The mariner transposons belonging to the irritans subfamily were maintained in chordate genomes by vertical transmission.

Mariner-like elements (MLEs) belong to the Tc1-mariner superfamily of DNA transposons, which is very widespread in animal genomes. We report here the first complete description of a MLE, Xtmar1, within the genome of a poikilotherm vertebrate, the amphibian Xenopus tropicalis. A close relative, XlMLE, is also characterized within the genome of a sibling species, Xenopus laevis. The phylogenetic analysis of the relationships between MLE transposases reveals that Xtmar1 is closely related to Hsmar2 and Bytmar1 and that together they form a second distinct lineage of the irritans subfamily. All members of this lineage are also characterized by the 36- to 43-bp size of their imperfectly conserved inverted terminal repeats and by the -8-bp motif located at their outer extremity. Since XlMLE, Xlmar1, and Hsmar2 are present in species located at both extremities of the vertebrate evolutionary tree, we looked for MLE relatives belonging to the same subfamily in the available sequencing projects using the amino acid consensus sequence of the Hsmar2 transposase as an in silico probe. We found that irritans MLEs are present in chordate genomes including most craniates. This therefore suggests that these elements have been present within chordate genomes for 750 Myr and that the main way they have been maintained in these species has been via vertical transmission. The very small number of stochastic losses observed in the data available suggests that their inactivation during evolution has been very slow.