Yaks in Mongolia have more than 1 percent cattle DNA despite male hybrid sterility

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Last year an interesting paper by Ivica Medugorac and coworkers presented data on introgression in domesticated yaks in Mongolia: “Whole-genome analysis of introgressive hybridization and characterization of the bovine legacy of Mongolian yaks”.

I have a long interest in the hybridization and introgression of genes among cattle and related species. It all started back in the 1980s when some family members were raising beefalo, a cattle breed that has a substantial fraction of bison ancestry. Hybrids of cattle, zebu, banteng, gaur, and bison featured in my 2006 paper on the feasibility introgression in Neandertals.

What’s interesting about yaks is that they have a good fraction of cattle genes despite hybrid male sterility.

Hybrid males are sterile, however, preventing the establishment of stable hybrid populations, but not a limited introgression after backcrossing several generations of female hybrids to male yaks. Here we inferred bovine haplotypes in the genomes of 76 Mongolian yaks using high-density SNP genotyping and whole-genome sequencing. These yaks inherited ∼1.3% of their genome from bovine ancestors after nearly continuous admixture over at least the last 1,500 years

A number of scientists have discussed whether some degree of reduced hybrid fertility is a possibility for Neandertals. The human X chromosome exhibits less introgression from Neandertals than the autosomes. Further, there are large “deserts” on the X chromosome with no evidence of Neandertal introgression in any living human samples.

However, in principle these features of X chromosome introgression may reflect selection over many generations rather than a great reduction in the fertility of male F1 hybrids.

The yak lineage diverged from the ancestors of cattle an estimated 4 million years ago, and male hybrid sterility is a common feature of sister species of mammals that have been separated for such a long time. Neandertals and modern humans have been separated for around 700,000 years.

An interesting historical story unfolds when thinking about female crossing and fitness within yaks. A relaxation of the intensity of breeding selection can in some instances increase the fraction of the genome that results from introgression:

Introgression was more intense during two periods (897–1121 CE and 1695–1828 CE), which coincide with the Medieval Climate Anomaly (900–1200 CE) and the Dzungar–Qing Wars (1687–1758 CE). These periods of intense introgression are most likely due to increased mortality of livestock during these difficult times that forced yak herders to breed all of the females available to restore their herds, including backcross-derived animals (Supplementary Note).

I am sure that many other species of domesticated animals have similarly interesting histories. The times that human populations had to struggle actually leave a mark in the genomes of the domesticated species.