Ancient Hepatitis Virus Found in Bird Genome
Hepatitis-B, a deadly virus that causes liver disease and is responsible for approximately 600,000 human deaths a year, is equally virulent in other species of mammals as well as birds. Though the modern version of this virus — one of a family called hepadnaviruses — is estimated to have arisen about 6,000 years ago, a recent study at the University of Texas at Arlington demonstrates that an ancient relative of the virus was circulating at least 19 million years ago, and has since incorporated itself into the genomes of at least two species of birds.
Viruses and Genes
Viruses are non-living replicators that use a host’s own genes to produce more of themselves. If they insert themselves into the germ line of a particular organism, it is also possible for them to pass themselves along into that organism’s offspring. In this way, the viruses may eventually become integrated into the genome of a species and tag along in the DNA as the species evolves.
Ancient Avian Viruses
The recent study looked at species of birds known to play host to the modern version of the hepatitis-B virus; these included an Australian species, the zebra finch, and a North American finch called the dark-eyed junco.
Researchers Clément Gilbert and Cédric Feschotte found more than a dozen fragments of a hepadnavirus — about 75% similar to the modern hepatitis-B virus — that had over time become harmlessly incorporated into the birds’ genomes. Since these two bird species diverged from one another about 25 million years ago, researchers theorize that the hepadnavirus must have infected a common ancestor of both birds, making the virus at least 25 million years old, though due to uncertainties in calculating the exact time of the divergence, they gave a more conservative estimate of the virus’s age at 19 million years.
Slow Mutation of Virus
The ancient hepadnavirus found in the birds’ DNA is so far the only virus of its kind discovered to have become integrated into its host’s genome. According to researchers, one of the most interesting aspects of the finding is that it suggests a very slow mutation rate for the virus; when compared to the modern version, the ancient relative appeared to have mutated at a rate about 1,000 times slower than the current virus. More research will be necessary to determine why this might be the case.