When to be sexual: sex allocation theory and population density-dependent induction of sex in cyclical parthenogens

The timing of sex is a critical fitness component in the cyclically parthenogenetic life-cycle of rotifers. It has been hypothesized that sex in rotifers is optimally timed to high population density because male-female encounters are more probable. Because sexual females produce either males or, if inseminated, diapausing eggs, the advantage of a higher male-female encounter rate is that allocation to male production can be lower. This is paradoxical in the context of the sex allocation theory developed for rotifers, as the theory predicts equal numbers of male-producing and diapausing-egg producing females. We investigated this paradox using both empirical data and theoretical analysis. Laboratory and field data show that the proportion of male-producing sexual females decreases with population density, as expected from the male-female encounter probability hypothesis. However, this observation is shown to be consistent with the equal average sex allocation hypothesis. We conclude that the threshold age of fertilization (the oldest age for a sexual female to be fertilized) and the average population density at which sex occurs interact to produce an equal sex allocation. The evolutionary analysis of an equal sex allocation has to consider these two factors. We found that, if sex occurs at sufficiently low population density, it imposes a cost due to delayed maturity. We conjecture that the natural selection has adjusted the threshold age of fertilization to achieve an equal sex allocation, and that initiation of sex at high population densities has evolved for some other reason, for example, to increase the number of sexual females.