Transposons traced through time
Transposable elements (TLEs) have mobility properties that make them potential tools for moving genes into new organisms (transgenesis) and mutagenesis (creation of a mutation in a cell). Detailed studies of TLEs were performed on Xenopus tropicalis (X. tropicalis) and X. laevis. The TLE Mariner is an endogenous element and it was studied in the two Xenopus species used as developmental models to gain a potential insight into any regulatory interactions between engineered and endogenous active transposons. Genomic sequencing data were used to identify multiple lineages of TLEs. A number of distinct TLE lineages were found to co-exist in Zebra fish, salmon and amphibians. Seven new TLEs were analysed and phylogenetic studies indicate that their closest relatives are found in the genomes of ray-finned fish and amphibians. One of these TLEs, named Xeminos, is strongly related to an invertebrate lineage. Moreover, these newly discovered TLEs contain an intact transposase open reading frame (ORF) which is a sequence that can be translated into a protein. This suggests that these elements may still be active. Furthermore, transposons that belong to a separate lineage were found to be located at both extremities of the vertebrate evolutionary tree, and are found in chordate genomes including most craniates. This suggests that they have been present in chordate genomes for around 750 million years. This study of the Xenopus vertebrate model system has considerable potential for biomedical research in developing transposable elements as molecular tools for functional genomics. One such artificial element reconstructed by molecular engineering is 'Sleeping beauty'. This element, which dates back some fifteen million years, is transposable and has potential for the delivery of genes for genetic disease therapy.
