Genotypic characteristics of cyclic parthenogens and their obligately asexual derivatives

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Abstract
Cyclic parthenogenesis is a mode of reproduction which involves the more or less regular alternation of asexual and sexual reproduction. During the asexual phase of the life cycle individuals develop from unfertilized eggs as opposed to other forms of asexuality in which new individuals develop from somatic tissues. In organisms employing this latter breeding system the timing of asexual reproduction varies from immediately after zygote formation (polyembryony) until late in adult life (fission). Many somatic asexuals also reproduce sexually and hence employ a breeding system which is very similar to cyclic parthenogenesis. While the distinction of somatic and cyclic parthenogens may be hard to justify on genetic criteria, their separation has a long historical basis (von Siebold, 1857; Weismann 1893) founded on the unique ability of cyclic parthenogens to produce both sexual and asexual eggs. Bell (1982) has advocated extending the use of the term cyclic parthenogenesis to include organisms which alternate between obligate self-fertilization and outcrossing. This seems unwise, for such a breeding system has genetic consequences very different from those of conventional cyclic parthenogenesis. Selfing leads to a rapid loss of heterozygosity (Wright, 1969), while during apomictic parthenogenesis heterozygosity is maintained or even amplified (Berger, 1976). On this basis the present review considers only traditional cyclic parthenogens.

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