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Mongolian Yak Genomes Reveal Cattle Mixing History

NEW YORK (GenomeWeb) – Mongolian yaks carry a small but significant proportion of bovine cattle DNA in their genomes, reflecting past efforts to breed yak and cattle at low altitudes in Asia.

An international team led by investigators in Germany and France used a combination of array-based genotyping and whole-genome sequencing to assess ancestry patterns across stretches of sequence inherited together in 76 Mongolian yaks. Their analyses, outlined today in Nature Genetics, indicated that around 1.3 percent of sequences in the yak genome came from bovine cattle, despite the sterility that's been documented in male yak-bovine hybrids.

"The introgressed regions are enriched in genes involved in nervous system development and function, and particularly in glutamate metabolism and neurotransmission," first author Ivica Medugorac, an animal genetics and husbandry researcher at Ludwig Maximilians University Munich, and his co-authors wrote. "We also identified a novel mutation associated with a polled (hornless) phenotype originating from Mongolian Turano cattle."

The Mongolian yak is known for adaptations that make it adept at surviving in high-altitude environments. And while it belongs to a lineage that diverged from the bovine lineage some 4.9 million years ago, the team explained, the yak is raised and sometimes bred with cattle at lower altitudes.

Moreover, the researchers reasoned that "analysis of bovine introgression in Mongolian yaks represents an appealing model to identify exchange of traits of interest between domesticated species."

They used Illumina HiSeq 1500 or 2000 instruments to sequence the genomes of two Mongolian yaks — one yak carrying two copies of polled locus mutations associated with the hornless phenotype and one horned yak — as well as a Turano cow with mutations affecting one copy of the Polled locus.

Using allele-frequency patterns in the genomes, along with available bovine sequence data, the team searched for yak-specific SNPs and began teasing out potential bovine sequences in the yak genomes. With Illumina BovineHD BeadChip genotyping arrays, the group went on to consider introgression patterns in another 40 yaks with horns and 36 without.

Bovine cattle ancestry turned up across 0.67 percent to 2.82 percent of the yak genomes, the researchers reported, spanning 1.3 percent of the sequences in the yak genome, on average. And roughly one-third of cattle genome sequences turned up in at least one of the yaks.

Their results hinted that the Turano cattle from Mongolia are related to bovines behind the yak-cattle mixing. Such admixture stretched back at least 1,500 years and appeared to peak in around the 10th and 11th centuries and again in the 18th century.

When the team took a look at cattle sequence introgression patterns, it identified cattle genes involved in nervous system function and domestication. From these and other findings, the authors concluded that "introgressive hybridization contributed to the improvement of yak management and breeding."

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