Timing the initiation of sex: Delay mechanisms alter fitness outcomes in a rotifer population model

Species that inhabit variable environments have complex mechanisms to precisely time their life-history transitions as conditions change. One such mechanism in rotifers is a block on sexual reproduction that extends across multiple asexual generations after emergence from diapause. It has been hypothesized that this delay is advantageous in competitive and stochastic environments. Here, we develop a model of cyclically parthenogenic rotifer populations with a novel formulation of a "mictic block" that prevents sexual reproduction by females that are not sufficiently distant, generationally, from a stem ancestor that was produced sexually. We find that mictic blocks are indeed adaptive but that the most successful phenotypes have shorter blocks than previously reported and that the success of different delay phenotypes is highly dependent on the duration of the growing season. For a fixed environmental regime, coexistence of distinct mixis phenotypes is possible, wherein a phenotype with a longer block performs better in years with an average-length growing season, and a phenotype with a shorter block and lower mixis ratio performs better in years with an "extreme" growing season, whether short or long. Our model provides an eco-evolutionary framework for the study of Brachionus rotifers, a model system for non-genetic maternal effects and the evolution of sex.