Sampling from natural populations with RNAI reveals high outcrossing and population structure in Caenorhabditis elegans

Despite a nearly worldwide distribution in nature, Caenorhabditis elegans exhibits low levels of genetic polymorphism, possibly as an indirect consequence of low levels of outcrossing. In the laboratory, Caenorhabditis elegans males are produced at low rates and are steadily eliminated from cultures, so that reproduction happens largely through self-fertilization in hermaphrodites. C. elegans is increasingly the focus of evolutionary research; however, natural outcrossing rates are difficult to measure because mating tests with laboratory strains are usually required to identify C. elegans. We sampled natural populations of C. elegans with an RNA interference (RNAi) assay. Heterozygosities and polymorphism patterns revealed surprisingly high levels of population structure and outcrossing (approximately 22% of individuals are estimated to be the result of outcrossing and not self-fertilization). The finding of strong local population structure, together with low levels of diversity on local and global scales, suggests a metapopulation model of frequent extinction and recolonization of local populations. The occurrence of substantial outcrossing suggests that the extinction of local populations is probably not driven by the accumulation of harmful mutations.