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Abstract

Why do so many organisms bother with sex? Sexual outcrossing, or mating between two individuals, is far less efficient than simply cloning or mating with oneself. Yet it’s ubiquitous amongst eukaryotes. The Red Queen Hypothesis argues that this penchant for sex all comes down to parasitism. It proposes that antagonism between hosts and parasites selects for: 1. Rare host genotype that can escape their rapidly adapting parasites, and 2. Rare parasite genotypes that can keep up with these escaping hosts. Outcrossing can generate a diversity of these rare genotypes and should thus be favored, while cloning or selfing tends to propagate static, common lineages that fail to rapidly adapt. Here, I’ll specifically test the oft-neglected prediction that parasitic taxa are more likely to be outcrossing than their free-living relatives. I focus on the Nematode phylum, where I use phylogenetic comparative methods to determine if sexual outcrossing is favored in lineages of parasitic nematodes. This approach broadly supports parasitism as an important force shaping the macroevolutionary distribution of outcrossing. Interestingly, there are striking differences between nematode parasites that infect animals vs. plants, the latter having a high frequency of selfing/cloning that is inconsistent with theory.