Genetics of coxal chaetae in Drosophila melanogaster II. Responses to selection

Abstract

The average number of chaetae borne by the coxae is lowest on the rear (R) legs, intermediate on the middle (M) legs and highest on the front (F) legs. Mather & Hanks (1978) have shown that the numbers of coxal chaetae are mediated by a polygenic system. On the basis of evidence from a survey of inbred lines, chromosome assays of the differences among three of these lines, and a survey of the distribution of genic differences along the X chromosomes of the same three lines they postulated three kinds of gene in this system : α genes active on all three legs; β genes inactive on the rear legs, but active on the middle and front legs; and γ genes active only on the front legs. From this hypothesis, expectations are now derived for the responses of chaeta number on all three kinds of leg to selection for increased (H) and decreased (L) numbers of chaetae on the rear legs ( R H and R L ), on the middle legs ( M H and M L ) and on the front legs ( F H and F L ); and for increased and decreased differences between the chaeta numbers of the front and middle legs (( F – M ) H and ( F – M ) L ), of the front and rear legs (( F – R ) H and ( F – R ) L ), and of the middle and rear legs (( M – R ) H and ( M – R ) L ). The basic expectations are subject to prospective disturbance by certain complicating factors, notably by differences among the legs in heritability of the variation in chaeta number and by correlated responses to the selection arising from linkage between genes of the different kinds. Starting with the F 2 of a cross between two true-breeding stocks, twelve selection lines were initiated, one for each of the twelve types of selection, and continued for a minimum of 10 generations. It is shown that the responses in the chaetae numbers of the three kinds of leg to the twelve types of selection accord with expectation. Indeed, the results of the selection experiments of themselves lead directly to the hypothesis of the three kinds of gene.