Genetic analysis of rainbow trout susceptibility to the myxosporean, Ceratomyxa shasta

Six genetic groups, a high susceptibility strain (Mt. Shasta), a low susceptibility strain (Pit River), and their F₁, F₂, and backcrosses, were exposed to Ceratomyxa shasta for 7 days and continuously (> 120 d). No simple Mendelian model of inheritance fit the observed mortality data, and segregation was not detectable in the distributions of time to death, indicating that the genetic control for these two traits involves more than one locus. Significantly lower mean liabilities (threshold units) resulted from a 7-day exposure than from continuous exposure. Regardless of experimental exposure the mean liabilities for the six genetic groups showed a linear pattern along a scale of liability, with the F₁ and F₂ (F₁-F₂) being intermediate to the parental strains, and the backcrosses being intermediate to the F₁-F₂ and their corresponding parental strains. Phenotypic classification of each genetic group by means of a threshold model indicated that the mean liability of the susceptible Mt. Shasta strain was not significantly different from an "acute" class; the BC-Shasta was intermediate in susceptibility between the "acute" and "subacute" classes; the F₁-F₂ was intermediate between the "subacute" and "chronic" classes; and the BC-Pit and the Pit River strain were not different from a "chronic" phenotypic class. "True" time to death was estimated for all groups from observed partial distributions of time to death. Analysis of genetic components of these means as well as analysis of genetic components of survival (logit transformed) indicated that significant additive, dominance, and digenic interaction effects are involved in both traits by which we measured susceptibility, and that the Pit River alleles are dominant over the alleles of susceptibility (Mt. Shasta). The analysis of the genetic components of means also indicated that the nature of genetic causal components differed with time of exposure to C. shasta.