Sexual Selection Operating in a Wild Population of Drosophila Robusta.

Almost 60 years have elapsed since the discovery of variation in the polytene chromosomes of Drosophila (Sturtevant and Dobzhansky 1936). Inversion polymorphism studies have revealed some of the intricacies of evolutionary change, but even now, when interest in them has waned, there is little understanding of the particular alleles contained within them that alone or in relation to other genes influence any component of fitness. Perhaps the most important rule about inversion polymorphisms learned from a long history of laboratory experiments (cf. Lewontin et al. 1981) is their sensitivity to environmental features such as temperature and nutrition. Therefore, laboratory studies alone may not predict the consequences due to genotype (karyotype) by environment interactions of selection in wild populations. Only a few studies have documented selective mechanisms associated with preserving inversion polymorphisms in natural populations beyond establishing correlations with temperature, rainfall, humidity, elevation, and other environmental features. Despite the long history of interest in the ecology of Drosophila breeding and feeding sites (cf. Carson 1951, 1971; Heed 1957, 1968, 1971), experimental analyses of inversion polymorphisms involving natural breeding substrates are few because the mating sites and larval ecology of most species with well described inversion polymorphisms are poorly known, e.g., D. pseudoobscura. An early exception to this problem was the comprehensive study of inversion polymorphism in D. jlavopilosa, a flower breeder from South America (Brncic 1962). Egg to adult viability differences among karyotypes were observed among preadult stages in flowers returned to the lab and exposed to contrasting temperatures (Brncic 1968). More recently, analysis of the "sexratio" meiotic drive system in natural populations of D. pseudoobscura suggested that most selection responsible for maintaining this polymorphism must be operating in preadult stages (Beckenbach 1996). A notable non-Drosophila example is the chromosome I polymorphism in seaweed flies, Coelopa frigida, that carry out their life cycle on decomposing seaweed on beaches around the north Atlantic and North Sea. Gene arrangement-based differences in egg to adult development time, adult body size, and mate preference have been demonstrated in seaweed-reared flies (Day et al. 1983; Engelhard et al. 1989; Gilburn et al. 1992). For species that can be collected in nature throughout their life cycle, karyotype frequencies can be estimated from life stages reared in the wild and comparisons can be made between stages of the life cycle in selection component analyses Accepted January 9, 1996.