Evolution of Anisogamy in Organisms with Parthenogenetic Gametes

The two sexes are defined by the sizes of the gametes they produce, anisogamy being the state with two differing gamete sizes (hence, females and males). The origin of this divergence has received much research interest, both theoretically and empirically. The gamete dynamics (GD) theory is a widely accepted theoretical explanation for anisogamy, and green algae have been an important empirical testing ground for the theory. However, some green and brown algae produce parthenogenetic gametes (gametes that can develop without fusing with another gamete), in contrast to an assumption in GD theory that unfused gametes do not develop. Here, we construct a GD model accounting for parthenogenetic gametes. We find that under conditions of panmixia and highly efficient fertilization (i.e., conditions of classical GD models from 1972 onward), the results remain largely unaltered by parthenogametes. However, under gamete-limited conditions anisogamy evolves less easily in the new model, and a novel result emerges: whereas previous models typically predict the evolution of either anisogamy or small isogamy, the current model shows that large isogamy can evolve when parthenogenetic gametes evolve under conditions of inefficient fertilization. Our analyses uncover unexplored complications relating to sex ratios under this relatively uncharted gametic system. We discuss limitations these complications impose on our models and suggest avenues for future research. We compare model results to algae with parthenogenetic gametes in nature.