Mx mRNA expression and RFLP analysis of rainbow trout Oncorhynchus mykiss genetic crosses selected for susceptibility or resistance to IHNV

Three interferon-inducible Mr genes have been identified in rainbow trout O ncorhynchus mykiss and their roles in virus resistance have yet to be deter mined. In mice, expression of the Mx1 protein is associated with resistance to influenza virus. We report a study to determine whether there was a cor relation between the expression of Mr in rainbow trout and resistance to a fish rhabdovirus, infectious hematopoietic necrosis virus (IHNV). A compari son of Mr mRNA expression was made between different families of cultured r ainbow trout selected for resistance or for susceptibility to IHNV. A trout -specific Mr cDNA gene probe was used to determine whether there was a corr elation between Mr mRNA expression and resistance to the lethal effects of IHNV infection. Approximately 99% of trout injected with a highly virulent strain of the fish rhabdovirus, IHNV, were able to express full length Mr m RNA at 48 h post infection. This is markedly different from the expression of truncated, non-functional Mr mRNA found in most laboratory strains of mi ce, and the ability of only 25% of wild mice to express functional Mr prote in. A restriction fragment length polymorphism (RFLP) assay was developed t o compare the Mr locus between individual fish and between rainbow trout ge netic crosses bred for IHNV resistance or susceptibility. The assay was abl e to discriminate 7 distinct RFLP patterns in the rainbow trout crosses. On e cross was identified that showed a correlation between homozygosity at th e Mr locus and greater susceptibility to IHN-caused mortality.