The role of chromosomal inversions in local adaptation and ecotype formation across a variable landscape in an intertidal snail

Adaption to environmental variation can be discrete or continuous, responding to sharp or gradual habitat transitions. Chromosomal inversions accentuate these adaptive responses as multiple loci respond in tandem across a habitat transition. Earlier studies of Littorina saxatilis (Olivi, 1792) have shown that three ecotypes segregate in different habitats supported by polymorphisms in a few inversions. However, many more large inversions are polymorphic in this species and potentially have adaptive effects in response to uncharacterised environmental gradients. We sampled 200 random shores representing all kinds of shore habitats present in a 12x12 km area of a Swedish archipelago, with L. saxatilis found at 196 sites. Environment and shell morphology were summarised using dimension reduction analyses, and Bayesian regression models were used to analyse the relationships between phenotype, environment and inversions. Phenotypic variation was a continuum with previously described ecotypes positioned in different parts. 18 inversions responded to at least one dimension of environmental variation, and over (12) half influenced the measured phenotypes. Overall, inversion arrangements explained 15% of phenotypic variation, with limited direct contributions from the environment (<1%). Chromosomal inversions thus are a major factor for local adaptation in L. saxatilis, supporting adaptation across a landscape more complex than the previously described ecotypes.