Population genetics of a sex-linked locus in Drosophila melanogaster

Abstract

In population cages set up from laboratory stocks of white (w) and whiteblood (wbl), w frequency increased rapidly, rather than decreasing as expected. However, gene frequency changes in two environmental treatments (light and dark) were different. In the light, w frequency increased over four generations to about 0.65, remained polymorphic for about 12 generations, and then declined to zero. In the dark, w frequency increased to higher values over a longer time, but subsequent changes varied among populations. Two of these populations remain polymorphic, but at different frequencies, some 200 generations after initiation. Additional experiments involving 61 cage populations were done to test the hypothesis that the initial polymorphism resulted from associative overdominance due to linkage disequilibrium, and to analyse the nature of this disequilibrium. Results are interpreted in terms of an initial complete and complex linkage disequilibrium, with some loci affecting fitness in one environment, but not in the other. The experiments demonstrate that a complex linkage disequilibrium resulting from the mixing of previously isolated populations can lead to a variety of different polymorphisms, some of which may persist for very long periods of time, and that our understanding of the dynamics of multi‐locus systems is at a very low level.

Bottlenecks, population differentiation and apparent selection at microsatellite loci in Australian Drosophila buzzatii

“…Given the relatively low (albeit significant) genetic differentiation among these populations, the latter possibility is most unlikely. However, the former is very compatible with the prediction (Barker, 1982;Sokal et al, 1987) of a population explosion of D. buzzatii immediately following its introduction in the 1930s. Garza and Williamson (2001) argued that their M ratio should retain information about past demographic history for longer than methods such as BOTTLENECK, and our results appear to validate this.…”

“…By 1931, the density of C. cactorum was estimated at 2.5 Â 10 7 larvae per hectare, over the thousands of square kilometers of dense prickly pear (Dodd, 1940). Thus, D. buzzatii then had extremely large areas of suitable habitat, probably spread rapidly to all the Opuntiainfested area, and must have had an enormous population size (Barker, 1982;Sokal et al, 1987). By 1940, as the Opuntia was controlled and its distribution reduced more or less to the habitat islands found today, D. buzzatii also contracted to the spatially isolated populations that still exist.…”

“…D. buzzatii is a cactophilic species, feeding and breeding in the rotting tissues of Opuntia cactus (prickly pear) species, and is restricted to this niche in Australia (Barker and Mulley, 1976). The Chaco region of Argentina is the presumed center of origin of D. buzzatii (Vilela et al, 1980), and its introduction to Australia was apparently from this region in the period 1931-1936(Barker, 1982Barker et al, 1985), during the program for the biological control of prickly pear (Mann, 1970). Cactoblastis cactorum (Berg) (Lepidoptera: Pyralidae), which was introduced from Argentina in 1925 as a potential control agent, proved extremely effective-the larvae destroying plant tissue, leading to rot and death of the plants.…”