Genetic architecture of antibody responses of chickens to sheep red blood cells

Genetic resistance to disease development is complex and involves several systems of the body with the immune system being an important component (WARNER et al. 1987; MALE and ROITT 1993). Selection for resistance to particular diseases tends to be specific and has little effect on general disease resistance required in modern livestock and poultry production (CARSON 1951; GAVORA 1990). Correlated responses to selection for antibody titres to foreign proteins provides resistance to a wider range of diseases than would be achieved by selection for individual disease resistance, however, the enhanced resistance can not be generalized to ail diseases. In a two-way selection experiment for high or low antibody response to sheep red blood cell(SRBC) antigens, the high line exhibited stronger antibody to Newcastle disease, was more resistant to Mycoplasma gallisepticum, Eimeria necatrix, a splenomeglia virus, and feather mites, but was more susceptible to Escherichia coli and Staphylococcus aureus infection than the low line (GROSS et al. 1980). Subsequent studies with these lines showed greater resistance by the high than low line to Eimeria tenella (MARTIN et al. 1986) and that the high line exhibited greater immunoresponsiveness to Brucella abortus than the low line (DUNNINGTON et al. 1992). There is need to understand more about relationships between general immune response and disease resistance. Working with immune responses has an added benefit in that commercial breeding populations need not be exposed to infectious agents. Efficient utilization of genetic variation in immunoresponsiveness in livestock and poultry breeding where crossing of populations predominates requires an understanding of the modes of inheritance of immune traits. Such information on poultry is rather limited. Considerable responses to selection and moderate realized heritability estimates (SIEGEL and GROSS 1980; MARTIN et al. 1990; PINARD et al. 1992) suggest substantial additive genetic variation for antibody response to SRBC antigens. In addition, the influence of major genes of the MHC on immunoresponsiveness has been reported (GUNTHER et al. 1974; DUNNINGTON et al. 1989), although its importance relative to the rest of the genome appeared minor (PINARD and VAN DER ZIJPP 1993). Heterosis appeared important in some studies (SIEGEL et al. 1982), but was minimal in others (PINARD and VAN DER ZIJPP 1993). Through long-term divergent selection, lines of chickens have been developed that differ markedly in antibody response to SRBC. This paper reports on the genetic architecture of response to SRBC antigen based on various crosses of these lines of chickens.