The structure and evolutionary role of the Penelope mobile element in the Drosophila virilis species group

The mobile element Penelope is activated and mobilizes several other transp osons in dysgenic crosses in Drosophila virilis. Its structure proved to be complex and to vary greatly in all examined species of the virilis group. Phylogenetic analysis of the reverse transcriptase (RT) domain assigned Pen elope to a new branch, rather than to any known family, of LTR-lacking retr oelements. Amino acid sequence analysis showed that the C-terminal domain o f the Penelope polyprotein is an active endonuclease, which is related to i ntron-encoded endonucleases and to bacterial repair endonuclease UrvC, and may act as an integrase. Retroelements coding for a putative endonuclease t hat differs from typical integrase have not been known thus far. Phylogenet ic analysis divided the Penelope copies from several virilis species into t wo subfamilies, one including virtually identical full-length copies, and t he other comprising highly divergent defective copies. The results suggest both vertical and horizontal transfer of the element. Possibly, Penelope in vasion recurred during evolution and contributed to genome rearrangement in the virilis species. Chromosome aberrations detected in D. virilis, which is now being invaded by Penelope, is direct evidence for this assumption.