DETAILS OF RETROPOSITIONAL GENOME DYNAMICS THAT PROVIDE A RATIONALE FOR A GENERIC DIVISION - THE DISTINCT BRANCHING OF ALL THE PACIFIC SALMON AND TROUT (ONCORHYNCHUS) FROM THE ATLANTIC SALMON AND TROUT (SALMO)

Salmonid species contain numerous short interspersed repetitive elemen ts (SINEs), known collectively as the HpaI family, in their genomes. A mplification and successive integration of individual SINEs into the g enomes have occurred during the evolution of salmonids. We reported pr eviously a strategy for determining the phylogenetic relationships amo ng the Pacific salmonids in which these SINEs were used as temporal la ndmarks of evolution. Here, we provide evidence for extensive genomic rearrangements that involved retropositions and deletions in a common ancestor of all the Pacific salmon and trout. Our results provide gene tic support for the recent phylogenetic reassignment of steelhead and related species from the genus Salmo to the genus Oncorhynchus. Severa l other informative loci identified by insertions of HpaI SINEs have b een isolated, and previously proposed branching orders of the Oncorhyn chus species have been confirmed. The authenticity of our phylogenetic tree is supported both by the isolation of more than two informative loci per branching point and by the congruence of all our data, which suggest that the period between succesive speciations was sufficiently long for each SINE that had been amplified in the original species to become fixed in all individuals of that species.