American Journal of Human Genetics (online early)
The Himalayas as a Directional Barrier to Gene Flow
Tenzin Gayden et al.
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 8.1 ± 2.9 thousand years ago (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 (5.1 ± 1.8 and 11.3 ± 3.7 KYA, respectively) that are more recent than those 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.