Data from: Coevolutionary interactions with parasites constrain the spread of self-fertilization into outcrossing host populations

Given the cost of sex, outcrossing populations should be susceptible to
invasion and replacement by self-fertilization or parthenogenesis.
However, biparental sex is common in nature, suggesting that
cross-fertilization has substantial short-term benefits. The Red Queen
hypothesis (RQH) suggests that coevolution with parasites can generate
persistent selection favoring both recombination and outcrossing in host
populations. We tested the prediction that coevolving parasites can
constrain the spread of self-fertilization relative to outcrossing. We
introduced wild-type Caenorhabditis elegans hermaphrodites, capable of
both self-fertilization, and outcrossing, into C. elegans populations that
were fixed for a mutant allele conferring obligate outcrossing. Replicate
C. elegans populations were exposed to the parasite Serratia marcescens
for 33 generations under three treatments: a control (avirulent) parasite
treatment, a fixed (nonevolving) parasite treatment, and a copassaged
(potentially coevolving) parasite treatment. Self-fertilization rapidly
invaded C. elegans host populations in the control and the fixed-parasite
treatments, but remained rare throughout the entire experiment in the
copassaged treatment. Further, the frequency of the wild-type allele
(which permits selfing) was strongly positively correlated with the
frequency of self-fertilization across host populations at the end of the
experiment. Hence, consistent with the RQH, coevolving parasites can limit
the spread of self-fertilization in outcrossing populations.