VARIATION AND HETEROZYGOSITY IN SEXUALLY VS. CLONALLY REPRODUCING POPULATIONS OF POECILIOPSIS.

Abstract

The adaptive value of sexual reproduction is not well understood. Numerous authors have attempted to identify the reasons for its widespread occurrence in both plants and animals but conflicting opinions have emerged (see Williams, 1975, for a review). Discussions have been largely theoretical and entirely debatable because little or no concrete data have been available. Widespread use of electrophoresis in recent years has revealed much about the genetics of sexual populations; yet little is known about asexual species. The effects of sexual and asexual reproduction on the genetic structure of populations can best be understood by comparing related species that share a common environment but utilize different methods of reproduction. Fishes of the genus Poeciliopsis (Poeciliidae) are ideal for comparing sexually and clonally reproducing populations since both forms coexist in the rivers of northwestern Mexico. Three distinct diploid all-female forms of Poeciliopsis arose as hybrids between sexual species in this genus. Each unisexual form contains a maternal genome derived from P. monacka, and a paternal genome from one of three species, P. lucida, P. occidentalis, or P. latidens, with which the unisexuals live and mate (Schultz, 1969, 1971, 1973). The F1 hybrid genotype of the unisexuals is sustained through a reproductive process termed hybridogenesis (Schultz, 1969) in which segregation is not random; the monacka genome passes intact to the haploid egg while the paternal genome is lost during meiosis