In new research, an international team of scientists sequenced and analyzed mitochondrial and nuclear genomes of living and extinct caballine horses (Equus spp.) to explore the possible impacts of the Bering Land Bridge on genetic diversity within, and connectivity among, populations of this once wide-ranging group. They found that Eurasian horse populations initially diverged from those in North America, their ancestral continent, around 1 million years ago; subsequent to this split, they identified two bi-directional long-range dispersals across the Bering Land Bridge 875,000-625,000 and 200,000-50,000 years ago.
Paleontologists have long known that horses evolved and diversified in North America.
One lineage, the caballine horses (which includes domestic horses), dispersed into Eurasia over the Bering Land Bridge about 1 million years ago, and the Eurasian population then began to diverge genetically from the horses that remained in North America.
The new study shows that after the split, there were at least two periods when horses moved back and forth between the continents and interbred, so that the genomes of North American horses acquired segments of Eurasian DNA and vice versa.
“This is the first comprehensive look at the genetics of ancient horse populations across both continents,” said Dr. Alisa Vershinina, a postdoctoral researcher at the University of California, Santa Cruz.
“With data from mitochondrial and nuclear genomes, we were able to see that horses were not only dispersing between the continents, but they were also interbreeding and exchanging genes.”
Dr. Vershinina and colleagues sequenced 78 new mitochondrial genomes from ancient horses found across Eurasia and North America.
Combining those with 112 previously published mitochondrial genomes, they reconstructed a phylogenetic tree, a branching diagram showing how all the samples were related.
With a location and an approximate date for each genome, they could track the movements of different lineages of ancient horses.
“We found Eurasian horse lineages here in North America and vice versa, suggesting cross-continental population movements,” Dr. Vershinina said.
“With dated mitochondrial genomes we can see when that shift in location happened.”
The analysis showed two periods of dispersal between the continents, both coinciding with periods when the Bering Land Bridge would have been open.
In the Middle Pleistocene, shortly after the two lineages diverged, the movement was mostly east to west.
A second period in the Late Pleistocene saw movement in both directions, but mostly west to east.
The researchers also sequenced two new nuclear genomes from well-preserved horse fossils recovered in Yukon Territory, Canada.
These were combined with 7 previously published nuclear genomes, enabling the team to quantify the amount of gene flow between the Eurasian and North American populations.
“The usual view in the past was that horses differentiated into separate species as soon as they were in Asia, but these results show there was continuity between the populations,” said Dr. Ross MacPhee, a paleontologist at the American Museum of Natural History.
“They were able to interbreed freely, and we see the results of that in the genomes of fossils from either side of the divide.”
“The new findings help reframe the question of why horses disappeared from North America,” said Dr. Grant Zazula, a paleontologist with the Government of Yukon.
“It was a regional population loss rather than an extinction. We still don’t know why, but it tells us that conditions in North America were dramatically different at the end of the last Ice Age. If horses hadn’t crossed over to Asia, we would have lost them all globally.”
The results were published in the journal Molecular Ecology.
Alisa O. Vershinina et al. Ancient horse genomes reveal the timing and extent of dispersals across the Bering Land Bridge. Molecular Ecology, published online May 10, 2021; doi: 10.1111/mec.15977
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