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Millions of domestic yaks are helping the people in the highland of the Himalaya, Pamir and over to the Altai Mountains to survive in their harsh environment. Recent studies reported very similar traits in both domestic cattle and yak and single examples of gene flow from cattle to yak have been presented for such traits. However, there was no systematic genome-wide comparative analysis of yak and cattle so far. Recently scientists from INRA, ALLICE (France) and LMU Munich (Germany) in collaboration with researchers from Germany, Austria, Great Britain, Russia, USA and Mongolia were able to determine the origin, extent, distribution and age of bovine introgression in Mongolian yak. Using extensive population genomic analyses they found evidence for non-random distribution of genes with bovine origin in the yak genome. Introgressed regions were clearly enriched for genes involved in the development and function of the nervous system, and especially in glutamate metabolism and neurotransmission. The researchers now present their findings in the scientific journal Nature Genetics.

With whole genome sequencing, high-density genotyping and bioinformatic analyses they demonstrated, for a large sample collection from European and Asian origin, that all investigated yaks carried a small (~1.3%) proportion of bovine genes. Interestingly this result was comparable to the proportion of Neanderthal genes found in modern non-African humans. The absence of pure yak as well as the distribution and length of the bovine chromosome fragments indicated that introgression most probably began more than 1500 years ago and was maintained with changing intensity over time.

In their research, the scientists identified a new sequence variant determining the genetic absence of horn, i.e. polledness, and simultaneously demonstrated that it has been introgressed from Mongolian Turano cattle to Mongolian yak a long time ago. In the past, the team of researchers had already described two other genetic variants responsible for the same phenotype in European cattle. Polledness is an especially important trait in horn-bearing domestic animals as it avoids the common -yet ethically undesired- practice of dehorning in modern management systems.

Yet polledness is only one among many traits that allowed the ancient Asian breeders to tame the ferocious yaks. Through long-term admixture with a low proportion of cattle gametes (around 1/11000 per generation throughout the last 1500 years) they introduced more or less intentionally numerous bovine variants that contributed to the improvement of yak. These variants are predominantly located in non coding - and presumably regulatory - regions of genes that are predominantly involved in nervous system development and function. Each Mongolian yak carries on average about 33 of such bovine variants and thus harbour a critical mass of modifications affecting intelligence, behaviour, instinct, perception etc.

This study makes an important contribution to understanding the genes and pathways which have been the most affected by domestication and highlights the beneficial role played by introgressive hybridization in transferring favorable polymorphisms from one domestic species to another.

The scientific team led by Ivica Medugorac (LMU) and Aurelien Capitan (INRA and ALLICE) launched this study without explicit research project designed for comparative study of yak and cattle. However, contributions of numerous research partners and funders (Apis-Gène and LMU excellence programme), and comprehensive sample collections created during previous projects helped to achieve these results.

REFERENCE

Alexander Graf, Cécile Grohs, Sophie Rothammer, Yondon Zagdsuren, Elena Gladyr, Natalia Zinovieva, Johanna Barbieri, Doris Seichter, Ingolf Russ, André Eggen, Garrett Hellenthal, Gottfried Brem, Helmut Blum, Stefan Krebs, Aurélien Capitan. Whole-genome analysis of introgressive hybridization and characterization of the bovine legacy of Mongolian yaks, Ivica Medugorac, Nature Genetics. 30 janvier 2017.