Contrasting segregation patterns among endogenous retroviruses across the koala population

Koalas (Phascolarctos cinereus) are subject to retroviral infections by the koala retrovirus (KoRV), resulting in accumulation of heritable endogenous KoRV in the koala germline. Like other vertebrates, koalas have experienced a history of retroviral epidemics leaving marks as endogenous retroviruses (ERVs) in their genomes; another recently identified ERV lineage, named phaCin-β, shows a pattern of recent, possibly current, activity with high insertional polymorphism in the koala population that predates the establishment of endogenous KoRV. We investigate geographic patterns of retrovirus activity, focusing on three distinct ERV lineages of increasing estimated ages from KoRV to phaCin-β and to phaCin-β-like, using a whole-genome sequencing dataset of 430 koalas produced by the Koala Genome Survey. We find thousands of ERV loci across the koala population and identify contrasting patterns of polymorphism. In the youngest established ERV-lineage (KoRV), we identify thousands of integrations among northern individuals. In the phaCin-β ERV lineage, which is slightly older yet with overlapping time of activity with KoRV, we observe hundreds of integrations in northern individuals. In southern koalas, however, phaCin-β loci are found at higher frequencies possibly reflecting a bottleneck effect. Overall, the findings suggest high ERV burden in koalas that represent historic retrovirus-host interactions. Importantly, the ERV catalogue supplies improved markers for population conservation genetics of this endangered species.