Genetic structure and shell shape variation within a rocky shore whelk suggest both diverging and constraining selection with gene flow

摘要

Variation in snail shell shape has provided evolutionary biologists with excellent material for the study of local adaptation to local environments. However, the assumption that shell shape variation is evidence of distinct lineages (species) might have led to taxonomic inflation within some gastropod lineages. Here, we compare shell shape variation and genetic structure of two independent lineages of New Zealand rocky shore whelks in order to understand the process that leads to an unusual disjunct distribution. We examined the Buccinulum vittatum complex (three subspecies plus Buccinulum colensoi) using mitochondrial DNA sequences, 849 single nucleotide polymorphisms and geometric morphometric data on shell shape. Specimens within the range of B. colensoi shared nuclear markers and mitochondrial DNA haplotypes with the southern subspecies Buccinulum vittatum littorinoides, whereas the northern samples (Buccinulum vittatum vittatum) had distinct genotypes. We infer that gene flow between B. colensoi and B. v. littorinoides is greater than between either of these taxa and B. v. vittatum. The distinctive shell sculpturing of B. colensoi is maintained despite gene flow, suggesting a role for selection. Although the shell form of B. colensoi appears to be an adaptation to local conditions, we did not observe convergent evolution in the sympatric whelk species Cominella maculosa.