Incomplete induction of mixis in Brachionus calyciflorus:: patterns of reproduction at the individual level

In the rotifer Brachionus calyciflorus mictic-female production is density- dependent and appears to be induced by a chemical - a quorum sensing molecule - produced by the females themselves. Even at the highest densities, however, populations never become entirely mictic: i.e., some amictic females continue to be produced. Surprisingly, the phenomenon also occurs in clonal laboratory populations with genetically identical individuals. Here, we study how this ecologically adaptive phenomenon is generated at the level of individual reproducing females. In a life- history experiment we subjected 123 amictic females of a clone of B. calyciflorus separately to a daily renewed stimulus of culture medium conditioned at a density of 30 females ml(-1). For each of these mothers we isolated the lifetime offspring individually and recorded whether these females were amictic or mictic. Mothers produced on average 16 offspring but none of the mothers produced 100% mictic offspring; the average proportion of mictic females was 30%, despite the extremely strong stimulus. The distribution of amictic vs. mictic offspring was not uniform over the mothers' lifetime. Early and late offspring had a low probability of being mictic whereas mid-aged mothers produced the highest proportion of mictic daughters ( up to 56%). We conclude that not all oocytes of B. calyciflorus can be turned into mictic females, even when the micticfemale-inducing stimulus is extremely high. Propensity to become a mictic female also depends on the rank of an egg within a female's offspring production. Despite these regularities, we observed considerable stochastic variability with respect to individual mothers' life histories.