Bursts and Horizontal Evolution of DNA Transposons in the Speciation of Pseudotetraploid Salmonids

Background: Several genome duplications have occurred in the evolutionary history of teleostfish. In returning to a stable diploid state, the polyploid genome reorganized, and large portions arelost, while the fish lines evolved to numerous species. Large scale transposon movement has beenpostulated to play an important role in the genome reorganization process. We analyzed the DNAsequence of several large loci in Salmo salar and other species for the presence of DNA transposonfamilies.Results: We have identified bursts of activity of 14 families of DNA transposons (12 Tc1-like and2 piggyBac-like families, including 11 novel ones) in genome sequences of Salmo salar. Several ofthese families have similar sequences in a number of closely and distantly related fish, lamprey, andfrog species as well as in the parasite Schistosoma japonicum. Analysis of sequence similaritiesbetween copies within the families of these bursts demonstrates several waves of transpositionactivities coinciding with salmonid species divergence. Tc1-like families show a master gene-likecopying process, illustrated by extensive but short burst of copying activity, while the piggyBac-likefamilies show a more random copying pattern. Recent families may include copies with an openreading frame for an active transposase enzyme.Conclusion: We have identified defined bursts of transposon activity that make use of masterslaveand random mechanisms. The bursts occur well after hypothesized polyploidy events andcoincide with speciation events. Parasite-mediated lateral transfer of transposons are implicated.