Prevalent sex ratio bias in Caenorhabditis nematodes

Abstract

Although equal sex ratio represents an equilibrium in evolutionary theory, cases of sex ratio bias are accumulating, mostly reported in single species. Here, we surveyed progeny sex ratios in 23 species of the nematode genus Caenorhabditis. In experiments where males and females had unlimited access to each other (unlimited mating), we found 15 species out of the 23 species had female(hermaphrodite)-biased sex ratios. Phylogenetic mapping indicates female-bias to represent the ancestral state, with the occurrence of seven independent transitions from female-bias to non-bias along the phylogeny. As sperm competition could underlie the female bias, we also assayed progeny sex ratios in an experiment where mating was limited to a few hours. Of the 15 species that showed female-biased ratios under unlimited mating, six species showed no sex ratio bias when mating was limited. This result is consistent with sperm competition whereby X-bearing sperm take precedence over nullo-X during fertilization, yielding more female progeny when sperm are unlimited but equal sex ratio when sperm are limited. The other nine species showed sex ratio bias in both experiments, but the day-by-day profiles suggest sperm competition may also play a role. Our study shows that sex ratio evolution within Caenorhabditis nematodes is dynamic and that sex ratio bias is common not only in parasites as previously found but also in free-living nematodes. Our results also suggest that sperm competition could be a mechanism underlying sex ratio bias.