Patterns of genetic variation across the distribution range of the cockle Cerastoderma edule inferred from microsatellites and mitochondrial DNA

The edible cockle Cerastoderma edule is an ecologically and economically important species found along the northeastern Atlantic coast. In this study, microsatellite and mitochondrial cytochrome b (cyt b) gene variation was examined in 19 localities across the species' distribution range to test whether the patterns of genetic diversity, population structure and demographic inferences deduced from the cytochrome oxidase I gene in a previous study were supported by these molecular makers. High levels of genetic diversity were detected, with allelic richness, heterozygosity and haplotype diversity, showing different geographic patterns. Through Bayesian clustering analysis, Fst estimates and AMOVA, microsatellites indicated the existence of a southern group of populations that expands up to France, a northern one composed of localities from Denmark to Russia plus one from Scotland, and a third central group comprising localities from Ireland, Great Britain and southern North Sea. Regarding the cyt b gene, Fst data supported the pattern inferred from microsatellites, but haplotype distribution and AMOVA revealed a different three-group scenario where the central group includes only the southern North Sea localities. Both marine currents and historical processes, such as a recent population expansion of the southern group, the existence of northern cryptic refugia during Pleistocene glaciations and secondary contact between the southern and northern groups of populations, may explain the genetic composition of C. edule populations. Overall, the available data suggest that in C. edule nuclear markers and even different mitochondrial genes reacted differently to the same population history.