HETEROZYGOTE DEFICIENCIES UNDER LEVENE'S POPULATION SUBDIVISION STRUCTURE.

Abstract

This note is intended to serve two purposes. First I wish to show that maintenance of genetic variability by means of spatial environmental heterogeneity could give rise to a deficiency of heterozygous genotypes compared with the numbers predicted by Hardy-Weinberg in random mating populations. This is placed in the context of Levene's (1953) spatial model. Second, I discuss the often-observed heterozygote deficiencies at some enzyme loci in cactophilic Drosophila (Barker and Mulley 1976; Barker 1981; Barker et al. 1986a; Thomas and Barker 1990; but see Quezada-Diaz et al. 1992). A simulation investigation, inspired by experiments comparing the genetic variation among Drosophila buzzatli breeding sites (Santos et al. 1989; Thomas and Barker 1990; Quezada-Diaz 1993), is carried out to evaluate the magnitude of heterozygote deficiency as a function of the allele frequencies, the strength of selection, and the number of females ovipositing on a niche. Finally, empirical studies to test the likelihood that the niche-variation hypothesis could explain the observed deficiency of heterozygotes are suggested. Levene (1953) considered a model of selection and extreme migration (i.e., there is a single pool of mating individuals who mate at random) in which the population is subdivided into several discrete units (niches). Two alleles, A, and A2, with frequencies p and q = 1 p, respectively, were considered. The genotypes are distributed randomly among the spatially isolated patches with different environments, where they become subject to selection such that the fitnesses in the ith (i = 1, 2, . . . , s) subpopulation are w111, 1, and w22,i for A Al, A A2, and A2A2 genotypes, respectively. After selection is completed, the proportion of the mating pool that subpopulation i contributes is ci (Mici = 1). Under this model, it is well known that the sufficient conditions for a protected polymorphism (i.e., the property of persistence of an allele even when initially rare) are