Item

Abstract

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.