Parthenogenesis, sexual conflict and the evolution of oogamy

Abstract

AbstractOrganisms with external fertilization exhibit a variety of reproductive modes, from simple parthenogenesis to isogamy, anisogamy and oogamy. Here, we develop a mathematical model that helps to explain the evolution of these modes through the co-evolution of cell size and fertilization rate. By assuming that gametes can develop parthenogenetically should they fail to fertilize, and that survival of a propagule (zygote/unfertilized gamete) depends on size, we find that an isogamous population can evolve to anisogamy through evolutionary branching. Oogamy can then evolve from an anisogamous population under sexual conflict. Furthermore, we derive analytic results on the model parameters required to arrest evolution on this isogamy-oogamy trajectory. Low fertilization rates stabilise isogamy, while low fertilization costs stabilise anisogamy. Additionally we show using adaptive dynamics that isogamy can be maintained as a bet-hedging strategy in a stochastically switching environment.Graphical AbstractHighlightsAdapting the classic Parker-Baker-Smith model for the evolution of anisogamy (gametic sex cells of differing sizes), we provide the first analysis of the co-evolution of fertilization rate and gamete cell mass in organisms that can reproduce both sexually and asexually through parthenogenesis.We identify a novel mechanism by which oogamy (motile microgametes and sessile macrogametes) can be selected for, arising from sexual conflict between microgametes (sperm) and macrogametes (eggs).We identify a novel mechanism by which a state of isogamy (equal sized gametes) can be stabilized under evolution, resulting from a bet-hedging strategy in switching environments.For all of the above insights, we develop a mathematical analysis with predictions that are supported by detailed computational simulations.