Analysis of MHC class II and class IV restriction fragment length polymorphism in chicken lines divergently selected for multitrait immune response

In the present study, chickens of four lines divergently selected for high (H) and low (L) immunocompetence in replicate were analyzed to investigate polymorphisms of MHC class II and MHC class IV on the molecular level assoc iated with selection. The long-term selection experiment for multitrait imm unocompetence was carried out in replicates and allows, therefore, the oppo rtunity to distinguish effects of selection from other genetic factors. The SacI-digested DNA was hybridized individually with MHC class LI and MHC cl ass IV gene probes. The MHC class II RFLP analysis revealed four polymorphi c bands and only one of tl;em showed a significant difference between the s election directions H and L pooled between replicates. The small frequency differences of this band relative to the long-term selection suggest that t his MHC class II fragment may contain genetic elements that are only slight ly associated with the immune response traits used for selection. The hybri dization with the MHC class IV probe displayed 26 scorable bands, of which 18 were polymorphic. Tn most instances, the differences between the lines w ere likely caused by the influence of genetic factors other than selection for multitrait immunocompetence. Only one band displayed a consistency in d ifference between selection directions in both replicates and no frequency difference between replicates. This band was almost completely absent in bo th ii sublines, but at a frequency of about 50% in both L sublines. The gen eral results of this study did not reveal major differences in band frequen cies that indicate a close association of MHC class II and MHC class TV pol ymorphic markers to the divergent selection for multitrait immune response. Although the MHC makes a crucial contribution in immune response, it may h ave been difficult to detect single-gene associations with the selection cr iteria of this study, because of the myriad of components contributing to g eneral immune responses measured in vivo.