THE MHC CLASS-II REGION AND RESISTANCE TO AN INTESTINAL PARASITE IN SHEEP

RFLP analysis was used to investigate the effect of genetic variation in the Major Histocompatibility Complex (MHC) class II region on resistance of sheep to the intestinal parasite Trichostrongylus colubriformis, using faecal egg count (fee) as the measure of resistance. RFLP analysis of DNA from 335 sheep with the restriction enzymes TaqI, PvuII and HindIII, and DRB, DQA and DQB human class II MHC cDNA clones, revealed 238 bands, of which 233 were polymorphic. Sixteen bands were associated with a significant effect on fee, when analysed using a mixed model, best linear unbiased prediction statistical methods. However, when p values mere corrected for the number of bands tested the associations were no longer significant. While no individual band had a significant effect on fee, Fisher's X(2) test of the distribution of p-values showed that RFLP bands, when considered together, do account for a significant. proportion of variation in fee. One hundred and thirteen offspring from 11 halb-sib families were classified according to which MHC haplotype they had inherited from their sire. While there was overall no convincing statistical evidence of an effect of MHC haplotype on resistance, results in one family showed evidence for a repeatable MHC effect.