IDENTIFYING INFERTILE HOMOZYGOUS INVERDALE (FECX(I)) EWE LAMBS ON THEBASIS OF GENOTYPE DIFFERENCES IN REPRODUCTIVE HORMONE CONCENTRATIONS

Introduction of the Inverdale prolificacy gene (FecX(1)) could markedl y improve reproductive efficiency in commercial flocks, but as homozyg ous carrier Inverdale ewes are infertile, it is imperative that these animals are identified at an early age and excluded from breeding stoc k. As the ovaries of homozygous carrier ewes are nonfunctional, there are wide differences in reproductive hormone levels between these and other Inverdale genotypes. This study assesses the accuracy of using h ormone concentrations alone, to identify infertile homozygous ewe lamb s. Ewe lambs were blood sampled at 2, 5 and/or 8 months of age, and pl asma analyzed for follicle-stimulating hormone (FSH), luteinizing horm one (LH) and inhibin content. These animals were either the offspring of both known carrier rams and known carrier ewes, and therefore would be either homozygous (II) or heterozygous (I+)for the Inverdale gene (group 1, N=122), or had one parent that was a carrier and therefore w ould be either heterozygous or noncarriers (++) of the gene (group 2, N=32). Animals were designated as either II or I+/++ on the basis of t heir plasma hormone concentrations. Inverdale genotype was also assign ed from laparoscopic observation of the ovaries at each of these occas ions. Definitive assignment of genotype was made at laparoscopy as adu lts during the breeding season. On the basis of laparoscopy as adults, 62 (51%) lambs in group 1 were identified as homozygous and 60 (49%) as heterozygous. At all three ages, both mean FSH and mean LH concentr ations were significantly higher in II than in Ii lambs. Mean inhibin concentrations were significantly lower in II lambs at 8 months, but d id not differ significantly between genotypes at 2 or 5 months of age. The use of discriminant analysis techniques to segregate individual a nimals in group 1 on the basis of their plasma FSH and LH concentratio ns, correctly identified Inverdale genotype in 50/52 (96%) lambs at 2 months, 75/79 (95%) at 5 months and 118/122 (97%) at 8 months of age. Discriminant analysis was equally effective for segregating II ewe lam bs (group 1) from fertile ewe lambs of I+ and ++ genotype (group 2, 97 % correct at 5 months and 98% at 8 months). At no stage did inclusion of inhibin concentrations into the discriminant function alter the num ber of homozygous ewes misclassified. This demonstrates that infertile homozygous ewe lambs can accurately be distinguished from their ferti le flockmates by using plasma concentrations of gonadotrophins alone, and that this can be achieved from as early as 2 months of age. (C) 19 97 Elsevier Science B.V.