Delayed mixis in rotifers: an adaptive response to the effects of density-dependent sex on population growth

Abstract

In most cyclically parthenogenetic life cycles, sex is needed to produce resting stages. In several species of cyclically parthenogenetic rotifers, some generations of clones are not responsive to a density-dependent signal that triggers sexual female production. These unresponsive rotifers hatch from resting eggs and typically pass 8–12 generations of female parthenogenesis before becoming receptive to the mixis signal. We addressed the selection for mixis delay using a simulation model. A delay of sexual reproduction could increase population growth through parthenogenesis and thus the number of resting eggs ultimately produced. In a monomorphic population without mixis delay, we determined the optimal ratio of mictic to amictic females (mixis ratio) to be 45%, and the optimal population density threshold for induction of mictic females (mixis threshold) to be 82 rotifers L−1. This mixis pattern, however, was not an evolutionarily stable strategy. A mixis ratio of 14% and threshold of 70 rotifers L−1 proved to be resistant to invasion by other mixis patterns. When we gave this phenotype a mixis delay of 8–10 days, it could invade a population with the same mixis pattern, but lacking a mixis delay. The advantage of delaying mixis was relatively small, suggesting that a polymorphism is possible.