Female brown trout × Atlantic salmon hybrids produce gynogens and triploids when backcrossed to male Atlantic salmon

Abstract

We examined protein electromorphs and karyotypes of progeny produced from interspecific mass spawnings between female brown trout ( Salmo trutta) and male Atlantic salmon ( S. salar). Species-specific allelic differences for the loci Gpi-1, Gpi-3, Ldh-5, Mpi, and Pgm-2 enabled assessment of species contributions in the hybrids. All of the F 1 hybrids examined showed equal expression of both parental alleles at all loci, as would be expected for codominant alleles. However, unexpected results were obtained for backcross progeny. Seventy of 177 backcross individuals had electrophoretic phenotypes identical to F 1 phenotypes; 107 appeared to be triploid based on isozyme dosage relationships. All triploids expressed one copy of brown trout alleles and two copies of Atlantic salmon alleles at all loci examined. Karyotypes of four F 1 and nine backcross hybrids were consistent with the ploidy levels and species contributions deduced from the electrophoretic phenotypes. In addition, a novel hybrid generated by crossing a female F 1 hybrid to a male brook trout ( Salvelinus fontinalis) appeared to contain and express the haploid genomes of all three species. The disparate parental chromosome complements of the F 1 hybrids (brown trout 2 n=80, Atlantic salmon 2 n=54–60) most likely led to a disruption of meiosis. Meiotic mechanisms that could account for the unusual backcross progeny are discussed. The type of inheritance exhibited by these F 1 hybrids has not been reported before in salmonid fish but appears similar to that reported in female hybrids of other vertebrate species. These results show the importance of using genetic markers to determine parental contributions and gene segregation in species hybrids.