Evolution of the Xenopus piggyBac transposon family TxpB: domesticated and untamed strategies of transposon subfamilies

A new family, termed TxpB, of DNA transposons belonging to the piggyBac superfamily was found in 3 Xenopus species (Xenopus tropicalis, Xenopus laevis, and Xenopus borealis). Two TxpB subfamilies of Kobuta and Uribo1 were found in all the 3 species, and another subfamily termed Uribo2 was found in X. tropicalis. Molecular phylogenetic analyses of their open reading frames (ORFs) revealed that TxpB transposons have been maintained for over 100 Myr. Both the Uribo1 and the Uribo2 ORFs were present as multiple copies in each genome, and some of them were framed by terminal inverted repeat sequences. In contrast, all the Kobuta ORFs were present as a single copy in each genome and exhibited high evolutionary conservation, suggesting domestication of Kobuta genes by the host. Genomic insertion polymorphisms of the Uribo1 and Uribo2 transposons (nonautonomous type) were observed in a single species of X. tropicalis, indicating recent transposition events. Transfection experiments in cell culture revealed that an expression vector construct for the intact Uribo2 ORF caused precise excision of a nonautonomous Uribo2 element from the target vector construct but that for the Kobuta ORF did not. The present results support our viewpoint that some Uribo2 members are naturally active autonomous transposons, whereas Kobuta members may be domesticated by hosts.