Genotypic characteristics of cyclic parthenogens and their obligately asexual derivatives.

Among extant taxa it appears that cyclic parthenogenesis has evolved on fewer than ten occasions. Evidence suggests that these groups are all derived from bisexual ancestors with genetic systems possessing specific preadaptations for the breeding system shift. Its apparently independent origin in two cecidomyiid lineages suggests that, when such preadaptations exist, the adoption of cyclic parthenogenesis may be a highly probable event. The genetic consequences of this transition are not dramatic; the genotypic characteristics of most cyclic parthenogens are similar to those of bisexuals. Specifically, genotypic frequencies at polymorphic loci are generally in Hardy-Weinberg equilibrium and linkage disequilibrium between variants at different loci is rare. Levels of variation in local populations of cyclic parthenogens are lower than those typical of sexual outcrossers, but the lack of variation appears to be a consequence of founder effects, as regional gene pool diversity is not similarly impoverished. An unusual genotypic structure occasionally develops in populations from habitats in which sustained parthenogenetic reproduction is possible and sexual recruitment is low. Under these circumstances low fitness genotypes are winnowed from the population by natural selection. The truncation of genotypic diversity often produces Hardy-Weinberg deviations and linkage disequilibrium. The depletion of clonal diversity rarely leads to the domination of a habitat by a single clone; an array of ecologically diversified clonal variants remains. The level of clonal variation in such populations is occasionally augmented by sexual recruitment, but the persistence of clonal diversity in local populations of obligate parthenogens makes it clear that clonal coexistence is not a consequence of such recruitment. Cyclic parthenogens have made the transition to obligate asexuality with high frequency, but there is little evidence to support the argument (Williams, 1975) that such shifts result from the relaxation of the short-term selection pressures supposedly necessary to sustain the sexual phase of the life cycle. Maynard Smith (1978) has recognized that in groups such as the aphids, which have never evolved the ability to produce their diapausing eggs asexually, sexuality may simply be retained as a consequence of its role in the formation of these eggs. However, he accepted that short-term selection was required to maintain sexuality in groups such as the cladocerans in which asexual forms have demonstrated their ability to retain life cycle complexity.(ABSTRACT TRUNCATED AT 400 WORDS)