Poised for contagion: Evolutionary origins of the infectious abilities of invertebrate retroviruses

Phylogenetic analyses suggest that long-terminal repeat (LTR) bearing retro transposable elements can acquire additional open-reading frames that can e nable them to mediate infection. Whereas this process is best documented in the origin of the vertebrate retroviruses and their acquisition of an enve lope (env) gene, similar independent events may have occurred in insects, n ematodes, and plants. The origins of env-like genes are unclear, and are of ten masked by the antiquity of the original acquisitions and by their rapid rate of evolution. In this report, we present evidence that in three other possible transitions of LTR retrotransposons to retroviruses, an envelope- like gene was acquired from a viral source. First, the gypsy and related LT R retrotransposable elements (the insect errantiviruses) have acquired thei r envelope-like gene from a class of insect baculoviruses (double-stranded DNA viruses with no RNA stage). Second, the Cer retroviruses in the Caenorh abditis elegans genome acquired their envelope gene from a Phleboviral (sin gle ambisense-stranded RNA viruses) source. Third, the Tas retroviral envel ope (Ascaris lumricoides) may have been obtained from Herpesviridae (double -stranded DNA viruses, no RNA stage). These represent the only cases in whi ch the env gene of a retrovirus has been traced back to its original source . This has implications for the evolutionary history of retroviruses as wel l as for the potential ability of all LTR-retrotransposable elements to bec ome infectious agents.