Phylogeography of Y-chromosome haplogroup I reveals distinct domains of prehistoric gene flow in Europe

To investigate which aspects of contemporary human Y-chromosome variation in Europe are characteristic of
primary colonization, late-glacial expansions from refuge areas, Neolithic dispersals, or more recent events of gene
flow, we have analyzed, in detail, haplogroup I (Hg I), the only major clade of the Y phylogeny that is widespread
over Europe but virtually absent elsewhere. The analysis of 1,104 Hg I Y chromosomes, which were identified in
the survey of 7,574 males from 60 population samples, revealed several subclades with distinct geographic distributions. Subclade I1a accounts for most of Hg I in Scandinavia, with a rapidly decreasing frequency toward both
the East European Plain and the Atlantic fringe, but microsatellite diversity reveals that France could be the source
region of the early spread of both I1a and the less common I1c. Also, I1b*, which extends from the eastern Adriatic
to eastern Europe and declines noticeably toward the southern Balkans and abruptly toward the periphery of
northern Italy, probably diffused after the Last Glacial Maximum from a homeland in eastern Europe or the
Balkans. In contrast, I1b2 most likely arose in southern France/Iberia. Similarly to the other subclades, it underwent
a postglacial expansion and marked the human colonization of Sardinia ∼9,000 years ago.