A family of Tcl-like transposons from the genomes of fishes and frogs: evidence for horizontal transmission

Tcl-like transposons are very widely distributed within the genomes of anim al species. They consist of an inverted repeat sequence flanking a transpos ase gene with homology to the mobile DNA element, Tcl of the nematode Caeno rbabditis elegans. These elements seem particularly to infest the genomes o f fish and amphibian species where they can account for 1% of the total gen ome. However, all vertebrate Tcl-like elements isolated so far are non-func tional in that they contain multiple frameshifts within their transposase c oding regions. Here I describe a Tcl-like transposon (PPTN) from the genome of a marine flatfish species (Pleuronectes platessa) which bears conserved inverted repeats flanking an apparently intact transposase gene. Closely r elated, although degenerate, Tcl-like transposons were also isolated from t he genomes of Atlantic salmon (SSTN, Salmo salar) and frog (RTTN, Rana temp oraria). Consensual nucleic acid sequences were derived by comparing severa l individual isolates from each species and conceptual amino acid sequences were thence derived for their transposases. Phylogenetic analysis of these sequences with previously isolated Tcl-like transposases shows that the el ements from plaice, salmon and frog comprise a new subfamily of Tcl-like tr ansposons. Each member is distinct in that it is not found in the genomes o f the other species tested. Plaice genomes contain about 300 copies of PPTN , salmon 1200 copies of SSTN and frog genomes about 500 copies of RTTN. The presence of these closely related elements in the genomes of fish and frog species, representing evolutionary lines, which diverged more than 400 mil lion years ago, is not consistent with a vertical transmission model for th eir distributions. (C) 2001 Elsevier Science B.V. All rights reserved.