Latitudinal variation in wing size in Drosophila subobscura and its dependence on polygenes of chromosome O

Abstract

The genetic variability of natural populations of Drosophila subobscura derived from localities situated approximately along a north-to-south diameter through the distribution area of the species was studied. The polygenic and adaptive trait ‘wing size’ shows a continuous clinal reduction from north to south. The comparison between flies homozygous or randomly heterozygous for wild chromosomes showed that heterozygous flies are on an average bigger than homozygous flies and that overdominance exists for increased wing size. The genetic variance proved lower in central than in ecologically marginal populations. From the analysis of left-right asymmetry it could be shown that heterozygous flies are more homeostatic than homozygous flies but no differences appeared between populations. Therefore, lower phenotypic variability in central populations must be due to lower genetic variation. An hypothesis was put forward that normalizing selection eliminates + and-alleles in the central populations; in marginal populations, on the other hand, either + (north) or-(south) alleles are favoured and the genetic variability is augmented.