Genetics, immunoresponsiveness, and disease resistance in chickens

The experiments reported in this dissertation explored the effects of selection for antibody response on other immunological measures and on production traits. The role of thyroid hormones in antibody response was also studied. Selection for high (HA) and low (LA) antibody response to sheep erythrocytes altered subclasses of antibodies in different ways. In line LA antibody response was primarily mercaptoethanol-susceptible (IgM), while the line HA response was primarily mercaptoethanol-resistant antibody (IgG).

Sublines of HA and LA were developed with all possible combinations of major histocompatibility complex haplotypes B¹³ and B²¹. An experiment was conducted to test Marek’s disease resistance of these haplotypes in line LA. Mortality from a natural exposure was high for all three groups, and there was no difference among haplotypes.

Correlated responses of growth and reproductive traits in lines HA and LA were due to genetic correlations with antibody response. These genetic correlations were generally negative and are suggestive of differential allocation of resources. Phenotypic correlations were generally very small. Changes in allelic frequencies at alloantigen systems were also observed in response to selection.

Experiments designed to study the role of thyroid hormones on antibody responses showed no direct relationship. Chickens from lines HA and LA fed thiouracil exhibited hypothyroidism but did not differ from controls in antibody response. Differences in thyroid hormone concentrations between lines of dwarf and non-dwarf White Rocks selected for high and low juvenile body weight bore no relationship to differences in antibody responses.