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The major histocompatibility ( B ) complex of a distinct commercial pure White Leghorn chicken line was characterized using serological, biochemical and restriction fragment length polymorphism (RFLP) typing. Line B chickens displayed a high recombination frequency within the B complex. Three recombinant haplotypes were identified. The influence of these haplotypes was determined in relation to the haplotypes B" and BZ1 on their resistance to Marek's disease (MD) in an experimental infection with the virus. Offspring of sires with a recombinant haplotype in combination with B" or BZ1, and dams, which were homozygous BlglBlg or BZ'lBZ1 were infected. The B type of the offspring had a significant effect upon survival. Animals with B complex types BZ1/Bz1, B134/B21 and B2341B21 were relatively resistant to MD (24432% mortality), whereas Blg/Blg birds were highly susceptible (68% mortality). Animals with a recombinant halpotype B19r21 (B-GZ1, B-F19) were equally susceptible to MD as birds with the complete Blg haplotype. In contrast to earlier publications, resistance was not inherited as a dominant trait. Apparently, B" was associated with a dominant susceptibility. The gene(s) associated with the B complex and involved in resistance to MD were localized within the B-FIB-L region. However, the association with a presumably non-coding subregion of B-G could not be excluded.

In commercial pure white leghorn lines, A, B, and C, the effects on resistance against a virulent strain of Marek's disease virus were assessed for B19 and B21 haplotypes of the chicken major histocompatibility complex. B haplotypes were identified by direct hemagglutination using alloantisera raised against erythrocyte antigens. In homozygous B21 female chicks from lines A and B, mortality upon challenge with virus was 16% and 9%, respectively; in B19 chicks, mortality was 42% and 60%, respectively. Intermediate mortality was observed in heterozygous B19/B21 birds. When line A and B hens were crossed with B15/B15 or B5/B19 cocks from line C, differences between B19 and B21 were significant only in the progeny from B5/B19 sires. Therefore, it was concluded that selection for major histocompatibility complex-associated disease resistance markers may be useful only when B haplotypes complement each other in commercial line crosses and when interactions with genetic background do not severely obscure the differential haplotype effects, as are observed within pure lines.

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