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River Valleys Influenced Mitochondrial Landscape Among Han Chinese Populations

NEW YORK (GenomeWeb) – Three river valleys have influenced the mitochondrial genomic makeup of Han Chinese populations, a new study has found.

Han Chinese comprise about 92 percent of the Chinese population, and previous studies have suggested that there is a north-south genetic divide within Han Chinese populations. To explore this variation, researchers from the Kunming Institute of Zoology examined mitochondrial DNA from more than 21,000 Han Chinese individuals living across China.

As they reported yesterday in Molecular Biology and Evolution, Kunming's Qing-Peng Kong and his colleagues found three rivers in China — the Yellow, Yangtze, and Zhujiang (Pearl) Rivers — influenced the distribution of mitochondrial haplotypes there. These river valleys served as centers of agriculture, and the researchers traced the appearance of these genetic differences to when farming emerged there.

"Our study implies that the maternal gene pools of contemporary Han Chinese populations have retained the genetic imprint of early Neolithic farmers from different regions, thus highlighting the important roles of the three ancient agricultural traditions in shaping the genetic landscape of Han Chinese," Kong said in a statement.

The researchers collected saliva samples from 21,668 unrelated individuals of Han ethnicity living in nearly every Chinese province. They genotyped these mitochondrial genomes at about 4,000 diagnostic variants and, using the MitoTool scoring system, assigned each sample to a haplogroup.

The most common haplogroup in this cohort was haplogroup D4, to which 16.5 percent of samples belonged, followed by haplogroups B4, F1, and M7.

However, the prevalence of these haplogroups varied by region. Haplogroup D4, for instance, was more frequent in northern and northeastern China, reaching 22.7 percent of samples from Neimenggu, 21.8 percent of samples from Liaoning, and 21.5 percent of samples from Heilongjiang. Haplogroup B4, meanwhile, was more common in southern and southwestern populations, reaching 22.2 percent of Hong Kong samples, 16.4 percent of Yunnan samples, and 14.4 percent of samples from Shanghai.

While they noted no correlation between haplogroup and dialect, the researchers did find through a principal components analysis a divide between south and north Han Chinese samples.

They further linked this divide to three river valleys. Samples from people living in the Yellow, Yangtze, and Zhujiang River valleys tended to each cluster together. The researchers' analysis estimated that these three river valley clusters exhibited the highest among-group variance.

Using additional data from nearly 5,000 additional mitogenomes, the researchers traced the timing of these splits between these various Han Chinese populations, which suggested that these divisions were present by the late Holocene.

The Han Chinese population likely underwent population growth about 18,870 years ago, just after the last glacial maximum, they reported. They then noted another time of population growth in Han Chinese that took place about 9,430 years ago during the early Holocene. Similar population growth occurred in northern and southern Han groups, as well as in Han groups from the three river valleys, the researchers reported.

According to the researchers, this last time of population growth among the river valley populations corresponds to the origination of agriculture in those regions: millet in the Yellow River valley, rice in the Yangtze River valley, and roots and tubers in the Zhujiang River valley.

They further noted that some of the genetic affinity they observed between the Yellow River valley populations and those in other locales could be due to the northward spread of millet agriculture out of that region.

The researchers also noted that additional analyses of paternally inherited Y chromosome could provide more insight into the Neolithic population expansion.