Cytogenetic evolution of genus Salmo

The ancestral genome duplication that occurred around 80-100 MYA in the common ancestor of extant salmonids induced a stressed auto-tetraploid genome. This was followed by expansion of transposable elements which is believed to facilitate large-scale chromosomal rearrangements. The species radiation did not occur until around 40-50 million years later when earth’s climate was going into a cooling stage. As the result of the large-scale genome reorganization, present-day salmonids share large syntenic collinear blocks in a species-specific order. In the present study, our first aim was to visualize these rearrangements by taking advantage of genus Salmo (including two sole members, namely, Atlantic salmon (S. salar) and brown trout (S. trutta)) where species not only exhibit distinct karyotypes, but also intra-species chromosomal polymorphisms. Moreover, another aim of present study was to provide cytogenetic support for confirming the identity of sex determining gene, sdY, in Atlantic salmon by revealing its physical locations in both European and North American derived Tasmanian Atlantic salmon. In the genus of Salmo, large scale rearrangements including fusions, translocations, fissions and possible chromosome arm loss were evident in the present study, giving rise to the extant karyotypes of S. salar and S. trutta. The fact that no polymorphic karyotype of the S. trutta was observed in our study and other publications suggests a relative stable genomic background. In contrast, polymorphic karyotypes due to Robertsonian rearrangements of three pairs of chromosomes were found in Canadian subpopulations of S. salar. The fixation of the homozygous translocation in most Canadian populations indicates possible fixation of advantageous mutations and suggests a mild to near-neutral underdominance of this rearrangement in its ancestral heterozygous state. On the other hand, the observation of potentially deleterious Robertsonian tandem fusions in all Canadian populations indicate the initial rearrangement likely took place in a small effective population with subsequent spreading into other river populations through colonization. In our effort to visualize the physical locations of sdY, a single-copy gene, the gene was indeed found residing on the sub-telocentric region of Ssa02q in European Atlantic salmon, consistent with results from previous SEX mapping studies. In comparison, multiple locations including Ssa02q, Ssa03p and Ssa06p were shown to be locations of sdY in a North American derived Tasmanian population, supporting a previous study suggesting a jumping gene theory.