Postglacial dispersal, glacial refugia, and clonal structure in Russian/Siberian populations of the arctic Daphnia pulex complex

Results from an extensive allozymic survey of the genetic (clonal) structure of Russian/Siberian populations in the arctic Daphnia pulex complex yielded high clonal diversity/richness estimates, at both the intrapopulational and intraregional levels. Highest levels of clonal diversity were detected in the heart of the Beringian glacial refuge, with lower levels detected in glaciated regions (western Russia). mtDNA haplotype diversity (based on RFLPs) showed similar spatial trends, with the highest levels of haplotype diversity again being found in Beringia. Several haplotypes were extremely widespread (in the order of thousands of kilometres across the breadth of Eurasia), and evidence for presumed mutationally derived haplotypes was plentiful. Spatial autocorrelation analysis revealed a significant clinal pattern in the degree of genetic similarity among mtDNA haplotypes. In addition, similarity of clonal arrays (based on allozymes) showed a significant inverse association with geographical distance; i.e. similarity of clonal arrays between populations increased with decreasing geographical distance between populations. Sporadic sexual reproduction (based on Hardy-Weinberg expectations) was detected in Beringia, and has undoubtedly played a role in fuelling the production of novel genotypes. Our results suggest that vast expanses of northern Eurasia have been colonized by a small number of geographically widespread haplotypes, with more regionally restricted haplotypes constituting the remaining lineages. We discuss our results within the general framework of biogeographical colonization events of the Arctic by members of the D. pulex complex, and place these data into the broader picture of the importance of Beringia as a glacial refuge for many taxa during the Pleistocen