Pre-Cambrian origin of <i>envelope</i>-carrying retrotransposons in metazoans

In a study examining the origin of retrotransposons carrying envelope genes in metazoans, researchers discovered that these elements are widespread across various ancient animal taxa, including cnidarians, ctenophores, and tunicates. These retrotransposons, similar to Ty3/gypsy errantiviruses found in Drosophila, contain gag and pol open reading frames necessary for their replication in the host germline genome.

Additionally, they carry envelope genes, which are classified into two types resembling glycoprotein F from paramyxoviruses and glycoprotein B from herpesviruses. Despite their structural and functional similarities to retroviruses, the envelope-containing Ty3/gypsy elements have an unclear evolutionary relationship. Were they recent acquisitions of viral fusogens, or did they have an ancient association with envelope-like genes?

By systematically searching invertebrate metazoan genomes for intact envelope-containing ERVs homologous to Ty3/gypsy, the researchers found that such elements are prevalent across many taxa. Many elements consist of multiple highly similar copies within their genomes, suggesting recent genomic expansion in some host lineages. The envelope genes, though present in both glycoprotein F and B types, are equally abundant and widespread.

Through phylogenetic and structural analyses, the study revealed that envelope genes have largely diverged from pol genes and the host organisms throughout their evolutionary history. Furthermore, recombination events between these genes are infrequent, suggesting that the envelope acquisition to ERVs likely occurred before the split of bilaterian and non-bilaterian animals in the Pre-Cambrian era.