Coexisting overwintering strategies in Daphnia pulex: Clonal differences in sexual reproduction

Using two sets of clones of Daphnia pulex derived from overwintering parthenogenetic females and neonates hatched from dormant embryos, we tested for a genetic basis of differing overwintering strategies. While in an earlier study we had tested if the two clonal groups differed with respect to their growth rates, which would give them different opportunities during the spring development, we now performed experiments on possible differences in traits related to sexual reproduction (i.e., male production and ephippia formation). Our working hypothesis was that clones isolated from the sediment egg bank show a greater tendency to reproduce sexually than females overwintering in the open water. Although individual clones showed large differences in the proportion of males among offspring under short-day conditions (zero to > 40 %), the clonal groups did not differ significantly. Ephippia production was also significantly different among clones, but due to the large variances the group means did not differ. Ephippia production and male production in individual clones were not correlated, hence the tendencies to produce males or ephippia varied independently. Although there were considerable differences in the mean reproductive characteristics between the groups, the low power of the tests prevented support of any of the hypotheses. Rather, it seems that there is incomplete temporal differentiation while some females may follow a mixed strategy implying the production of sexual eggs as insurance against catastrophic events and successive attempts to survive the winter as parthenogenetic adults, which would result in a reproductive advantage during early spring