Abstract
BackgroundSeveral genome duplications have occurred in the evolutionary history of teleost fish. In returning to a stable diploid state, the polyploid genome reorganized, and large portions are lost, while the fish lines evolved to numerous species. Large scale transposon movement has been postulated to play an important role in the genome reorganization process. We analyzed the DNA sequence of several large loci in Salmo salar and other species for the presence of DNA transposon families.ResultsWe have identified bursts of activity of 14 families of DNA transposons (12 Tc1-like and 2 piggyBac-like families, including 11 novel ones) in genome sequences of Salmo salar. Several of these families have similar sequences in a number of closely and distantly related fish, lamprey, and frog species as well as in the parasite Schistosoma japonicum. Analysis of sequence similarities between copies within the families of these bursts demonstrates several waves of transposition activities coinciding with salmonid species divergence. Tc1-like families show a master gene-like copying process, illustrated by extensive but short burst of copying activity, while the piggyBac-like families show a more random copying pattern. Recent families may include copies with an open reading frame for an active transposase enzyme.ConclusionWe have identified defined bursts of transposon activity that make use of master-slave and random mechanisms. The bursts occur well after hypothesized polyploidy events and coincide with speciation events. Parasite-mediated lateral transfer of transposons are implicated.
Highlights
Several genome duplications have occurred in the evolutionary history of teleost fish
Two genome duplications are thought to have occurred in the evolutionary history of ancient vertebrates, a third in ancestral teleosts, with a fourth occurring more recently in the genome of salmonid fishes (Salmonidae), 25 to 100 million years ago (Mya) [1,2]
DNA transposon families To explore the role of transposons in speciation events in fish, nearly 3 Mbp of BAC sequence from Salmo salar as well as from Oncorhynchus mykiss was analyzed for the presence of DNA transposon-like sequences
Summary
Several genome duplications have occurred in the evolutionary history of teleost fish. After duplication and subsequent mutation of the duplicated chromosomes, the polyploid genome began to reduce to a more stable diploid form, losing approximately 50 percent of its DNA [3]. In this model, rediploidization preceeded and is concurrent with speciation events of the Salmonidae line to Salmoninae (salmon and trout), Coregoninae (whitefish), and Thymallinae (grayling). The mechanism by which this restabilization occurs is not well understood, but large scale movement of repetitive sequences, in particular transposable elements, are postulated to play a major role in reshaping genomes and the genome's evolution. Significant increases in transposon activity have been noted in plants after allopolyploidization [7], during chromosome rearrangements in Drosophila [8], and after species hybridization [9]
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