Fish retroposons related to the Penelope element of Drosophila virilis define a new group of retrotransposable elements

Poseidon and Neptune are two ancient lineages of retroposons related to the Penelope element from Drosophila virilis. They have been identified in var ious teleost fish species, including the medakafish (Oryzias latipes), and the pufferfishes Fugu rubripes and Tetraodon nigroviridis, whose genomes ar e currently being sequenced. Some of these elements are highly reiterated i n fish genomes. Penelope-related elements were also identified in blood flu ke, shrimp, sea urchin, cichlid fish and frog, showing that they are widesp read in animals. Penelope-related retroposons were not detected among seque nces from the Drosophila melanogaster and human genome projects, suggesting that they have been lost from certain animal lineages. A sequence encoding a putative Uri (also called GIY-YIG) endonuclease domain was detected down stream from the gene for reverse transcriptase. To the best of our knowledg e, this type of endonuclease sequence has previously been identified in gro up I introns and in genes for prokaryotic excinucleases but not in retrotra nsposable elements. Penelope-related elements are frequently truncated at t heir 5' ends and can also be flanked by long terminal repeat-like structure s. Phylogenetic analysis of the reverse transcriptase domain failed to assi gn Penelope-related retroposons to one of the major groups of retroelements . Overall, therefore, the evidence strongly suggests that these sequences r epresent a new group of retrotransposable elements.