New environment, new invaders - repeated horizontal transfer of LINEs to sea snakes

While numerous studies have found horizontal transposon transfer (HTT) to be widespread across metazoans, few have focused on HTT in marine ecosystems. To investigate potential recent HTTs into marine species we searched for novel repetitive elements in sea snakes, a group of elapids which transitioned to a marine habitat at most 18 Mya. Our analysis uncovered repeated HTTs into sea snakes following their marine transition. Such major shifts in habitat should require significant genomic changes.The seven subfamilies of horizontally transferred LINE retrotransposons we identified in the olive sea snake (Aipysurus laevis) are transcribed, and hence are likely still active and expanding across the genome. A search of 600 metazoan genomes found all seven were absent from other amniotes, including terrestrial elapids, with the most similar transposons present in fish and marine invertebrates. The one exception was a similar transposon found in sea kraits, a lineage of amphibious elapids which independently transitioned to a marine environment following their divergence from terrestrial species 25 Mya. Our finding of repeated horizontal transfer events into separate lineages of marine snakes greatly expands past findings of frequent horizontal transfer in the marine environment, suggesting it is ideal for the transfer of transposons.Transposons are drivers of evolution as sources of genomic sequence and hence genomic novelty. This provides evidence of the environment influencing evolution of metazoans not only through specific selection pressures, but also by contributing novel genomic material.