Incidence of endogenous viral genes in Leghorn strains of different origin, each with sublines of a different genotype for resistance to avian leukosis virus infection

SUMMARY: The identification of genes affecting disease resistance in domestic fowl has challenged research workers in various countries, in the study of their effects and in the evaluation of their potential for commercial poultry breeding. This study concerns endogenous viral genes (ev genes), which can play an important role in the response to avian leukosis virus infection. The incidence of ev genes was determined in three experimental White Leghorn strains of different origin, each consisting of two sublines genotypically either susceptible or resistant to infection from avian leukosis virus (ALV) of the subgroups A and B. The three pairs of sublines had been used extensively for investigations of the effects of leukosis infections in laying hens. Among the 11 ev genes found, only three were present in all three strains - ev1 and ev3, which appear to be very common in White Leghorn, and ev6, which has been associated with reduced immune response to ALV infection. Five of the 11 ev genes found occurred only in one of the three strains, while the number of different ev genes in each of the strains amounted to either six or seven. The frequency of each ev gene present in a strain was surprisingly similar in the genotypically resistant and susceptible subline. With the exception of ev1 and ev3, which were present in all three strains at a relatively high frequency, there were striking differences between the strains in the level of frequencies of the additional ev genes. In one of the strains, which originated from a commercial hybrid, this level was exceptionally low, while the additional ev genes had much higher frequencies in the other two strains. The information about the incidence of ev genes, and specifically that about ev6, suggested a plausible explanation for the striking difference observed between the three Leghorn strains in their respect to ALV infection and, in particular, the rate of congenital ALV infection.