Horizontal transfer and southern migration: the tale of Hydrophiinae′s marine journey.

Transposable elements (TEs), also known as jumping genes, are sequences able to move or copy themselves within a genome. As TEs move throughout genomes they can be exapted as coding and regulatory elements, or can promote genetic rearrangement. In so doing TEs act as a source of genetic novelty, hence understanding TE evolution within lineages is key in understanding adaptation to their environment. Studies into the TE content of lineages of mammals such as bats have uncovered horizontal transposon transfer (HTT) into these lineages, with squamates often also containing the same TEs. Despite the repeated finding of HTT into squamates, little comparative research has examined the evolution of TEs within squamates. The few broad scale studies in Squamata which have been conducted found both the diversity and total number of TEs differs significantly across the entire order. Here we examine a diverse family of Australo-Melanesian snakes (Hydrophiinae) to examine if this pattern of variable TE content and activity holds true on a smaller scale. Hydrophiinae diverged from Asian elapids ~15-25 Mya and have since rapidly diversified into 6 amphibious, ~60 marine and ~100 terrestrial species which fill a broad range of ecological niches. We find TE diversity and expansion differs between hydrophiines and their Asian relatives and identify multiple HTTs into Hydrophiinae, including three transferred into the ancestral hydrophiine likely from marine species. These HTT events provide the first tangible evidence that Hydrophiinae reached Australia from Asia via a marine route.