A group of deuterostome Ty3/gypsy-like retrotransposons with Ty1/copia-like pol-domain orders

Here we report the existence of an unusual group of LTR retrotransposons. termed Gmrl-like elements. The members of this group are most similar in sequence to elements of the Ty3/gypsy group. yet, unlike typical Ty3/gypsy elements. they have pol genes in which the integrase domain lies upstream, rather than downstream, of the reverse transcriptase domain, Such an arrangement was formerly believed to be a characteristic peculiar to Ty1/copia elements. The group includes the previously described retrotransposon Gmrl from the Atlantic cod Gadus morhua, together with elements from a variety of other vertebrates, such as the zebrafish Danio rerio, the pufferfish Fugu rubripes and the clawed toad Xenopus laevis, as well as elements from non-vertebrate deuterostomes such as the sea squirt Ciona intestinalis. No Gmrl-like elements were found outside of the deuterostomes, nor were any other groups of elements with atypical pol-domain orders identified. Phylogenetic analyses show that the Gmrl-like elements form a monophyletic group within the larger group of Tv3/gypsy elements. Some of the newly identified elements appear to be structurally intact and may still be active. The identification of this group challenges some previously held beliefs concerning the structure and evolution of LTR retrotransposons.