Impact of ice ages on circumpolar molecular diversity: insights from an ecological key species.

We address the impact of the ice age cycles on intraspecific cpDNA diversity, for the first time on the full circumboreal-circumarctic scale. The bird-dispersed bog bilberry (or arctic blueberry, Vaccinium uliginosum) is a key component of northern ecosystems and is here used to assess diversity in previously glaciated vs. unglaciated areas and the importance of Beringia as a refugium and source for interglacial expansion. Eighteen chloroplast DNA haplotypes were observed in and among 122 populations, grouping into three main lineages which probably diverged before, and thus were affected more or less independently by, all major glaciations. The boreal 'Amphi-Atlantic lineage' included one haplotype occurring throughout northern Europe and one occurring in eastern North America, suggesting expansion from at least two bottlenecked, glacial refugium populations. The boreal 'Beringian lineage' included seven haplotypes restricted to Beringia and the Pacific coast of USA. The 'Arctic-Alpine lineage' included nine haplotypes, one of them fully circumpolar. This lineage was unexpectedly diverse, also in previously glaciated areas, suggesting that it thrived on the vast tundras during the ice ages and recolonized deglaciated terrain over long distances. Its largest area of persistence during glaciations was probably situated in the north, stretching from Beringia and far into Eurasia, and it probably also survived the last glaciation in southern mountain ranges. Although Beringia apparently was important for the initial divergence and expansion of V. uliginosum as well as for continuous survival of both the Beringian and Arctic-Alpine lineages during all ice ages, this region played a minor role as a source for later interglacial expansions.