Ancient virus may have given mammals the edge during dinosaur extinction era

A virus that incorporated itself into mammal genomes over 70 million years ago may have helped humanity’s ancestors survive the cataclysmic conditions that led to the extinction of the dinosaurs at the end of the Cretaceous era.

Research published by Jim Wellehan, associate professor of zoological medicine at UF’s College of Veterinary Medicine and a member of the UF Emerging Pathogens Institute, suggests the protection offered through the integration of bornaviruses into the genomes of Cretaceous-era mammals may have given them an advantage after the dinosaur extinction event that occurred 66 million years ago.

Working in collaboration with an international cohort of veterinary scientists, Wellehan helped identify new species of bornaviruses — a family of viruses that cause neurologic disease in a variety of species — that were impacting Australian carpet pythons.

When comparing these new viruses to available genetic data, the researchers found the viruses varied significantly from previously known bornaviruses; in fact, the most closely related sequences of these python bornaviruses were bornavirus genes that had integrated into mammal genomes.

These bornavirus genes, called “endogenous bornaviral-like elements,” or EBLs, have been found in other species. According to a 2014 study published in “Proceedings of the National Academy of Sciences,” an EBL in thirteen-lined ground squirrels protects them against infection from related bornaviruses. Several studies have demonstrated that endogenous viral elements protect their hosts from related viruses.

“While studying the genetic history of these EBLs and their locations in the genomes of various species, we concluded that they likely integrated into mammal DNA  approximately 70 million years ago,” Wellehan said.

Sixty-six million years ago, the end-Cretaceous extinction led to the disappearance of all non-avian dinosaurs. Although the cataclysm following the Chicxulub meteor impact receives much of the blame for the extinction in popular culture, climate shifts following the volcanic release of carbon dioxide also helped bring about the end of the dinosaur age.

Bornaviruses are in the same order of viruses as Ebola, measles and rabies, all of which are capable of causing significant population-level effects. During the end-Cretaceous extinction, EBLs within mammal genomes may have protected mammals from bornaviruses that affected birds and reptiles, allowing them to step into the ecological niche left by extinct dinosaur species.

“Before the end-Cretaceous extinction, mammals had a poor track record for adapting to extreme climate shifts,” Wellehan said. “During times of environmental stress, infectious diseases can have greater impacts on populations. Mammals living during the end of the Cretaceous era, therefore, had a level of protection against bornaviruses that was lacking in birds and reptiles.”

The end-Permian extinction, which preceded the end-Cretaceous extinction by nearly 200 million years, occurred due to major climate warming from volcanic carbon dioxide release. The fossil record suggests that climate changes occurring during this time resulted in the extinction of nearly all species.

Before the end-Permian extinction, the dominant species were part of the mammal lineage. The avian/reptile lineage has renal and respiratory physiological advantages over the mammal lineage, however. In the hot, dry conditions of the time, reduced water loss gave reptiles the advantage, setting the stage for the age of dinosaurs.

Recent studies suggest that significant global warming occurred at the end of the Cretaceous era as well, due to volcanic carbon dioxide release by a major volcanic eruption just prior to the meteor impact. Unlike the period following the end-Permian extinction, however, mammals overtook reptiles as the predominant terrestrial vertebrates following the end of the Cretaceous era.

“I find this surprising given the physiological advantages of birds and reptiles,” Wellehan said. 

EBLs, he said, help explain why mammals fared so much better than their predecessors did in the Permian era.

Wellehan worked on this project with colleagues in both the United States and Australia before publishing results in PLOS Pathogens. Timothy Hyndman, Ph.D., a veterinary medicine professor at Murdoch University in Australia, collected the snakes and performed the initial diagnoses. Mark Stenglein, Ph.D., a veterinary microbiologist at Colorado State University, was responsible for assembling the virus so that it could be analyzed in more detail. Catherine Shilton, Ph.D., a research scientist at the Berrimah Veterinary Laboratories in Australia, described the pathology of the bornavirus strain identified in the pythons, and Wellehan was responsible for the phylogenetics, forming the connection between the strain found in the carpet pythons and the EBLs that have existed in mammal DNA for millions of years.

Written by: Evan Barton