Parthenogenesis in Rotifers: The Control of Sexual and Asexual Reproduction

SYNOPSIS. The class Rotifera includes species which reproduce solely by apomictic female parthenogenesis and species which alternate this “asexual” reproduction with ordinary sexual reproduction. The transition between asexual and sexual reproduction is controlled by the environment. Laboratory studies with the genus Asplanchna have shown that it is possible to identify specific molecules as inducers, which act on embryos in utero to modify their development and determine whether they will mature as sexually or asexually reproducing females. Moreover, an evolutionary rationale can be provided for the response to these particular environmental controlling agents, which are such that sexual reproduction will occur only when it will result in successful fertilization. Rotifers are opportunistic or colonising organisms, which implies selection for rapid reproduction. We suggest that this may account, at least in part, for the origin of both apomicitc parthenogenesis and certain features of the pattern of macromolecular syntheses during development. To account for the success of those rotifers which have lost sexual reproduction entirely, we note that accumulation of mutations during periods of exponential apomictic parthenogenetic reproduction, together with “mitotic” crossing-over, could theoretically produce sufficient genotypic diversity to provide evolutionary flexibility. This would eliminate a major advantage of sexual reproduction.