Evidence for recent invasion of the medaka fish genome by the Tol2 transposable element

Tol2 is a transposable element of the terminal-inverted-repeat class, resid ing in the genome of the medaka fish Oryzias latipes. The genus Oryzias con tains more than 10 species for which phylogenetic relationships have previo usly been estimated. To infer the history of Tol2 in this genus we performe d genomic Southern blots and PCR analyses of 10 of the species. It was reve aled that Tol2 occurs in 2 of the 10 species (O. curvinotus and O. latipes) and that the length and the restriction map structure of Tol2 are identica l in the two cases. Further, sequencing analysis revealed an extremely low level of divergence compared with that in a nuclear gene. These results sug gest recent incorporation of Tol2 into one or both of the two species, impl ying horizontal transfer of Tol2 from one species to the other or into them both from a common source.