The invasion of a host germline by infectious retroviruses has been a common evolutionary event that shaped the genomes of all vertebrates, including humans. Endogenous retroviruses (ERVs) comprise ca. 8% of the human genome, where they play a role in placentation and in a number of diseases. Despite their ubiquity, until recently all identified ERVs had entered their host germline thousands or millions of years ago. Many ERVs have undergone mutations, deletions and recombination, and many retroviruses that gave rise to ERVs may be extinct. Thus, the evolutionary events that occurred as retroviruses invaded host genomes can be difficult to infer. Recently, an emerging koala retrovirus (KoRV), linked to a high incidence of leukemia and immune system suppression in Australian koalas, was discovered to be in the midst of transitioning from being an infectious exogenous virus to becoming an endogenous part of the koala genome. To date, KoRV is the only known exogenous retrovirus that is currently in the midst of invading a host germ line. Our objective is to use KoRV to understand the process by which an infectious retrovirus invades its host germline, a process that has shaped the human genome, as it actually happens. We will determine how the genetic diversity of KoRV is evolving across loci, individuals, and populations. Museum samples of koalas will be used to determine how the genetic diversity of KoRV has evolved over time. Differences between ancient and modern KoRV will be determined, and examined for their effects on functional properties of the virus. The specific aims are: (1) Examine the sequence diversity and insertional polymorphisms of KoRV across the geographic range of the host population, from which the origins, spread and evolution of KoRV may be inferred. (2) Use archival samples of koalas to directly determine the evolution of genetic and functional diversity of KoRV across the time period during which KoRV emerged in a new host and invaded the germline. (3) Use complete proviral genomes and known host pedigrees to examine the evolutionary events that are shaping KoRV. The complete genomes of individual KoRVs will be sequenced and analyzed to determine the degree to which KoRVs invading their host germ line are being affected by selection, recombination, open reading frame disruptions, host anti-viral mechanisms, and host population structure.