Signals of Natural Selection Across Regions of Low Recombination in Wild Populations of the Purple Sea Urchin, Strongylocentrotus purpuratus.

Structural variants (SVs) are increasingly recognized as important components of genetic architecture. Yet our understanding of the evolutionary forces maintaining SVs in natural populations is limited. Chromosomal inversions in particular can facilitate local adaptation in populations with high gene flow, including many marine species. The purple sea urchin (Strongylocentrotus purpuratus) is a powerful system to study these dynamics due to its high gene flow, lack of population structure, and broad latitudinal range. We analyzed whole genome sequence data from 137 individuals sampled across seven populations to identify regions of low recombination using scans for elevated linkage disequilibrium and genetic differentiation. Such regions may arise from structural variants, including chromosomal inversions. We identified nine regions showing signatures of reduced recombination, including three way genotype clustering, long range linkage, and hanging bridge patterns frequently associated with inversion polymorphisms. The regions were polymorphic within locations and along the species range with three loci showing concordant signatures of balancing and spatially heterogeneous selection based on enrichment of outliers and distinct patterns of allelic age. Additionally, these loci showed enrichment for genes associated with biomineralization and development. Our results provide the first evidence for regions of low recombination in the purple sea urchin genome, several of which display genomic signatures consistent with structural variants such as chromosomal inversions. These findings add to growing evidence that regions of reduced recombination constitute an important component of standing genetic variation in natural populations and may play a key role in adaptation to heterogeneous environments.