Impact of glaciations on genetic diversity of pelagic mollusks: Antarctic Limacina antarctica and Arctic Limacina helicina

Contemporary genetic constitution of marine species carries signatures of Pliocene-Pleistocene glacial cycles. Molecular studies of polar organisms show that isolation in refugia during glaciation often results in loss of genetic diversity. However, recent studies of marine organisms from the Southern Ocean have highlighted their remarkably high level of infraspecific genetic differentiation and the presence of cryptic species. Thus, demographic responses to climate change vary substantially with geography and life history. To elucidate the relative role of glacial period in driving the evolution of Antarctic and Arctic fauna we examined the genetic diversity and historical demography of the pelagic marine gastropods Limacina antarctica from Drake Passage in the Antarctic and Limacina helicina from Spitsbergen fjords in the Arctic. Diversity was assessed by comparing nucleotide sequences from part of the mitochondrial gene encoding the cytochrome c oxidase subunit I (COI). Sequences from 60 individuals of L. antarctica collected at 7 stations along Drake Passage were compared with those of 67 individuals of L. helicina from the fjords Hornsund and Isfjorden. We identified 47 different haplotypes for L. antarctica and 25 for L. helicina. No spatial genetic structure was found in either species, indicating that studied populations in each species belong to a single evolutionary unit. Demographic analyses of haplotype networks and significant negative Tajima's D and Fu's FS indices suggest recent rapid population expansion in both species. However, L. antarctica populations displayed a higher level of haplotype and nucleotide diversity than L. helicina populations, which suggests that the impact of glaciations was less prominent in L. antarctica.