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Genetic Analysis Uncovers Indian Ancestry Among Many Present-Day Thai Populations

NEW YORK — Researchers have uncovered traces of Indian genetic ancestry among some populations living in present-day Thailand.

During the first millennium of the Common Era, Indian culture influenced the establishment of early states in mainland Southeast Asia and evidence of that influence can still be observed today. To tease out genetic influence on the region, an international team of researchers generated genome-wide SNP data on 10 ethnic groups from Thailand that they analyzed in combination with other published data.

As they reported in PLOS Genetics, the researchers examined the genetic makeup of those populations, finding that many groups, including ones known to have historical ties with South Asia, showed evidence of admixture with South Asian populations, possibly Bengali-related populations. Some isolated Southeast Asian groups, meanwhile, did not have any signs of South Asian genetic heritage.

"Indian genetic heritage in Southeast Asian populations suggests multiple waves of migrations from India to Southeast Asia in the past, which may have been responsible for the spread of Indian culture in the region," first author Piya Changmai from the University of Ostrava in the Czech Republic, said in a statement.

Changmai and colleagues generated genome-wide data for 119 individuals representing 10 ethnic groups from different regions of Thailand. Six of the populations spoke an Austroasiatic language, one population spoke a Hmong-Mien language, one a Kra-Dai language, and two Sino-Tibetan languages. They further merged their data with published data on both ancient and present-day populations.

Following principal components and admixture analyses, the researchers pieced together the genetic ancestry of the 10 different Thai populations.

They found South Asian admixture, ranging between 2 percent and 16 percent, within four of the Thai ethnic groups they sampled and within four groups from the published literature. For instance, the Khmer from Thailand group as well as the Khmer from Cambodia group shared ancestry from a Mlabri-related source — a hunter-gatherer population in Thailand — and a South Asian source, while the Mon and Nyahkur populations of Thailand also have genetic ancestry stemming from a Mlabri-related source and a South Asian source, though the Mon also have genetic ancestry from a Tibetan-related source.

Some isolated populations like the Karen and Hmong, who live mostly in the northern and western highland regions of Thailand, meanwhile, did not harbor signs of genetic ancestry from South Asia.

The researchers uncovered eight different South Asian populations that contributed more than 1 percent of ancestry to the Southeast Asian populations. A Bengali-related population appeared to be the more prominent South Asian source population, though the researchers cautioned the Bengali population was the only South Asian population surrogate with detectable East Asian ancestry, which could lead to excessive haplotype sharing.

At the same time, the researchers uncovered genetic ties between Kra-Dai language family speakers and Austronesian language family speakers. Previous linguistic analyses had suggested that the two language families might be cladal.

The researchers also noted that many South Asian populations, especially in Myanmar, Laos, and Cambodia, are understudied and that there are few genome-wide analyses of ancient people from the region. "Further data from present-day and ancient groups are expected to provide further insights into the genetic population history of Mainland Southeast Asia," the researchers wrote in their paper.