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Multiple origins of circumboreal taxa in Pyrola (Ericaceae), a group with a Tertiary relict distribution.

BACKGROUND AND AIMS
In the Northern Hemisphere, Tertiary relict disjunctions involve older groups of warm affinity and wide disjunctions, whereas circumboreal distributions in Arctic-Alpine taxa tend to be younger. Arctic-Alpine species are occasionally derived from Tertiary relict groups, but Pyrola species, in particular, are exceptional and they might have occurred multiple times. The aim of this study was to reconstruct the biogeographic history of Pyrola based on a clear phylogenetic analysis and to explore how the genus attained its circumboreal distribution.


METHODS
Estimates of divergence times and ancestral geographical distributions based on neutrally evolving DNA sequence variation were used to develop a spatio-temporal model of colonization patterns for Pyrola.


KEY RESULTS
Pyrola originated and most diversification occurred in Asia; North America was reached first by series Scotophyllae in the late Miocene, then by sub-clades of series Pyrola and Ellipticae around the Pliocene. The three circumboreal taxa, P. minor, P. chlorantha and the P. rotundifolia complex, originated independently of one another, with the last two originating in Asia.


CONCLUSIONS
Three circumboreal Pyrola lineages have arisen independently and at least two of these appear to have originated in Asia. The cool, high-altitude habitats of many Pyrola species and the fact that diversification in the genus coincided with global cooling from the late Miocene onwards fits a hypothesis of pre-adaptation to become circumboreal within this group.