Selfing is the safest sex for <i>Caenorhabditis tropicalis</i>.

Abstract

Mating systems have profound effects on genetic diversity and compatibility. The convergent evolution of self-fertilization in three <i>Caenorhabditis</i> species provides a powerful lens to examine causes and consequences of mating system transitions. Among the selfers, <i>Caenorhabditis tropicalis</i> is the least genetically diverse and most afflicted by outbreeding depression. We generated a chromosomal-scale genome for <i>C. tropicalis</i> and surveyed global diversity. Population structure is very strong, and islands of extreme divergence punctuate a genomic background that is highly homogeneous around the globe. Outbreeding depression in the laboratory is caused largely by multiple Medea-like elements, genetically consistent with maternal toxin/zygotic antidote systems. Loci with Medea activity harbor novel and duplicated genes, and their activity is modified by mito-nuclear background. Segregating Medea elements dramatically reduce fitness, and simulations show that selfing limits their spread. Frequent selfing in <i>C. tropicalis</i> may therefore be a strategy to avoid Medea-mediated outbreeding depression.