POLYGENIC MUTATION IN DROSOPHILA-MELANOGASTER - GENETIC-ANALYSIS OF SELECTION LINES

We have conducted genetic analyses of 12 long-term selection lines of Drosophila melanogaster derived from a highly inbred base population, containing new mutations affecting abdominal and sternopleural bristle number. Biometric analysis of the number of effective factors differe ntiating the selected lines from the base inbred indicated that with t he exception of the three lines selected for increased number of abdom inal bristles, three or more mutations contributed to the responses of the selection lines. Analysis of the chromosomal distribution of effe cts revealed that mutations affecting abdominal bristle number occurre d on all three major chromosomes. In addition, Y-linked mutations with effects ranging from one to three bristles occurred in all three line s selected for decreased number of abdominal( )bristles, as well as in one line selected for increased abdominal bristle number. Mutations a ffecting sternopleural bristle number were mainly on the X and third c hromosomes. One abdominal and one sternopleural selection line showed evidence of a segregating lethal with large effects on bristle number. As an indirect test for allelism of mutations occurring in different selection lines, the three lines selected in the same direction for th e same trait were crossed in all possible combinations, and selection continued from the F-2 hybrids. Responses of the hybrid lines usually did not exceed those of the most extreme parental lines, indicating th at the responses of the parental lines may have been partly due to mut ations at the same loci, although other interpretations are possible.