Intact, recombinant, and spliced forms of endogenous mouse mammary tumor viruses in inbred and wild mice

Endogenous retroviruses (ERVs) are copies of viral genomes inserted into host chromosomes, producing a fossil record of past infections and virus-host co-adaptations. ERVs of mouse mammary tumor viruses (<i>Mtvs</i>) were found in all common laboratory strains, all <i>Mus musculus</i> subspecies, and a sister species, <i>Mus spretus</i>. Most laboratory mouse <i>Mtvs</i> predate inbred strain origins and were acquired by <i>M. musculus domesticus</i>, but although widely shared among strains, none of these were found in wild mice. Among wild mouse <i>Mtvs</i>, only one showed a broad geographic distribution. All <i>M. musculus</i> subspecies carry <i>Mtvs</i> with a large envelope deletion corresponding to the processed mRNA for the viral <i>rem</i> gene; such <i>Mtvs</i> likely derive from spliced viral mRNA. The <i>Mtv sag</i> gene responsible for resistance to exogenous infection is under purifying selection and has been subject to recombination, whereas the <i>Mtv</i> envelope and its cellular receptor show no evidence of genetic conflicts.