North-east India has always been a hotspot for population geneticists due to its unique, strategic geographic location and the presence of linguistically, culturally and demographically diverse populations practicing varied occupations (from hunter-gathering to settled agriculture).

There are an estimated 532 tribal communities who inhabit the different geographical regions, who vary in their morphological features and ethnic origins (Australian, African, East Asian) of varied cultural features and who belong to three linguistic families (Austro-Asiatic, Dravidian and Tibeto-Burman).

These are important in understanding the genetic history and peopling of the Indian subcontinent, and to derive further insights into the antiquity and past human migrations to other parts of Asia and their genetic relatedness.

In a new study published in PLoS ONE on July 2, Dr T.S. Vasulu and colleagues at the Indian Statistical Institute examine, for the first time, the genetic status of sub-tribes of one such remotely located tribal cluster - the Adi, a Tibeto-Burman-speaking tribe of Arunachal Pradesh in the north-east of India.

Based on 15 autosomal microsatellite (STR) markers, the authors studied the genetic affinity, differentiation and sub-structuring among six Adi subgroups, as well as their genetic affinity with other, neighbouring, Tibeto-Burman-speaking tribes of India and with the linguistically divergent east and south-east Asian populations, with whom they share common ethno-historical and cultural attributes.

The researchers investigated to what extent the six Adi subgroups are genetically divergent or affiliated. The results of the pair wise and locus wise comparison indicate that of the six groups, the Adi Pasi Upper sows significantly differ from others in case of two loci: D7S820 and D13S317, whereas locus D8S1179 shows no significant differences between the six groups compared.

However, the overall results of the AMOVA and STRUCTURE analyses based on the 15 autosomal STR loci indicates a low degree of genetic differentiation and the least sub-structuring among the sub-tribes. This indicates that the recognized subgroups are more geographical and cultural constructs and show the least genetic differentiation, although the clustering tree shows some tendency for the Adi Pasi Upper to deviate from the rest.

A comparison with the 16 Tibeto-Burman-speaking tribes of the neighbouring region in northern and north-eastern parts of the country as revealed by the cluster analyses indicates geographically proximate populations forming a close cluster. This is to be expected if these populations have indeed diverged from a common source after their settlement in different regions of the country in the recent past.

In a comparison of the 50 populations (including populations from east and south-east Asia) for genetic diversity based on the autosomal loci, the resultant clustering tree showed some of the Tibeto-Burman tribes clustering with the populations from Tibet and China and whereas other Tibeto-Burman tribes of India cluster with linguistically different Southeast Asian populations.

These results support the possibility that Tibeto-Burman populations have derived from more than one common source. Overall, the Adi and other Tibeto-Burman speaking populations of India are regionally well differentiated and exhibit genetic affinity with the neighboring populations of East/Southeast Asia, based on their shared ethno-history.

However, a clearer picture may well emerge from the analysis of increased number of informative genetic markers and from the uniparental markers like mitochondrial DNA and Y chromosome.