부산대학교 도서관

Biparental sex is widespread in nature, yet costly relative to uniparental reproduction. It is generally unclear why self-fertilizing or asexual lineages do not readily invade outcrossing populations. The Red Queen hypothesis predicts that coevolving parasites can prevent self-fertilizing or asexual lineages from invading outcrossing host populations. However, only highly virulent parasites are predicted to maintain outcrossing, which may limit the general applicability of the Red Queen hypothesis. Here, we tested whether the ability of coevolving parasites to prevent invasion of self-fertilization within outcrossing host populations was dependent on parasite virulence. We introduced wild-type Caenorhabditis elegans hermaphrodites, capable of both self-fertilization and outcrossing, into C. elegans populations fixed for a mutant allele conferring obligate outcrossing. Replicate C. elegans populations were exposed for 24 host generations to one of four strains of Serratia marcescens parasites that varied in virulence, under three treatments: a heat-killed (control, noninfectious) parasite treatment, a fixed-genotype (nonevolving) parasite treatment, and a copassaged (potentially coevolving) parasite treatment. As predicted, self-fertilization invaded C. elegans host populations in the control and fixed-parasite treatments, regardless of parasite virulence. In the copassaged treatment, selfing invaded host populations coevolving with low- to mid-virulence strains, but remained rare in hosts coevolving with highly virulent bacterial strains. Therefore, we found that only highly virulent coevolving parasites can impede the invasion of selfing. Lay Summary: The dominance of biparental sex in plant and animal populations is difficult for evolutionary biologists to explain given the high costs associated with sex. The Red Queen hypothesis provides a possible explanation, predicting that interactions with coevolving parasites produce fluctuating environments that can select for biparental sex despite the costs. However, an important limitation of the Red Queen hypothesis is that coevolving parasites might only impose sufficiently strong selection to maintain biparental sex when parasite virulence is very high. We explored this potential limitation by testing whether uniparental sex (i.e., self-fertilization) could spread into outcrossing populations of the nematode host Caenorhabditis elegans that were exposed to strains of the bacterial parasite Serratia marcescens that varied in virulence. We found that self-fertilization spread in all conditions except when the pathogen was highly virulent and coevolving. Our results support the Red Queen hypothesis by demonstrating that virulent coevolving parasites can select for biparental sex in hosts. They also highlight the limitations of the Red Queen hypothesis, showing that the conditions under which parasites can maintain biparental sex may be limited to cases of high virulence. [ABSTRACT FROM AUTHOR]