Inheritance of susceptibility to Ceratomyxa shasta (Myxozoa) in rainbow trout and the effect of length of exposure on the liability to develop ceratomyxosis

Abstract

Susceptibility to C. shasta among a susceptible and a resistant strain of rainbow trout ( Oncorhynchus mykiss), and their reciprocal F1's was measured as mortality and time to death after exposure to the parasite infective stage, in two experimental settings: a short (7-day) and a continuous exposure. Regardless of experimental setting, mortalities among the two strains and their reciprocal F1's were significantly different, with the reciprocal F1's being intermediate. Time to death was significantly shorter for the susceptible strain than for the reciprocal F1's and resistant strain, which were not different. The effect of length of exposure on increasing liability to develop ceratomyxosis was not the same for the two strains and their reciprocal F1's; regardless of length of exposure the resistant strain showed a low level of mortality, while mortalities among the susceptible strain and the reciprocal F1's were significantly larger when continuously exposed than when exposed for 7 days. However, an apparently larger effect of exposure length on mortality was seen among the reciprocal F1's than for the susceptible strain. This response is presumed to be the result of constraints imposed by the scale of measurement having a limit of 100%. Estimates of degree of dominance derived from cumulative mortalities at the end of a continuous exposure, and therefore, after the susceptible strain reached the limit of this scale, result in an apparent dominance for susceptibility, but comparisons of cumulative mortalities after a “pulse” exposure of 7 days indicated dominance for resistance. Results on mortality and time to death suggest that the reciprocal F1's have inherited a mechanism of defense against C. shasta from the resistant strain. This defense mechanism appears to be highly sensitive to parasite dose such that under continuous exposures, protection can be overwhelmed leading to an increasing liability to develop ceratomyxosis. However, the effect of that defense mechanism was still evident by the significantly longer time to death among the reciprocal F1's when contrasted with the shorter time to death of the susceptible strain.