Factors organising and maintaining polymorphism in a cyclic parthenogenetic species: Daphnia pulex

The possible causes of the evolutionary stability of cyclic parthenogenesis are considered. The large genotypic variability of Daphnia pulex clones for reproduction systems in natural populations is provided by the high probability of interclonal crosses. When reproduction is mainly parthenogenetic, the less adapted clones are eliminated, and balanced interclonal polymorphism is established among the survivors. The following selective forces maintain polymorphism for mode of reproduction: high reproduction efficiency of clones prone to parthenogenesis, on the one hand, short-term advantage (high resistance to crowding and unfavourable conditions) of those prone to bisexuality, on the other hand. This selection gives rise to a stable polymorphism maintained by several developmental programmes within a genotype. Activation of any of the programmes and of the corresponding gene groups depends on genotypic and environmental backgrounds.