Wednesday, November 05, 2008

Post-admixture selection among Mexican Americans?

This paper examines the variation in admixture proportions across the genome at 284 microsatellite markers among 392 Mexican Americans. They find increased European ancestry on a region of chromosome 1, and decreased African ancestry on Chromosomes 2 and 9 (see figure on right: "Genome-wide distribution of African (bottom line), European (top line) and Native American (middle line) ancestry. The dashed vertical lines correspond to chromosome boundaries. Chromosomes are listed in numerical order from 1 to 22")
Since the markers are widely spaced out, they couldn't pinpoint any specific genes. Interestingly, these results differ pretty dramatically compared to a previous similar study among Puerto Ricans. Also, interestingly, since they had people who were hypertensive or had diabetes, and people who didn't, they were able to determine that the identified regions were not associated with these health outcomes, since the local ancestry at those areas was no different between cases and controls.
They do make some mention at the end that they might expect selection at infectious-disease related genes given that the ancestral parental populations that had adapted to specific environments were suddenly faced with different environments.

Genome-wide distribution of ancestry in Mexican Americans
Analabha Basu, Hua Tang, Xiaofeng Zhu, C. Charles Gu, Craig Hanis, Eric Boerwinkle and Neil Risch
Human Genetics Volume 124, Number 3 / October, 2008
Abstract Migrations to the new world brought together individuals from Europe, Africa and the Americans. Inter-mating between these migrant and indigenous populations led to the subsequent formation of new admixed populations, such as African and Latino Americans. These unprecedented events brought together genomes that had evolved independently on different continents for tens of thousands of years and presented new environmental challenges for the indigenous and migrant populations, as well as their offspring. These circumstances provided novel opportunities for natural selection to occur that could be reflected in deviations at specific locations from the genome-wide ancestry distribution. Here we present an analysis examining European, Native American and African ancestry based on 284 microsatellite markers in a study of Mexican Americans from the Family Blood Pressure Program. We identified two genomic regions where there was a significant decrement in African ancestry (at 2p25.1, p less than 10−8 and 9p24.1, p less than 2 × 10−5) and one region with a significant increase in European ancestry (at 1p33, p less than 2 × 10−5). These locations may harbor genes that have been subjected to natural selection after the ancestral mixing giving rise to Mexicans.

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