Marek's Disease Resistance of B (MHC) Heterozygotes in a Cross of Purebred Leghorn Lines

The incidence of Marek's disease (MD) was determined for 404 progeny of five B genotypes resulting from a cross of two purebred Leghorn lines. All chicks were inoculated at 3 weeks of age with 1000 pfu of MD virus, strain HPRS 16. Birds dying during the ensuing 14 week period and any with symptoms of MD at the termination of the experiment were subjected to postmortem examination. The female parent line, B, possessed B21 in relatively high frequency (.66), with B19 being the most frequent of three remaining B alleles. Comparison of progeny genotypes within line B matings for mortality from MD revealed that chicks with B21/B21 genotype were more resistant than those of the genotype B19/B21. Homozygous B21 females of this line were selected for mating to males of the other Leghorn line, A, in which five B alleles were segregating – B2 (.61 frequency), B5 (.09), B15 (.18), B19 (.02), and Bx (.10). Seven line A males were individually mated to the line B females; two of the males were of the genotype B2/Bx and there was one of each of the genotypes B2/B15, B5/Bx, B15/B19, B2/B2, and B5/B5. Among the progeny exposed to MD were 59 chicks of the Houghton Poultry Research Station MD susceptible line, which suffered a mortality of 83%. The percent incidence of MD among single-cross progeny with each of the five possible genotypes B2/B21, B5/B21, B15/B21, B19/B21, and Bx/B21 was, respectively, 5 (7/129), 38 (42/112), 48 (22/46), 50 (16/32), and 48 (41/85). This highly significantly greater resistance (P<.001) of the B2/B21 among the five genotypes demonstrates that certain B alleles exclusively present in particular parent stocks may complement each other in heterozygous cross-line birds.