NEW YORK – An international research team has sequenced the ancient mitochondrial genome of bison and looked at changes in bison morphology recorded by cave art to determine that modern European bison (Bison bonasus) originally came from interbreeding between the steppe bison (Bison priscus) and an ancient type of cattle called an auroch (Bos primigenius).
The findings shed light on a mysterious hybrid of bison and cattle that was detailed on cave walls more than 15,000 years ago, according to researchers from the University of Adelaide's Australian Centre for Ancient DNA, which led the study, published today in Nature Communications.
"Finding that a hybridisation event led to a completely new species was a real surprise — as this isn't really meant to happen in mammals," Professor Alan Cooper, ACAD director and leader of the study, said in a statement. "The genetic signals from the ancient bison bones were very odd, but we weren't quite sure a species really existed — so we referred to it as the Higgs Bison."
European bison, distinct from its American counterpart, is among the few terrestrial megafauna to have survived the late Pleistocene extinctions. But despite extensive fossil records in Eurasia, the evolutionary history of the European bison before the Holocene period, approximately 12,000 years ago, is unknown.
In an effort to determine the potential existence of a previously unrecognized fossil bison species within Europe, the researchers sequenced ancient mtDNA and nuclear DNA from the bones and teeth of 64 Late Pleistocene/Holocene bison specimens. They obtained samples from three regions across Europe — the Ural Mountains in Russia, the Caucasus Mountains that span Eurasia, and sites in Western Europe —extracting mitochondrial DNA using either phenol-chloroform or silica-based methods. They then generated next-generation sequencing data on the samples using Illumina HiSeq or MiSeq instruments.
The researchers mapped the reads to the reference genomes of the European bison, American bison, or aurochs, and aligned the 60 newly sequenced regions with 302 published sequence to infer a phylogenetic tree using both maximum likelihood and Bayesian methods.
The team, which also included researchers from the University of California, Santa Cruz (UCSC), as well as Polish bison conservation researchers, and paleontologists from across Europe, also used materials collected over time in the Ural Mountains to reconstruct paleoenvironmental proxies for the region. They also compared two distinct morphologies of bison present in cave paintings between 15,000 and 18,000 years ago: a long-horned form similar to modern American bison, which are believed to be descended from steppe bison and have "very robust forequarters and oblique dorsal line," and a "second form with thinner double-curved horns, smaller hump and more balanced body proportions," they wrote.
"Combined evidence from genomic data, paleoenvironmental reconstructions, and cave paintings strongly suggest that the hybridization of steppe bison with an ancient aurochs lineage during the late Pleistocene led to a morphologically and ecologically distinct form, which maintained its integrity and survived environmental changes on the European landscape until modern times," the researchers wrote.
The researchers believe that this provides evidence of hybridization between aurochs and steppe bison, and supports the argument that both groups should be combined under the genus Bos.
"Further analyses of deeper ancient genome sequencing will be necessary to characterize the phenotypic consequences of such hybridization, this adds to recent evidence of the importance of hybridization as a mechanism for speciation and adaptation of mammals as is already accepted for plants," the researchers wrote.