NR AUMK
AU de Vries,F.
TI Zucht auf Scrapie-Resistenz beim Schaf
QU Inaugural-Dissertation zur Erlangung des Grades eines Doktors der Veterinärmedizin (Dr. med. vet.) durch die Tierärztliche Hochschule Hannover, 2.12.2004
PT Dissertation
AB
Breeding Scrapie resistant sheep
The objectives of this study were to investigate basic principles for the development of a breeding programme towards scrapie resistance in sheep. The results of this study should be useful to support the preparation of national breeding programmes towards transmissible spongiforme encephalopathy (TSE) resistance. Based on an EU decision these breeding programmes are voluntary until April 2005. After that date they are compulsory for all sheep flocks of high genetic merit in all EU member countries.
The ARR allele which is associated with scrapie resistance could be detected in 41 breeds. The frequencies were lowest for East Friesian milk sheep and a lot of land sheep breeds and highest for the most meat sheep breeds. A simple simulation study for selection of the ARR allele was performed in two hypothetical populations of different size. A successful selection towards the homozygous ARR/ARR genotype was feasible in 6 to 9 generations in all populations only by genotyping the rams. Additionally genotyping of females reduce the duration of the selection programme for 2 generations. Selection for the ARR/ARR genotype leads to a threefold increase of the inbreeding rate. In order to perform association studies, genetic parameters were estimated for performance traits in different meat, milk and land sheep breeds. For this analysis the data of sheep breeding organisations from Lower Saxony and Westphalia were used that had been collected on the days of licensing and of herdbook registration. The performance traits included score for muscle mass, wool quality and type, the reproduction traits early lambing (? 550 days), late lambing (>550 days), first lambing interval, second lambing interval and total number of lambs born.
Association studies were performed between the prion protein genotypes and different meat, type, wool, reproduction and milk performance traits of different sheep breeds. Linear animal models based on the genetic parameters estimated in previous analyses were employed for the analysis of the PrP-genotype effect. The models used for the analyses included systematic fixed and random effects, regression coefficients and the random additive genetic effect of the animal. In most cases, no significant associations were found between the PrP genotypes and the performance traits investigated. For the traits age at first early lambing in German Texel, second lambing interval, depth of back muscle, daily live weight gain in German black-headed mutton and second lambing interval in Suffolk a significant association for the PrP genotype could be observed, but the interpretation in these mentioned associations could be affected by the small number of animals involved. Significant associations were found for East Friesian milk sheep between the ARR allele and the traits withers height and heart girth. The average withers height and heart girth of ARR/ARR sheep were 2.7 cm and 5.1 cm lower, respectively, than in ARQ/ARQ animals, and the traits rump height and trunk length also showed the same tendency. The results indicate that selection towards scrapie resistance based mainly on ARR homozygous animals may lead to smaller East Friesian milk sheep. Moreover, breeding towards scrapie resistance has a significant effect on the distribution of ?S2- casein genotypes in East Friesian milk sheep and may positively affect milk fat yield through the genotypic effects of this milk protein. Some genotyped sheep showed significantly superior performance traits in comparison to the non-genotyped animals. An explanation for this might be that breeders seek to minimise genotyping costs by pre-selecting animals which do not meet the top breeding requirements.
The effects of different strategies to breed toward the ARR/ARR homozygous genotype in sheep on inbreeding, drift variance, possible negative side effects, bottle neck effects and breeding costs were demonstrated. The analyses were used to find an efficient strategy for the breeds East Friesian milk sheep, German Grey Heath, German black-headed mutton. In general breeds with low ARR allele frequency in the founder animals or in endangered breeds should not breed directly towards scrapie resistance. The strategy should be firstly to breed towards ARR heterozygous sheep until a threshold value is reached. The threshold value should be al least 30%. After this the strategy should be to breed towards ARR homozygous sheep. This is also the best strategy for breeds who will show a negative side effect.
SP englisch
PO Deutschland