04/07/2017

Tracking Genome Expansion in Giant Viruses

Piecemeal acquisition of genes from hosts may explain the rise of giant viruses.

The Science

The number of microbes in, on, and around the planet is said to outnumber the stars in the sky. The number of viruses found worldwide is at least an order of magnitude greater. As their name suggests, giant viruses are larger than many bacterial and eukaryotic cells. They were first discovered in 2003, and the true breadth of their diversity remains unknown. Researchers recently uncovered a new group of giant viruses after sifting through metagenomic datasets. Dubbed Klosneuviruses, these giant viruses contain a more complete set of translation machinery genes than any other virus known to date.

The Impact

Contrary to popular belief, most viruses do not affect humans. They do, however, impact microbes, which regulate biogeochemical cycles. Protists such as algae, for example, sequester large fractions of carbon in the atmosphere and are key components of the global carbon cycle. Viruses can significantly impact the productivity of the protist population, reducing their capabilities in regulating global cycles. As protists are thought to be the host of these Klosneuviruses, a better understanding of how viruses impact microbial survival and community interactions is relevant to Department of Energy (DOE) missions in bioenergy and environment.

Summary

While sifting through metagenomic sequence datasets for a DOE Joint Genome Institute (JGI) Community Science Program project, DOE JGI researchers identified genome sequences typically found in giant viruses. A group of giant viruses called Mimiviruses was first discovered in 2003, and a handful of such groups have been reported since. DOE JGI researchers assembled a 1.57-million base (Megabase) genome of a putative virus they called Klosneuvirus, and further searching through the metagenomic datasets uncovered three more related giant virus genomes. Three of the four Klosneuviruses were found with representatives of the protist phylum Cercozoa. This is unusual because until now, all giant viruses had been recovered with Acanthamoeba (amoebas found in soils and fresh waters), which was not seen with the Klosneuviruses. The team also found that the Klosneuviruses encoded components for a far more expansive translation system than had been seen with other giant viruses. Aside from increasing the known gene pool of giant viruses by nearly 2,500 additional gene families, comparing the genes to previously discovered giant viruses revealed that the Klosneuviruses are a subfamily of Mimiviruses. Starting then from their last shared ancestor with the Mimiviruses, the researchers suggest that over time, the Klosneuviruses picked up genes from various different hosts. Overall, the team’s findings lend credence to the theory that giant viruses evolved from much smaller viruses, rather than aligning with theories that they may instead be descended from a cellular ancestor. The consequences of Klosneuvirus infection of protist hosts remains to be explored.

Principal Investigator(s)

Tanja Woyke
DOE Joint Genome Institute
twoyke@lbl.gov

Related Links

Funding

This work was conducted by the U.S. Department of Energy’s (DOE) Joint Genome Institute, a DOE Office of Science user facility (contract number DE-AC02-05CH11231). Additional support was provided by the U.S. Department of Health and Human Services, European Research Council, Austrian Science Fund, and John Templeton Foundation.

References

Schulz, F., N. Yutin, N. N. Ivanova, D. R. Ortega, T. K. Lee, J. Vierheilig, H. Daims, M. Horn, M. Wagner, G. J. Jensen, N. C. Kyrpides, E. V. Koonin, and T. Woyke. 2017. “Giant Viruses with an Expanded Complement of Translation System Components,” Science 356(6333), 82-85. DOI: 10.1126/science.aal4657.