Retroposons of salmonoid fishes (Actinopterygii: Salmonoidei) and their evolution

Short and long retroposons, or non-LTR retrotransposons (SINEs and LINEs, respectively) are two groups of interspersed repetitive elements amplifying in the genome via RNA and cDNA-mediated reverse transcription. In this process, SINEs entirely depend on the enzymatic machinery of autonomous LINEs. The impact of retroposons on the host genome is difficult to overestimate: their sequences account for significant portion of the eukaryotic genome, while propagation of their active copies gradually reshapes it. In this way, the retropositional activity plays a role of important evolutionary factor. More than 100 LINE and nearly 100 SINE families have been described to date from the genomes of various eukaryotes, and it is salmonoid fishes (Actinopterygii: Salmonoidei) that are particularly noticeable for the diversity of transposons they host in their genomes, including two LINE and seven SINE families. Moreover, this group of ray-finned fish represents an excellent opportunity to study such a rare evolutionary phenomenon as lateral gene transfer, due to a great variety of transposons and other sequences salmons share with a blood fluke, Schistosoma japonicum (Trematoda: Strigeiformes)--a parasitic helminth infecting various vertebrates. The aim of the present review is to structure all knowledge accumulated about salmonoid retroposons by now, as well as to complement it with the new data pertaining to the distribution of some SINE families.