DEVELOPMENTAL INSTABILITY AND HIGH MERISTIC COUNTS IN INTERSPECIFIC HYBRIDS OF SALMONID FISHES.

Interspecific hybrids have been proposed to have reduced developmental stability in comparison to their parental species because the parental genomes have not undergone selection for the maintenance of developmental stability when they occur together. We present data from four interspecific hybrids of salmonid fishes that support this view. Natural hybrids of bull trout (Salvelinus confluentus) with brook trout (Salvelinus fontinalis) and laboratory hybrids of rainbow trout (Salmo gairdneri) with Yellowstone (Salmo clarki bouvieri), westslope (S. c. lewisi), and coastal (S. c. clarki) cutthroat trout all have higher levels of fluctuating asymmetry than either of their parental species raised in the same environment. Thus, the hybrids have reduced developmental stability. The hybrids do not have meristic counts intermediate to the counts of the parental species. The hybrids usually have counts as high as the species with the higher count for those characters that differ between the parental species and often have higher counts for those characters that do not differ between the parental species. We suggest that the tendency for interspecific hybrids to have high meristic counts may be related to differences between the species in the length and timing of the developmental periods during which the counts of the characters are determined.