Linkage Relationships Reflecting Ancestral Tetraploidy in Salmonid Fish

Fifteen classical linkage groups were identified in two salmonid species (Salmo trutta and Salmo gairdneri) and three fertile, interspecific hybrids (S. gairdneri X Salmo clarki, Salvelinus fontinalis X Salvelinus namaycush and S. fontinalis X Salvelinus alpinus) by backcrossing multiply heterozygous individuals. These linkage relationships of electrophoretically detected, protein coding loci were highly conserved among species. The loci encoding the enzymes appeared to be randomly distributed among the salmonid chromosomes. Recombination frequencies were generally greater in females than in males. In males, certain linkage groups were pseudolinked with other linkage groups, presumably because of facultative multivalent pairing and directed disjunction of chromosomes. Five such pseudolinkage groups were identified and they also appeared to be common among species and hybrids. Duplicate loci were never classically linked with each other, although some exhibited pseudolinkage and some showed evidence of exchanging alleles. Gene-centromere recombination frequencies estimated from genotypic distributions of gynogenetic offspring were consistent with map locations inferred from female intergenic recombination frequencies. These linkage relationships support the contention that all extant salmonids arose from a common tetraploid progenitor and that this progenitor may have been a segmental allotetraploid.