Phylogeographic implications for the North American boreal-arctic Arabidopsis lyrata complex

Background: Phylogeography describes the distribution of genes in space and time. Here we analyse the Pleistocene evolutionary history of the Arabidopsis lyrata species complex in arctic-boreal northern America and eastern Russia. Aims: We seek evidence for glacial survival on both sides of the Atlantic, with special emphasis on northern America, and ask how genetic variation within North American populations relates to the putative centre of genetic variation in central Europe. Finally, we comment on how genetic variation observed in North America corresponds to taxonomic units described for the area. Methods: We sequenced DNA markers from the nuclear genome (ribosomal internal transcribed spacers 1 and 2) and chloroplast genome (trnL intron and intergenic spacer trnL-F) from 68 accessions outside Europe. Phylogenetic analysis (tree reconstruction and network building algorithms) were used to infer relationships among DNA haplotypes. Results: We detected reduced genetic variation in northern America compared to Europe, although new DNA types were found. Strong genetic differentiation was found among major regions in northern America. Correlation of genetic data with taxonomy was weak, but for some taxa geographic distribution correlates with distribution of its genetic variation. Conclusions: The A. lyrata complex in North America is separated into two allopatric groups: one is distributed in the Pacific areas from southern Alaska to British Colombia, and the other is found in northern Alaska, central and east Canada, and Greenland. The results suggest glacial survival in and subsequent migration from two different refugia with allele fixation as a result of genetic drift.