THE ACCUMULATION OF GENETIC VARIATION IN A PARTHENOGENETIC SNAIL.

Abstract

It is becoming increasingly obvious that parthenogenesis in animals is an important evolutionary phenomenon providing unique opportunities for insights into evolutionary processes. However, in most groups the mechanisms of asexual evolution are poorly understood, leading to problems in taxonomy and uncertainty over the significance of patterns of variation. Points of major significance for evolutionary theory may be undermined by inadequate knowledge of evolutionary processes in parthenoforms (e.g., Williamson, 1981; Mayr, 1982). This study addresses the question of pattern in the evolution of a polyploid apomict. Apomixis is the most commonly occurring type of parthenogenesis (Suomalainen et al., 1976) and is represented in both vertebrate and invertebrate classes (White, 1970). Its essentially mitotic cytological basis leads to its placement in White's (1970) heterozygosity-promoting class. As there is no chance for segregation or recombination between genes, alleles newly acquired through mutation must remain in a heterozygous state unless a further identical mutation occurs. Asher and Nace (1971) conclude that, while apomictic populations will not become entirely heterozygous, an equilibrium with a high level of heterozygosity will be attained. Lokki (1976a) predicts, from a stochastic model of evolution in diploid parthenoforms, that, after the initial establishment of apomictic reproduction, functional heterozygosity will increase to a maximum and then start to decrease due to the accumulation of non-functional alleles. Extending this model to polyploid apomicts, he concludes that polyploids will exhibit a similar rise and fall but will retain a higher level of functional heterozygosity due to a greater buffering of non-functional alleles, with a consequent increase in the life expectancy of the species (Lokki, 1976b). This latter prediction considered with White's (1970, 1973) assertion that apomictic cytology favors the establishment of polyploidy may partly explain the preponderance of polyploids reported in apomictic groups (Suomalainen et al., 1976). The adaptiveness of the high levels of heterozygosity postulated for these groups has been presumed to result from associated heterosis and the development of a general purpose genotype (Jaenike and Selander, 1979; Vepsalainen and Jarvinen, 1979). If then apomictic parthenoforms are able to produce genotypes well adapted to some environments and also able to avoid the 'costs' of meiosis and maintenance of males, why do the majority of animal species reproduce sexually? In addition to books by Williams (1975) and Maynard Smith (1978) a wealth of publications attempt to explain this phenomenon (Crow and Kimura, 1965, 1969; Maynard Smith, 1968, 1971, 1974; Asher and Nace, 1971; Smith, 1971; Williams and Mitton, 1973; Felsenstein, 1974; Cuellar, 1977; Stanley, 1979). Explanations usually fall into two categories proposing either longor shortterm mechanisms. The bulk of theories in the former category revolve around some statement of the relative inability of parthenoforms to adapt to temporal changes in the physical or biotic environment due to constraints on their ability to evolve resulting from their lack of recombination. In the face of these concepts it should be rare to find an apomictic species able to accumulate variation in significant amounts.