Background: The Austro-Asiatic linguistic family, which is considered to be the oldest of all the families in India, has a substantial presence in Southeast Asia. However, the possibility of any genetic link among the linguistic sub-families of the Indian Austro-Asiatics on the one hand and between the Indian and the Southeast Asian Austro-Asiatics on the other has not been explored till now. Therefore, to trace the origin and historic expansion of Austro-Asiatic groups of India, we analysed Y-chromosome SNP and STR data of the 1222 individuals from 25 Indian populations, covering all the three branches of Austro-Asiatic tribes, viz. Mundari, Khasi-Khmuic and Mon-Khmer, along with the previously published data on 214 relevant populations from Asia and Oceania.

Northeast India, the only region which currently forms a land bridge between the Indian subcontinent and Southeast Asia, has been proposed as an important corridor for the initial peopling of East Asia. Given that the Austro-Asiatic linguistic family is considered to be the oldest and spoken by certain tribes in India, Northeast India and entire Southeast Asia, we expect that populations of this family from Northeast India should provide the signatures of genetic link between Indian and Southeast Asian populations. In order to test this hypothesis, we analyzed mtDNA and Y-Chromosome SNP and STR data of the eight groups of the Austro-Asiatic Khasi from Northeast India and the neighboring Garo and compared with that of other relevant Asian populations. The results suggest that the Austro-Asiatic Khasi tribes of Northeast India represent a genetic continuity between the populations of South and Southeast Asia, thereby advocating that northeast India could have been a major corridor for the movement of populations from India to East/Southeast Asia.

Because of the widespread phenomenon of patrilocality, it is hypothesized that Y-chromosome variants tend to be more localized geographically than those of mitochondrial DNA (mtDNA). Empirical evidence confirmatory to this hypothesis was subsequently provided among certain patrilocal and matrilocal groups of Thailand, which conforms to the isolation by distance mode of gene diffusion. However, we expect intuitively that the patterns of genetic variability may not be consistent with the above hypothesis among populations with different social norms governing the institution of marriage, particularly among those that adhere to strict endogamy rules. We test the universality of this hypothesis by analyzing Y-chromosome and mtDNA data in three different sets of Indian populations that follow endogamy rules to varying degrees. Our analysis of the Indian patrilocal and the matrilocal groups is not confirmatory to the sex-specific variation observed among the tribes of Thailand. Our results indicate spatial instability of the impact of different cultural processes on the genetic variability, resulting in the lack of universality of the hypothesized pattern of greater Y-chromosome variation when compared to that of mtDNA among the patrilocal populations.