Inheritance of Myxobolus cerebralis Resistance among F₁-Generation Crosses of Whirling Disease Resistant and Susceptible Rainbow Trout Strains

Abstract

A whirling disease resistant domestic strain of rainbow trout Oncorhynchus mykiss (German (GR)) was crossed with a susceptible wild strain of rainbow trout (Colorado River (CR)). Thirty-two families of F₁ crosses, along with five pure GR and two pure CR rainbow trout families, were then evaluated for resistance to whirling disease after experimental exposure to Myxobolus cerebralis. The pure domesticated GR strain was verified to have strong resistance to the parasite. In contrast, wild CR rainbow trout were highly susceptible. Crosses of these two strains resulted in offspring with a range of susceptibility. The resistance to whirling disease in some families was similar to that of pure GR rainbow trout, while other families were as susceptible as pure CR rainbow trout. Infection severity was significantly greater in the CR strain than in the pure GR and the GR × CR strains, as measured by both microscopic pathological scores (histology) and myxospore counts. Infection severity in the reciprocal crosses (CR female × GR male) was not significantly different from that of the pure CR rainbow trout as measured by histology but was significantly different with respect to lower myxospore counts. Future studies will examine the stability of inheritance of whirling disease resistance and the potential use of selective breeding to control whirling disease in free-ranging rainbow trout populations.