Cyclic parthenogenesis helps populations cross fitness valleys

Cyclic parthenogenesis is a form of occasional sexual reproduction. Here, we compare the evolvability of cyclic parthenogens and obligate sexuals in adaptive scenarios that entail the crossing of a fitness valley. We divide the valley crossing process into two phases. In the escape phase, a population produces genotypes that lie on the other side of the fitness valley. In the establishment phase, such genotypes are refined and promoted to high frequency. With individual-based models, we find that although cyclic parthenogens are slower to produce escape genotypes, they more efficiently establish such genotypes, and therefore more rapidly complete fitness-valley crossings. In obligate sexuals, genetic and mutational variance and covariances, G and M, are readily shaped by selection. In cyclic parthenogens, the shapes of G and M are less responsive to selection but have relatively little effect on the rate of fitness-valley crossing. In our model, the more adaptive evolution of G and M in obligate sexuals is not enough to overcome the establishment advantage of cyclic parthenogens. Nevertheless, the relative recalcitrance against selection on G and M in cyclic parthenogens could be an evolutionary cost that helps explain its paradoxical rarity.