Genetic Variation in Resistance to the HemoflagellateCryptobia salmositicain Coho and Sockeye Salmon

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Abstract Intra- and intraspecific differences in genetic variation for resistance to the pathogenic hemoflagellate Cryptobia salmositica were identified in coho salmon Oncorhynchus kisutch and sockeye salmon O. nerka. For each species, a strain sampled from a location enzootic to C. salmositica was more resistant than one sampled from outside the enzootic area. In coho salmon inoculated with 10-fold serial dilutions of 106 to 102 C. salmositica, survival was 95% in the resistant strain, 3% in the susceptible strain, and intermediate (30%) in the intraspecific hybrid strains. Genetic variation for survival detected in the hybrids was attributable to the susceptible strain only, indicative of strong selection pressure imposed on the fish by the parasite. For sockeye salmon treated identically, survival was 90% in the resistant, 40% in the susceptible, and 70% in the hybrid strains. Survival in both purebred sockeye salmon strains displayed additive and nonadditive genetic variation. Survival of intraspecifi...