Competitive gametic traits and their coevolution with strong and slight anisogamy

Gamete dynamics (GD) theory for the evolutionary transition from isogamy to anisogamy relies on the biophysics of fertilisation, combining the dynamics of gamete limitation and gamete competition with the provisioning requirements of gametes and zygotes. A recent development by Siljestam & Martinossi-Allibert which incorporates competitive gamete traits (motility and target size) into anisogamy evolution reaches very different conclusions from previous GD predictions, and challenges current views on sexual selection. We develop models of these traits under conventional GD theory assumptions showing that (i) unless gamete limitation is strong and the trait is more efficient in the larger gamete, such traits tend to arise in the male gamete, complying with previous analyses predicting that sexually selected expenditures are most likely to arise in males, (ii) gamete trait evolution does not alter the conditions under which anisogamy evolves from isogamy, (iii) the differences between our results and those of Siljestam & Martinossi-Allibert arise from their specific function for zygote survival, which is arguably less plausible than those used previous GD theory, and (iv) as a novel finding, we show that the coevolution of gamete size with other gamete-level traits (motility, jelly coats, chemoattractants) can result in the evolution of slight anisogamy.