by Bernie May, Research Professor, Department of Animal Science
The lineage that gave rise to current day salmonid fishes (salmon, trout, whitefish, grayling, and char) went through a polyploid event 30-50 million years ago that doubled their number of chromosomes. This phenomenon meant there were four copies of each chromosome (tetraploid) instead of the normal two copies that most organisms, including humans, have (diploid). Since that polyploidy event occurred, the salmonid genome has continued to evolve such that there are now only two copies of each chromosome, with some having residual tetrasomic regions on the ends of the chromosomes (telomeric regions) reflective of their polyploid origin. These residual tetrasomic regions allow normally disomic chromosomes to pair multivalently in salmonid males that upon gametic segregation give rise to residual tetrasomy for these telomeric loci and in some cases to pseudolinkage where loci on different chromosomes appear to be linked, although the segregation shows an excess of recombinant types over parental types. This change from a tetrasomic to a primarily disomic genome has provided the salmonids with more loci that have been able to evolve more specialized functions in metabolic pathways. The unusual meiotic pairing and gametic segregation of these telomeric regions in male salmonids is reviewed and explained in a recent paper by Bernie May (GVL founder) and Mary Delany (CAES Executive Associate Dean), a topic they studied together over 35 years ago when they first met at The Pennsylvania State University.