Endogenous retroviruses are a common ancestral feature of mammalian genomes with most having been inactivated over time through mutation and deletion1. A group of more intact endogenous retroviruses are considered to have entered the genomes of some species more recently, through infection by exogenous viruses2, but this event has never been directly proved. We have previously reported koala retrovirus (KoRV) to be a functional virus that is associated with neoplasia3. Here we show that KoRV also shows features of a recently inserted endogenous retrovirus that is vertically transmitted. The finding that some isolated koala populations have not yet incorporated KoRV into their genomes, combined with its high level of activity and variability in individual koalas, suggests that KoRV is a virus in transition between an exogenous and endogenous element. This ongoing dynamic interaction with a wild species provides an exciting opportunity to study the process and consequences of retroviral endogenization in action, and is an attractive model for studying the evolutionary event in which a retrovirus invades a mammalian genome.
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We thank J. Hanger, J. McKee, D. Hume and M. Eiden for discussions; Dreamworld Themepark, Queensland Parks and Wildlife Service, The South Australian Department of the Environment and Heritage, D. Higgins, S. Johnston and W. Ellis for access to, and supply of, samples; D. Irwin and staff at the Australian Genome and Research Facility for technical assistance with the single-cell PCR; B. Carroll and staff in the School of Molecular and Microbial Sciences, University of Queensland, for technical assistance with the Southern blotting analyses. This work was supported by Dreamworld Themepark and the Australian Research Council. Author Contributions R.E.T. carried out all experiments shown in Figs 1–3 and in Supplementary Information. All authors were involved in the conceptual design and interpretation of the experimental work with direction and supervision provided by J.M. and P.R.Y. All authors participated in the drafting of the manuscript.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
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