A novel transposable element in lampreys that is similar in sequence to Tol2 of teleost fishes

Tol2 is one of the few DNA-based transposable elements that reside in vertebrate genomes and retain transposition activity. It was first identified in the medaka fish (Oryzias latipes) and subsequently in another species of the genus Oryzias, which belongs to the order Beloniformes. Transposable elements similar in sequence to Tol2 also occur in several species of the order Cypriniformes, including the common carp. The low sequence variation of these elements relative to that of a nuclear gene provided evidence for past horizontal transfer events among host organisms. We collectively refer to Tol2 and these related elements as the Tol2 family. Here, we identified a novel transposable element in two European lamprey species (Lampetra fluviatilis and L. planeri), which we named Tel2. Lampreys belong to the jawless vertebrate lineage (infraphylum Agnatha), whereas all previously known host species of the Tol2 family are jawed vertebrates (infraphylum Gnathostomata). The nucleotide sequence similarity between Tol2 and Tel2 was higher than that observed for the nuclear gene, indicating at least one horizontal transfer event between the two infraphyla. Thus, the Tol2 family now includes Tel2 as a new member. These results indicate that Tol2 family elements have undergone multiple horizontal transfer events across a wide range of host organisms, including both jawless and jawed vertebrates. Because some members of this family still retain transposition activity, they provide an excellent and unique model for studying the role of horizontal transfer in the long-term persistence of DNA-based transposable elements.