes different from the rest of the tissues. The mosaic basitarsi thus produced had areas with yellow chaetae on a background of black chaetae. On the basis of cell lineage data, the male basitarsus can be divided in two main parts. Region A occupies most of the segment including the primary sex comb and the central bristle. It does not include the area of the longitudinal rows 1, 2, and 3 and that of the secondary sex comb which shows a close developmental relation to rows 1 and 2. These areas constitute region B. Within region A cell lineage data yield a subdivision into A-1 which includes longitudinal row 1′, the transverse rows, longitudinal rows 7 and 6 and 5.5, the primary sex comb and the central bristle; and A-2 the remaining area which includes longitudinal rows 4 and 5. In region A-1 the following developmental relations were found. (a) The descendants of a yellow cell tend to form longitudinal strips of tissue both along the longitudinal rows and in the area of the transverse rows. (b) The proximal teeth of the sex comb and the central bristle are closely related to the posterior part of the transverse rows. (c) The middle teeth are closely related to the middle and the anterior parts of the transverse rows and to row 7. (d) The bractless bristles 5.5 are closely related to the distal teeth within the middle part of the sex comb. (e) The distal teeth of the sex comb are closely related to longitudinal row 6, and less closely, to row 7. The data suggest that the prospective area of the primary sex comb initially occupies the same distal part of the basitarsus as the most distal transverse rows of the female basitarsus. They suggest further that in the male the tissue of this area shifts anteriorly, whereby it changes its direction by approximately 90 degrees. This not only is deduced from the results of the cell lineage studies, but explains the position of the bracts at the base of the sex comb teeth and the direction assumed by the teeth, both of which deviate by about 90 degrees from that of the other chaetae of the segment. The above results on pure male basitarsi were confirmed by cell lineage studies of gynandric basitarsi. Systematic deviations from the norm in the location and direction of the male sex comb teeth and of female chaetae at the distal part of the segment are the result of the differential morphogenesis of the male and female tissues in this area.