The Himalayas as a directional barrier to gene flow

High-resolution Y-chromosome haplogroup analyses coupled with Y - short tandem repeat (STR) haplotypes were used to ( 1) investigate the genetic affinities of three populations from Nepal - including Newar, Tamang, and people from cosmopolitan Kathmandu ( referred to as "Kathmandu" subsequently) - as well as a collection from Tibet and ( 2) evaluate whether the Himalayan mountain range represents a geographic barrier for gene flow between the Tibetan plateau and the South Asian subcontinent. The results suggest that the Tibetans and Nepalese are in part descendants of Tibeto-Burman - speaking groups originating from Northeast Asia. All four populations are represented predominantly by haplogroup O3a5-M134-derived chromosomes, whose Y-STR-based age ( +/- SE) was estimated at thousand years ago 8.1 +/- 2.9 (KYA), more recent than its Southeast Asian counterpart. The most pronounced difference between the two regions is reflected in the opposing high-frequency distributions of haplogroups D in Tibet and R in Nepal. With the exception of Tamang, both Newar and Kathmandu exhibit considerable similarities to the Indian Y-haplogroup distribution, particularly in their haplogroup R and H composition. These results indicate gene flow from the Indian subcontinent and, in the case of haplogroup R, from Eurasia as well, a conclusion that is also supported by the admixture analysis. In contrast, whereas haplogroup D is completely absent in Nepal, it accounts for 50.6% of the Tibetan Y- chromosome gene pool. Coalescent analyses suggest that the expansion of haplogroup D derivatives - namely, D1-M15 and D3-P47 in Tibet involved two different demographic events ( and KYA, respectively) that are more recent than those 5.1 +/- 1.8 11.3 +/- 3.7 of D2-M55 representatives common in Japan. Low frequencies, relative to Nepal, of haplogroup J and R lineages in Tibet are also consistent with restricted gene flow from the subcontinent. Yet the presence of haplogroup O3a5-M134 representatives in Nepal indicates that the Himalayas have been permeable to dispersals from the east. These genetic patterns suggest that this cordillera has been a biased bidirectional barrier.