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Population Study Expands View of Ancestral Groups in India

NEW YORK (GenomeWeb) – A study appearing online this week in the Proceedings of the National Academy of Sciences suggests present-day populations in India may carry twice as many distinct ancestral components as previously estimated.

Based on array-based genotyping data for hundreds of individuals from 20 populations on mainland India and the Andaman and Nicobar Islands — including representatives from diverse tribal populations — researchers from India they concluded that populations on their mainland are descended from at least four main ancestral groups.

"Our study … has provided a more robust explanation of the genomic diversities and affinities among extant populations on the Indian subcontinent, elucidating in finer detail the peopling of the region," senior author Partha Majumder, a researcher affiliated with the National Institute of BioMedical Genetics and the Indian Statistical Institute's Human Genetics Unit, and co-authors wrote.

Based on these findings, the study's authors argued that prior genetic research — in particular, a 2009 study by researchers in the US and India describing Ancestral North Indian and Ancestral South Indian groups — underestimated the number of ancestral components comprising current populations in India.

The current analyses spanned a wide range of geographical, sociocultural, and linguistic groups, including representatives from populations with traditionally hunter-gather, agricultural, priest, or warrior lifestyles, Majumder and co-authors noted.

The team also made an effort to analyze samples from Austro-Asiatic and Tibeto-Burman-speaking groups that had clustered outside of the Ancestral North Indian and Ancestral South Indian groups in the past.

As such, the investigators contended that their analyses contained "more representative population samples" than prior studies of Indian populations.

The researchers began by using Illumina Omni 1-Quad arrays to profile more than 800,000 SNPs across the genomes of 331 unrelated individuals from 18 sites in mainland India. They also generated similar genome-wide SNP data for 36 individuals from two populations from the Andaman and Nicobar Islands.

Amongst the individuals from mainland India, the team saw four genetic clusters consistent with Ancestral North Indian, Ancestral South Indian, and two other ancestral groups.

The new clusters were composed of individuals from the Austro-Asiatic or Tibeto-Burman language groups, the researchers explained, which were dubbed the Ancestral Austro-Asiatic and Ancestral Tibeto-Burman ancestry groups, respectively.

By looking at the ancestral component combinations and proportions, the team identified 69 sub-populations, most of which contained individuals from just one of the 18 original populations sampled.

Finally, the team used haplotype block patterns in the populations to retrace historical admixture between groups in different parts of India — an analysis that saw early between-population admixture that diminished with the introduction of a caste system starting some 70 generations ago.

For populations on the Andaman and Nicobar Island, meanwhile, the investigators saw ancestry patterns that overlapped to some extent with those on the mainland, but showed signs of long separation and differentiation. In particular, populations on the Andaman archipelago seemed to carry sequences that shared ancestry with populations in present-day Papua New Guinea and other parts of Oceania.