Variation of Scutellar Bristles in Drosophila X. Selection Limits

Abstract

The response to selection for scutellar bristles in three lines which originally shared a common gene pool is described. Genetic dissimilarities resulting from selection in these three lines could be adduced from the type of response patterns, the magnitude and rate of response, the effect of suspended and reverse selection, and from a genetic analysis of whole chromosomes. Two barriers to progress from selection were established. One appeared to be due to homozygosity of effective factors, while the other contained a considerable amount of genetic variability utilizable by reverse selection only. The two models proposed to account for the response patterns observed both invoke the role of recombination in the release of genetic variability. One model assumes the existence of at least two closely linked loci while the other takes account of the fact that recombination within an asymmetrically paired segment leads to the production of duplications. It is argued that such duplications may favour high bristle number.