SUMMARY Viruses of the subfamily Orthoretrovirinae are defined by the ability to reverse transcribe an RNA genome into DNA that integrates into the host cell genome during the intracellular virus life cycle. Exogenous retroviruses (XRVs) are horizontally transmitted between host individuals, with disease outcome depending on interactions between the retrovirus and the host organism. When retroviruses infect germ line cells of the host, they may become endogenous retroviruses (ERVs), which are permanent elements in the host germ line that are subject to vertical transmission. These ERVs sometimes remain infectious and can themselves give rise to XRVs. This review integrates recent developments in the phylogenetic classification of retroviruses and the identification of retroviral receptors to elucidate the origins and evolution of XRVs and ERVs. We consider whether ERVs may recurrently pressure XRVs to shift receptor usage to sidestep ERV interference. We discuss how related retroviruses undergo alternative fates in different host lineages after endogenization, with koala retrovirus (KoRV) receiving notable interest as a recent invader of its host germ line. KoRV is heritable but also infectious, which provides insights into the early stages of germ line invasions as well as XRV generation from ERVs. The relationship of KoRV to primate and other retroviruses is placed in the context of host biogeography and the potential role of bats and rodents as vectors for interspecies viral transmission. Combining studies of extant XRVs and “fossil” endogenous retroviruses in koalas and other Australasian species has broadened our understanding of the evolution of retroviruses and host-retrovirus interactions.

概要Orthoretrovirinae亚科病毒通过在细胞内病毒生命周期中将RNA基因组逆转录为DNA并整合到宿主细胞基因组中的能力定义。外源逆转录病毒（XRVs）在宿主个体之间水平传播，疾病的结果取决于逆转录病毒与宿主生物之间的相互作用。当逆转录病毒感染宿主的生殖细胞时，它们可能成为内源性逆转录病毒（ERV），它们是宿主生殖细胞中的永久性成分，会垂直传播。这些ERV有时仍具有传染性，它们本身会产生XRV。这篇综述整合了逆转录病毒系统发育分类的最新进展以及逆转录病毒受体的鉴定，以阐明XRVs和ERVs的起源和进化。我们考虑了ERV是否可能反复给XRV施加压力，以使受体的使用转移到避免ERV干扰。我们讨论了内源化后相关逆转录病毒如何在不同宿主谱系中经历替代命运，其中考拉逆转录病毒（KoRV）作为其宿主种系的近期入侵者而引起了广泛关注。KoRV具有遗传性，但也具有传染性，它提供了对种系入侵的早期阶段以及从ERV产生XRV的见识。KoRV与灵长类和其他逆转录病毒之间的关系被置于宿主生物地理环境中，以及蝙蝠和啮齿动物作为种间病毒传播载体的潜在作用。