Cellular genes derived from Gypsy/Ty3 retrotransposons in mammalian genomes

Gypsy/Ty3 retrotransposons, a group of long terminal repeat retrotransposons related to vertebrate retroviruses, are found in the genome of many fungi, plants, and animals. Although multiple families of such retroelements are present in fish, active Gypsy/Ty3 retrotransposons have all been eliminated from the lineage leading to mammals at least 180 million years ago. However, over 50 cellular genes related to Gypsy/Ty3 retrotransposons have been identified in mammalian genomes, indicating recurrent "molecular domestication" of these elements by their host during evolution. Most retrotransposon-derived proteins are conserved in divergent mammalian species and show sequence similarity to Gag proteins, major structural proteins for retroelement particle formation. Among the proposed and demonstrated biological functions for gag-derived genes, placenta formation in the mouse requires two gag-derived genes from the same family. Some forms of epigenetic regulation of gag-related genes might derive from host genome defense mechanisms that repelled retrotransposon ancestors. Together, such observations support a major role for transposable elements as a source of new coding sequences allowing important genetic innovations during evolution.