Genetic data from the red alga Palmaria palmata reveal a mid-Pleistocene deep genetic split in the North Atlantic

Aim We examined the biogeographical divergence, population-level genetic differentiation and historical genetic exchange of the red macroalga Palmaria palmata in order to test whether populations on opposite sides of the North Atlantic underwent different demographic histories. Location North Atlantic rocky intertidal zones between c. 42 and 65 degrees N. Methods We compiled mitochondrial (mtDNA) cox2-3 and plastid rpl12-rps31-rpl9 sequences from 333 and 310 individuals of P.palmata, respectively. Median-joining networks were constructed to evaluate the genetic relationships among haplotypes. Pairwise genetic differentiation (F-ST) was calculated to elucidate population genetic structure. Spatial analysis of molecular variance was used to define genetic differentiation between groups. Extended Bayesian skyline plots and IMa2 were used to estimate historical demography, genetic divergence and gene flow between regions. Results The mtDNA cox2-3 haplotype network and phylogenetic trees consistently showed that North Atlantic P.palmata divided into two genetic lineages in association with mid-Pleistocene (0.126-0.781Ma) climate change. The demographic expansion time for European populations was c. 0.169-0.320Ma. North American populations underwent demographic expansion c. 0.400Ma but, for the populations in the Bay of Fundy, a sharp population-size contraction happened c. 0.013-0.160Ma. IMa2 analysis revealed no gene flow between the European and North American coasts. Main conclusions Our results suggest significant genetic differentiation in P.palmata both at a trans-Atlantic continental scale and at a European basin scale, and that multiple glacial refugia may have existed for this species. Long-term isolation within the Atlantic may have contributed greatly to structuring the population-genetic differentiation of P.palmata. Our results also stress the importance of combining different gene markers to understand the phylogeographical history and genetic structure of marine biota.