The impact of ancestral tetraploidy on antibody heterogeneity in salmonid fishes

The immunoglobulin heavy chain locus in teleost fish is structurally simila r to that in mammals, comprising a series of variable gene segments upstrea m of two constant region genes coding for IgM and IgD. Atlantic salmon have been shown to possess two distinct heavy chain loci, related to the tetrap loid ancestry oi this fish family. The nature (and results) of the evolutio nary processes following the tetraploidization event are the focus of this review. Salmonid fish did not return quickly to a diploid state, but are st ill in the process of re-establishing disomic inheritance. Thus, a specific locus in one species may still be endowed with four alleles, while it may have been converted to a pair of isoloci in another species. Analyses of im munoglobulin heavy chain genes in Atlantic salmon (Salmo salar) have strong ly indicated that the ancestral heavy chain locus was subjected to tetrasom y throughout the radiation of the genera Oncorhynchus and Salmo, and that d isomic inheritance was established in the Salmo lineage in the comparativel y recent past. The introduction of disomic inheritance at these loci has re sulted in two subsets of IgM and IgD heavy chains in Atlantic salmon.