Newly discovered viral genome could rewrite history of virus
20 April 2012
A virus found in a high temperature acidic volcanic lake
belongs to a previously undetected group of viruses. More importantly,
it has a new type of viral genome that could have huge implications for
theories of viral emergence and evolution.
Viruses are the most
abundant organisms on earth, yet little is known about their
evolutionary history since they have exceptionally high rates of genetic
mutation which are difficult to track.
Viral metagenomics, however, is becoming an increasingly useful tool
with which to glimpse virus evolution, as it makes available vast
amounts of new sequence data for analysis.
Stedman’s team from Portland State University in Oregon, USA, used a
metagenomics approach to investigate virus diversity in Boiling
Springs Lake in Lassen Volcanic National Park, USA, an acidic, high
temperature lake (ranging from 52-95°C, with a pH of ~2.5) that
sustains a purely microbial ecosystem.
Unique hybrid RNA-DNA viral genome
Astonishingly, they found a unique viral genome that has never
before been reported — a circular, single-stranded DNA virus
encoding a major capsid protein seen previously only in RNA viruses.
This unusual genome provides proof that integration of an RNA virus
into a DNA virus may have occurred between two unrelated virus
groups at some point in evolution — something that has not been
Moreover, this suggests that entirely new virus types may emerge
via recombination of functional and structural modules between
vastly different viruses, using mechanisms that are as-yet unknown.
The team observed that the Boiling Springs Lake RNA-DNA hybrid
virus (BSL RDHV) genome is circular, but its size is roughly double
that of typical circoviruses, with the ORFs arranged in an uncommon
orientation. They compared the BSL RDHV genome to other metagenomic
DNA sequences from the Global Ocean Survey, and found strong
evidence to conclude that previously undetected BSL RDHV-like
viruses could be widespread in the marine environment and are likely
to be found in other environments as well.
No mechanism has
been proposed to account for the inferred instances of interviral
RNA-DNA recombination, but the team speculate that a DNA circovirus-like
progenitor may have acquired a capsid protein gene from a ssRNA
virus via reverse transcription and recombination.
author Kenneth M Stedman said, “As more viral metagenomic data are
generated and analyzed, additional evidence of recombination between
RNA and DNA virus groups will likely be discovered. The discovery
that novel virus groups can emerge via recombination between highly
disparate virus types will have broad implications for the early
evolution of viruses and extends the modular theory of virus
evolution to encompass a much broader range of possibilities than
Geoffrey S Diemer and Kenneth M Stedman. A novel virus genome
discovered in an extreme environment suggests recombination between
unrelated groups of RNA and DNA viruses. Biology Direct (in press).