Lambs with Scrapie Susceptible Genotypes Have Higher Postnatal Survival

BACKGROUND: Prion protein (PrP) alleles associated with scrapie susceptibility persist in many sheep populations even with high frequencies despite centuries of selection against them. This suggests that scrapie susceptibility alleles have a pleiotropic effect or are associated with fitness or other traits that have been subject to selection. METHODOLOGY/PRINCIPAL FINDINGS: We genotyped all lambs in two scrapie-free Scottish Blackface sheep flocks for polymorphisms at codons 136, 154 and 171 of the PrP gene. We tested potential associations of the PrP genotype with lamb viability at birth and postnatal survival using a complementary log-log link function and a Weibull proportional hazard model, respectively. Here we show there is an association between PrP genotype, as defined by polymorphisms at codons 154 ad 171, and postnatal lamb survival in the absence of scrapie. Sheep carrying the wild-type ARQ allele have higher postnatal survival rates than sheep carrying the more scrapie-resistant alleles (ARR or AHQ). CONCLUSION: The PrP genotypes associated with higher susceptibility to scrapie are associated with improved postnatal survival in the absence of the disease. This association helps to explain the existence, and in many instances the high frequency, of the ARQ allele in sheep populations

Autoregressive repeatability animal models for the analysis of first lactation test day records of Holstein cows

Animal models with first order autoregressive (AR(1)) covariance structures for permanent environmental effects (ARpe), residual effects (AR e) or genetic and residual effects (ARae) of test day records (TD) were examined. The models with AR(1) covariance structures were compared together and with a simple test day repeatability model with compound symmetry (CS) covariance structure for TD environmental effects and a 305-day lactation model. Data consisted of 106,472 records (TD data) of 12,071 first lactation Holstein cows (305-day data). Estimates of genetic and environmental components of variance and autocorrelation coefficients were obtained for milk, fat, and protein yields and somatic cell scores (SCS) using ASReml. Likelihood ratio tests indicated that models with the AR(1) covariance structures were significantly more appropriate for the TD data for all traits than the CS model. Estimates of heritability were slightly less with the models with the ARe or ARae covariance structures (0.09) than with the CS model (0.10 to 0.11) for yield traits. All TD models resulted in similar estimates of heritability for SCS (0.06). Estimates of residual variance may have been underestimated with the CS model compared with TD models with the ARe or the ARae covariance structures. Estimates of heritability with the 305-day model for all traits were in the range of 0.11 to 0.36. The predicted breeding values (PBV) with different TD models were highly correlated (0.98 to 1.00). The PBV with the ARae Model were not the same at different test days and tended to decrease with advancement of the lactation for SCS. The smallest estimates of accuracy of PBV were at the beginning and at the end of lactation. The AR(1) covariance structure for TD residual or for both genetic and residual effects may help to prevent overestimation of heritability and repeatability for milk, fat and protein yields and SCS compared with simple repeatability model with the CS covariance structure