Genetic polymorphism of complement component C3 in rainbow trout (Oncorhynchus mykiss) and resistance to viral haemorrhagic septicaemia

A possible relation between alloforms of complement component C3 in rainbow trout (Oncorhynchus mykiss) and resistance to viral haemorrhagic septicaemia was investigated by testing alloform and phenotype frequencies in different fish farm populations. Fish farm A, a non-infected production farm, supplied fry to the infected fish farm B and the non-infected fish farm C. Surviving trout of the infected fish farm B differed significantly (P<0·05) from fish of the non-infected farms A and C with respect to the distribution of the investigated alleles f1 and f2. The f2 allele was found in a significantly lower frequency in the infected fish farm. However, a postulated relation between certain phenotypes carrying the f2 allele could not be confirmed by testing phenotype frequencies in the different fish farms by a χ2-test. The distribution of phenotypes in fish farm C was not in agreement with that predicted from allele frequencies by the Hardy-Weinberg equation, suggesting the existence of other, unknown, selective forces affecting the phenotype distribution.