Understanding the adaptive role of chromosomal inversions across large geographical scales: The potential of pool-seq data

A growing body of research shows that chromosomal inversions, where each arrangement is associated with a certain environment and maintains a set of adaptive alleles, make an important contribution to local adaptation. However, inversions often remain unexplored across large geographical scales. It is unclear whether inversions contribute to adaptation across species ranges, which environmental factors affect arrangement frequencies, and whether the adaptive content of the same arrangement varies between locations. Here, we discuss ideas of how allele frequency data (e.g. pool-sequencing data) can be used to learn about inversions in a simple and cost-effective way. If populations connected by migration differ in arrangement frequency, plotting their SNP allele frequencies against each other will reveal a parallelogram whose corners reflect arrangement frequencies. We demonstrate the usefulness of this approach in locally-adapted populations of the intertidal snail Littorina saxatilis (Olivi). For twelve inversions, we estimate arrangement frequencies in 20 populations across the European species range. While roughly half of the inversions contribute to adaptation to the well-studied contrast between wave-exposed and crab-infested habitats, the other half likely contribute to adaptation to shore height. We also find evidence for geographical variation in arrangement content, suggesting variation in adaptive role.