We have carried out the first major infectivity trial to examine differential genetic resistance in fish for pathogens. We used captive-bred, endangered winter-run chinook salmon (Oncorhynchus tshawytscha) to determine resistance to three pathogens: the bacterium, Listonella (Vibrio) anguillarum, infectious hematopoietic necrosis virus (IHNV), and Myxobolus cerebralis, the parasite that causes whirling disease. We compared resistance to these three pathogens between inbred and outbred salmon and between siblings that were heterozygous or homozygous for a class II gene in the major histocompatibility complex (MHC). In two of five different comparisons, we found significant genetic effects on disease resistance. First, MHC heterozygotes had a higher survival than MHC homozygotes when exposed to IHNV and the selection disadvantage of homozygotes was estimated to be 8.5%. Second, outbred fish had a higher resistance (or lower infection severity) than inbred fish when exposed to M. cerebralis. Using a quantitative genetics approach, it appears that there are slightly more than three gene equivalents segregating that would result in no resistance to M. cerebralis when homozygous. Overall, our investigation suggests that pathogen susceptibility in the winter-run chinook salmon will increase if further genetic variation is lost in this endangered species.
Major histocompatibility complex (MHC) variation and susceptibility to the sea louse Lepeophtheirus salmonis in Atlantic salmon Salmo salar