Putative chromosomal rearrangements are associated primarily with ecotype divergence rather than geographic separation in an intertidal, poorly-dispersing snail

Littorina saxatilis is becoming a model system for understanding the genomic basis of ecological speciation. The parallel formation of crab-adapted ecotypes that exhibit partial reproductive isolation from wave-adapted ecotypes has enabled genomic investigation of conspicuous shell traits. Recent genomic studies suggest that chromosomal rearrangements may enable ecotype divergence by reducing gene flow. However, the genomic architecture of traits that are divergent between ecotypes remains poorly understood. Here, we use 11,504 single nucleotide polymorphism (SNP) markers called using the recently-released L. saxatilis genome to genotype 462 crab ecotype, wave ecotype, and phenotypically-intermediate L. saxatilis individuals with scored phenotypes. We used redundancy analysis to study the genetic architecture of loci associated with shell shape, shape corrected for size, shell size, and shell ornamentation, and to compare levels of co-association among different traits. We discovered 34...