“Genes that come from transposable elements and that are repurposed for cellular functions are typically no longer transposable,” said Cedric Feschotte, a senior author of the study. “BC200 was itself created from a mobile element but has retained its mobility and yet it is also clearly serving a cellular function. Somehow evolution hasn’t been able to untangle these two things.”

A Rare Genetic Jump Into a Virus

Scientists have documented a handful of cases in other species in which transposable elements moved into viruses. One early example came in the late 1980s, when researchers working with cultured moth cells observed a transposon moving from those cells into a baculovirus, a type of virus that infects insects.