The genetic basis and experimental evolution of inbreeding depression in Caenorhabditis elegans

Determining the genetic basis of inbreeding depression is important for understanding therole of selection in the evolution of mixed breeding systems. Here, we investigate howandrodioecy (a breeding system characterized by partial selfing and outcrossing) anddioecy (characterized by obligatory outcrossing) influence the experimental evolution ofinbreeding depression in Caenorhabditis elegans. We derived inbred lines fromancestral and evolved populations and found that the dioecious lineages underwent moreextinction than androdioecious lineages. For both breeding systems, however, there wasselection during inbreeding because the diversity patterns of 337 single-nucleotidepolymorphisms (SNPs) among surviving inbred lines deviated from neutral expectations. Inparallel, we also followed the evolution of embryo to adult viability, which revealedsimilar starting levels of inbreeding depression in both breeding systems, but alsooutbreeding depression. Under androdioecy, diversity at a neutral subset of 134 SNPscorrelated well with the viability trajectories, showing that the population geneticstructure imposed by partial selfing affected the opportunity for different forms ofselection. Our findings suggest that the interplay between the disruptions of coevolvedsets of loci by outcrossing, the efficient purging of deleterious recessive alleles withselfing and overdominant selection with outcrossing can help explain mixed breedingsystems.