MHC haplotypes have a remarkable influence on whether tumors form following infection of chickens with oncogenic Marek’s disease herpesvirus. Although resistance to tumor formation has been mapped to a subregion of the chicken MHC-B region, the gene or genes responsible have not been identified. A full gene map of the subregion has been lacking. We have expanded the MHC-B region gene map beyond the 92-kb core previously reported for another haplotype revealing the presence of 46 genes within 242 kb in the Red Jungle Fowl haplotype. Even though MHC-B is structured differently, many of the newly revealed genes are related to loci typical of the MHC in other species. Other MHC-B loci are homologs of genes found within MHC paralogous regions (regions thought to be derived from ancient duplications of a primordial immune defense complex where genes have undergone differential silencing over evolutionary time) on other chromosomes. Still others are similar to genes that define the NK complex in mammals. Many of the newly mapped genes display allelic variability and fall within the MHC-B subregion previously shown to affect the formation of Marek’s disease tumors and hence are candidates for genes conferring resistance.

Lines of chickens selected from a common ancestral population for either resistance or susceptibility to Marek's disease developed contrasting frequencies of particular B alloalleles. Comparison of inoculated sibs in backcross-families revealed that the B alloalleles characterizing the two lines accounted for an eightfold difference in tumor incidence. This genetic difference in tumorigenesis associated with the alloalleles of the major histocompatibility complex is probably expressed through the cell-mediated immune system.

The first standard nomenclature for the chicken (Gallus gallus) major histocompatibility (B) complex published in 1982 describing chicken major histocompatibility complex (MHC) variability is being revised to include subsequent findings. Considerable progress has been made in identifying the genes that define this polymorphic region. Allelic sequences for MHC genes are accumulating at an increasing rate without a standard system of nomenclature in place. The recommendations presented here were derived in workshops held during International Society of Animal Genetics and Avian Immunology Research Group meetings. A nomenclature for B and Y (Rfp-Y) loci and alleles has been developed that can be applied to existing and newly defined haplotypes including recombinants. A list of the current standard B haplotypes is provided with reference stock, allele designations, and GenBank numbers for corresponding MHC class I and class IIbeta sequences. An updated list of proposed names for B recombinant haplotypes is included, as well as a list of over 17 Y haplotypes designated to date.

Analyses of the major histocompatibility complex (Mhc) in chickens have shown inconsistencies between serologically defined haplotypes and haplotypes defined by the restriction fragment patterns of Mhc class I and class II genes in Southern hybridizations. Often more than one pattern of restriction fragments for Mhc class I and/or class II genes has been found among DNA samples collected from birds homozygous for a single serologically defined B haplotype. Such findings have been interpreted as evidence for variability within the Mhc haplotypes of chickens not detected previously with serological methods. In this study of a fully pedigreed family over three generations, the heterogeneity observed in restriction fragment patterns was found to be the result of the presence of a second, independently segregating polymorphic Mhc-like locus, designated Rfp-Y. Three alleles (haplotypes) are identified in this new system.

A reference population designed for molecular genetic mapping of the chicken genome was produced by backcrossing a partially inbred Red Jungle Fowl (JF) line to a highly inbred White Leghorn (WL) line. The parental lines were chosen to maximize the expected genetic polymorphisms between them. Two full-sib F^ males, produced by crossing a JF male with a WL female, were each individually mated to about 10 WL females to produce 400 progeny. All the progeny were classified for segregation of three loci controlling color phenotype and six blood group loci, some of which have been mapped by classical methods. Segregation of these nine loci did not differ significantly from the expected 1:1 ratio with one exception. At least 20 mL of whole blood was stored from all the parents and progeny to provide DNA for molecular analysis. Screening of the parental lines and Fi crosses by Southern blot with cloned genes and by the random amplified polymorphic DNA (RAPD) procedure revealed a large number of molecular markers mat were parental line-specific. A preliminary analysis of 16 backcross progeny classified for polymorphisms at 2 color loci, 6 blood group loci, 16 loci detected by cloned chicken genes, and 4 loci detected by the RAPD method has been completed. Segregation at 27 out of 28 loci did not differ significantly from the expected 1:1 ratio, showing that two alternative alleles were detected at each locus. Five pairs of linked loci were detected (P < .01). Thus, this population is polymorphic and gives simple segregation for two types of molecular probes, providing a good resource for collaborative mapping of the chicken genome. (