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The genomes of Drosophila melanogaster and Caenorhabditis elegans possess many retrovirus elements. The transition from ancestral non-viral retrotransposons to retroviruses has occurred by the acquisition of an envelope gene (that mediates infection) from other viral sources. This may be the underlying mechanism for the origin of all retroviral lineages, including those in vertebrate genomes. (For details, see Malik et al., p. 1307.)
