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The North Atlantic red alga Mastocarpus stellatus is characterized by two life histories (sexual-type and direct-type), which correspond to two geographically isolated breeding groups. These features enable M. stellatus to be an interesting model to investigate how environmental shift and apomictic propagation have influenced its population genetic structure, historical demography and distribution dynamic. To test these ideas, we obtained 456 specimens from 15 locations on both sides of the North Atlantic and sequenced portion of the nuclear internal transcribed spacer (ITS), mitochondrial cox 2–3 region (COX) and plastid RuBisCo spacer (RLS). Median-joining networks and ML trees inferred from COX and RLS consistently revealed two gene lineages (mtDNA: C N , C S ; cpDNA: R N , R S ). The concatenated COX and RLS markers yielded three cytotypes: a northern C N –R N , a southern C S –R S and a mixed cytotype C S –R N , which enabled us to roughly separate samples into D (direct-type life-cycle) and S (sexual-type life-cycle) groups (northern C N –R N and mixed cytotype C S –R N = D; southern C S –R S = S). Pairwise F ST analysis of the D group revealed a high level of genetic differentiation both along European coasts and across the Atlantic basin. Bayesian skyline plots (BSPs) and IMa analyses indicated that M. stellatus underwent slight demographic expansion at the late-Pleistocene, with the beginning of divergence between lineages dating to c . 0.189 Ma (95%HPD: 0.083–0.385 Ma). IMa analyses also revealed asymmetric genetic exchange among European populations and a predominant postglacial trans-Atlantic migration from Norway and Galway Bay to North America. Our study highlights the importance of phylogeographic approaches to discover the imprints of climate change, life histories and gene flow in driving population genetic connectivity and biogeographic distribution of intertidal seaweeds in the North Atlantic. [ABSTRACT FROM AUTHOR]

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° N. Methods We compiled mitochondrial (mt DNA) 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 ( FST) 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 IM a2 were used to estimate historical demography, genetic divergence and gene flow between regions. Results The mt DNA cox2-3 haplotype network and phylogenetic trees consistently showed that North Atlantic P. palmata divided into two genetic lineages in association with mid-Pleistocene (0.126-0.781 Ma) climate change. The demographic expansion time for European populations was c. 0.169-0.320 Ma. North American populations underwent demographic expansion c. 0.400 Ma but, for the populations in the Bay of Fundy, a sharp population-size contraction happened c. 0.013-0.160 Ma. IM a2 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. [ABSTRACT FROM AUTHOR]