We know that viruses can evolve and mutate very quickly over short periods of time, but what about ancient viruses?
A new paper in PLoS Biology describes a recent discovery of 19 million-year-old fossils of a Hepatitis B-like virus from modern zebra finches and related songbirds.
While ancient viruses have been found in human DNA, this study marks the first time that endogenous hepadnaviruses have been found in any organism. An endogenous virus is one that deposits itself or fragments of itself into the chromosome of an organism, allowing it to be passed from generation to generation. Previously, most of these known “fossilized” virus sequences have come from retroviruses.
Researchers from the University of Texas in Arlington dated the hepadnavirus fragments by locating them in the same spot on the genome of five species of passerine birds and then tracing those species to a common ancestor that lived more than 19 million years ago.
We asked the Academy’s Curator and Department Chair of Ornithology & Mammalogy, Jack Dumbacher, PhD, about the recent finding over email.
This is a really cool paper, and causes us to rethink long-term evolutionary rates. We mostly study viruses over short time periods, and we can measure substitution rates over hundreds or even dozens of years, and they appear exceedingly rapid. If we then multiply out these divergence rates over millions or even tens of millions of years, then we might expect that viruses living today would bear little resemblance to those from millions of years ago. This would essentially “erase” the phylogenetic similarities that researchers use to study evolutionary histories.
This study points out just the opposite, that hepadnaviruses from tens of millions of years ago are not that different from those of today. There appear to be huge differences between short-term evolutionary rates and long-term evolutionary rates. There are several possible reasons, but overall it gives some hope that perhaps viral evolutionary histories may be resolved much further back in time than previously expected.
Jack studies bird populations on the islands in Papua New Guinea and gently has been taking virus samples from them over the last two years. He’s studying birds on remote islands, as well as birds on islands populated with humans.
Presently, there are only three complete bird genomes available for analysis (chicken, turkey, and zebra finch). Thus, there is much to learn about endogenous viral DNA sequences found in birds. Our own work may help highlight the prevalence of hepadnaviruses in natural avian populations, about which very little is presently known.
You can learn more about Jack’s work in Papua New Guinea here.