The major histocompatibility complex (MHC) of mammals and birds is known to exert a major influence on both humoral and cellular immune responses. In nearly all species adequately investigated, the MHC is characterized by a high degree of polymorphism. In random breeding populations, most individuals would be heterozygous for MHC haplotypes and could thus benefit from the dominant expression of immune response genes in both of the haplotypes present. A homozygous individual, on the other hand, could benefit only from effects of the single haplotype. Thus, maximum immune response capabilities of a wild population would be favored by the continued segregation of a large number of diversified haplotypes. The primary objective of the proposed research is to determine the relative fitness of individuals homozygous versus those heterozygous for MHC haplotypes in species experiencing severe reduction in population size (endangerment). Because of the impracticality of subjecting individual members of an endangered species to the rigors of a full immunogenetic analysis, a wild species with declining population size that is also available from game farms, the ring- necked pheasant, has been selected as the primary experimental animal. Present knowledge and expertise gained from previous work with the twelve known alloantigen systems of the chicken should permit identification of corresponding or similar systems, including the MHC, in the pheasant and, to the extent possible in truely endangered avian species of special interest such as various species of cranes. The alloantisera produced in blood- typed chicken stocks will serve as serological probes in the identification of alloantigen systems in the other avian species. As haplotypes are determined for the MHC, pedigreed family groups of pheasants showing genetic segregation will be used in collaborative physiological, developmental, and disease exposure studies to investigate the specific effects of particular haplotypes in homozygous and heterozygous individuals. Much progress has been made in understanding the cellular and structural basis for the immune functions of the MHC in the mouse and human. Considerable evidence indicates that the MHC of the chicken influences several fitness traits such as hatchability of fertile eggs, growth rate, and survival in juvenile and /or adult periods of the life cycle. As yet, however, there is no clear understanding of the adaptive function of the MHC in wild populations. The ring-necked pheasant is a species living in the wild that can readily be used to investigate the role of the MHC in survival. From the apparent universal presence of an MHC in all vertebrate species and the essential similarities in immune function across species so far investigated, it is highly likely that a knowledge of the essential function of the MHC in the adaptability of a wild species will be important in genetic planning for preservation of endangered species.
