CAN B-HAPLOTYPE MARKERS BE USED FOR GENETIC SELECTION TO INCREASED DISEASE RESISTANCE IN BROILERS

The immune system is the major physiological defense against diseases in poultry. We hypothesized that viability of chickens can be genetica lly improved by selection. High polymorphism of Restricted Fragment Le ngth Polymorphism (RFLP) markers was revealed in the MHC's Class IV (B -G) region following digestion of DNA from meat-type chickens with the restriction enzymes Pvu-II or Bgl-II. These markers facilitate the de termination of MHC haplotypes (at the B-G region) in meat-type stock. Following these findings, the hypothesis that our lines selected for h igh (HC) or low (LC) antibody response at young age (10 days) may diff er in their MHC loci was evaluated by comparing the existence and freq uency of MHC markers (single RFLP bands) and haplotypes (patterns of R FLP bands), and their association with antibody titers. One hundred bi rds were examined for MHC evaluation, approximately a half from each s election direction (LC or HC). Twenty five RFLP bands were obtained us ing MHC Class-IV probe following digestion with the restriction enzyme s Pvu-II or Bgl-II. The frequency of several bands was significantly d ifferent between the two selection lines. Fixation of RFLP markers was not expected along the selection, and indeed most RFLP bands were pre sented in both LC and HC lines. Therefore data from all lines were com bined, and the difference in mean E. coli antibody titer was tested fo r each RFLP band between birds which carry it (+) and those which do n ot (-). A significant effect on antibody titers was found for those ba nds which were found at different frequencies in the selected lines. S ince no RFLP band was presented in one line only, and the distribution of individual titers within lines considerably overlapped, informatio n of LC and HC birds was combined for analysis of the association betw een bands and titers. Estimating the effects of all the significant ba nds simultaneously (by multiple regression) revealed that in most case s they are grouped to patterns which could be identified visually as w ell as by statistical means. The interpretation of these patterns as h aplotypes of the MHC B-G's region was followed by evaluation of their frequencies within the selection line, and their effect on antibody ti ters, similarly to the initial analysis of the individual RFLP bands' data. Several haplotypes existed only or mainly among either LC or HC birds. Their effects on antibody titers in the pooled data were estima ted by multiple regression analysis, conducted separately for each typ e of antigen. It was assumed that the pool data represent the base pop ulation. Five haplotypes were found to have a significant effect on th e level of antibody to E. coli and together they accounted for 48% (R2 ) of the antibody's total phenotypic variation. Three of these haploty pe had also significant effects, in the same direction, on antibody ti ters to NDV (R2 = 22%), and one haplotypes was associated with antibod y titers to SRBC (R2 = 12%). It is possible that the RFLP bands that f orm the patterns (i.e., haplotypes) with significant effects on the di fferent antibody titers, are markers of loci within the MHC with nonsp ecific effects on antibody response of young chicks. Such loci could b e responsible for the correlated changes in the early immune function due to selection of early antibody response to E. coli vaccination. Th e feasibility of selecting meat type chickens for earlier and faster i mmune response was demonstrated. There are indications that RFLP marke rs of the MHC may serve as an efficient selection criterion. The direc t and correlated effects suggest that such selection may improve the v iability of vaccinated broilers, reduce the need for further pharmaceu tical treatments and consequently improve production efficiency and qu ality of poultry meat.