Chromosomal Rearrangements in the Genus of Salmo

The common ancestor of salmon and trout experienced one additional whole genome duplication relative to models fish species such as medaka and stickleback. In the subsequent process of rediploidization, the genome needs to reorganize itself to deal with duplicated chromosomes. Brown trout and Atlantic salmon, which are the only two members in the genus of Salmo, have quite different karyotypes. Atlantic salmon has 54-58 chromosomes with 72-74 arms whereas brown trout has 80 chromosomes with 100-102 arms. As predicted from genetic mapping, fluorescent in situ hybridization (FISH) confirmed that there was a homozygous translocation event between chromosome (Chr) 1 and 23 in individuals from five strains of Canadian Atlantic salmon, while such an event was not observed in European salmon. Moreover, independent fusion events between Chr 26 and 28, Chr 8 and 29, were observed with and among the Canadian strains. The chromosome number of different individuals varied from 54 to 58, depending on the number of fusion events. We are beginning to understand why Atlantic salmon and brown trout have different karyotypes. For example, the fusion of three brown trout acrocentric chromosomes corresponds to a large metacentric Atlantic salmon chromosome. Thus, it appears that chromosomal variation in Salmo results from fusions and fissions of the ancestral chromosome arms.