The majority of DNA-based transposable elements comprise autonomous and nonautonomous copies, or only nonautonomous copies, where the autonomous copy contains an intact gene for a transposase protein and the nonautonomous copy does not. Even if autonomous copies coexist, they are generally less frequent. The Tol2 element of medaka fish is one of the few elements for which a nonautonomous copy has not yet been found. Here, we report the presence of a nonautonomous Tol2 copy that was identified by surveying the medaka genome sequence database. This copy contained three local sequence alterations that affected the deduced amino acid sequence of the transposase: a deletion of 15 nucleotides resulting in a deletion of 5 amino acids, a base substitution causing a single amino acid change, and another base substitution giving rise to a stop codon. Transposition assays using cultured human cells revealed that transposase activity was reduced by the 15-nucleotide deletion and abolished by the nonsense mutation. This is the first example of a nonautonomous Tol2 copy. Thus, Tol2 is in an early stage of decay in the medaka genome, and is therefore a unique element to observe an almost complete decay process that progresses in natural populations.
Keywords: autonomie; autonomy; horizontal transfer; transfert horizontal; transposase; transposition; vertebrate; vertébrés.