Stabilizing Selection for Number and Pattern of Extra Bristles in Drosophila Melanogaster

Abstract

Dorsocentral and scutellar bristles are rigidly canalized traits in wild-type Drosophila, showing a constant pattern of two anterior and two posterior bristles on the scutum and as many on the scutellum. The character results from a developmental process so well regulated that not only the number but also the position of all the bristles is constant. However, flies with one (or two) additional bristles on the scutum and/or the scutellum are often found at low frequencies in natural populations. Selective breeding of those deviant individuals quickly increases the mean number of extra bristles in the population. There have been reports of artificial directional selection resulting in an increase in the frequency of flies with two and four extra dorsocentral bristles (Ménsua, 1966; Pla, personal communication), suggesting the presence of a canalization threshold at those classes. Rendel and Sheldon (1960) have shown the possibility of artificial building up canalization for a phenotype other than the wild-type (two scutellar bristles) by using the variability of the scute mutant. The wild-type constancy of bristle number is presumably achieved through a polygenic system capable of stabilizing the phenotype despite the underlying genetic variation.