Deep sampling of Hawaiian Caenorhabditis elegans reveals high genetic diversity and admixture with global populations

Recent efforts to understand the natural niche of the keystone model organism Caenorhabditis elegans have suggested that this species is cosmopolitan and associated with rotting vegetation and fruits. However, most of the strains isolated from nature have low genetic diversity likely because recent chromosome-scale selective sweeps contain alleles that increase fitness in human-associated habitats. Strains from the Hawaii Islands are highly divergent from non-Hawaiian strains. This result suggests that Hawaiian strains might contain ancestral genetic diversity that was purged from most non-Hawaiian strains by the selective sweeps. To characterize the genetic diversity and niche of Hawaiian C. elegans, we sampled across the Hawaiian Islands and isolated 100 new C. elegans strains. We found that C. elegans strains are not associated with any one substrate but are found in cooler climates at high elevations. These Hawaiian strains are highly diverged compared to the rest of the global population. Admixture analysis identified 11 global populations, four of which are from Hawaii. Surprisingly, one of the Hawaiian populations shares recent ancestry with non-Hawaiian populations, including portions of globally swept haplotypes. This discovery provides the first evidence of gene flow between Hawaiian and non-Hawaiian populations. Most importantly, the high levels of diversity observed in Hawaiian strains might represent the complex patterns of ancestral genetic diversity in the C. elegans species before human influence.