Lymphoid organ size varies among inbred lines 6₃ and 7₂ and their thirteen recombinant congenic strains of chickens with the same major histocompatibility complex

The objective was to evaluate lymphoid organ size in chickens from a series of 13 recombinant congenic strains (RCS) and their highly inbred parental lines (6(3) and 7(2)). The parental line 6(3) was selected for resistance to tumors induced by Marek's disease virus and avian leukosis viruses, whereas line 7(2) was selected for susceptibility to these tumors. Each RCS on the average contains a random one-eighth of genome from the donor line 7(2). Previous studies have shown that lines 6(3) and 7(2) differ in the size of primary lymphoid organs; i.e., the bursa of Fabricius (BF) and the lobes of the thymus (T) are smaller in line 6(3) than line 7(2). In the current study, the relative size of the T, BF, and spleen was first examined in about 15 males from each of 13 RCS and the 2 parental lines at 60 to 69 d of age. The differences of relative BF, T, and spleen size among the RCS and the parental lines 6(3) and 7(2) differed significantly (P < 0.001). Males and females from 4 RCS and the 2 parental lines were evaluated a second time, and differences in the relative sizes in lymphoid organs among the RCS and parental lines were consistent. In 2 RCS, the size of the T and BF was comparatively large as in line 7(2), leading to the conclusion that different allelic forms at 1 or more loci in these RCS regulate the size of both organs. In 2 other RCS, the BF was large compared with the T, suggesting that allelic forms at some loci in these RCS influence the BF independent of the T. The relative lymphoid organ size among the RCS appeared to cosegregate with the concentration of IgG in the plasma measured previously. The evaluation of genomic variability of these lines is underway, and the RCS are available for research on traits that differ between lines 6(3) and 7(2).