Determining the relationship between restorative potential and size of a gene bank to alleviate the risks inherent in a scrapie eradication breeding programme

Roughsedge, T.; Villanueva, Beatriz; Woolliams, John

doi:10.1016/j.livprodsci.2005.09.005

Cited by 15 publications

(10 citation statements)

References 20 publications

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“…Primarily these risks are associated with novel TSE agents that are as yet unknown but may emerge in the future, as discussed above, and the possible loss of favourable attributes of a breed either through linkage or pleiotropic effects when selecting on a specific PrP alleles. To guard against this risk, semen gene banks have been proposed and designed [31], and now implemented in Great Britain and Northern Ireland, to provide the potential for future reintroduction of removed alleles or genotypes. Further, these semen banks have been designed accounting for the specific population dynamics and demographics of each sector of the UK sheep industry.…”

Section: Resultsmentioning

confidence: 99%

Progress and limits of PrP gene selection policy

Dawson¹,

Moore

Bishop

2008

Vet. Res.

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-Classical scrapie has proved to be a notoriously difficult disease to control due to a poor understanding of its natural history. The recognition of disease risk linkage to PrP genotype has offered the prospect of a disease control strategy, viz. genotyping and selective breeding, novel to veterinary medicine when first considered in the 1990s. The UK Spongiform Encephalopathy Advisory Committee recommended the exploitation of this approach in a voluntary, national programme to control classical scrapie and protect the public against food-borne exposure to bovine spongiform encephalopathy, should the national flock have been exposed via contaminated feed. The National Scrapie Plan for Great Britain was launched in 2001 and uptake has been widespread throughout the purebreeding sector of the sheep industry, with membership peaking at over 12 000 flocks in 2006. A total of 700 000 rams from 90 breeds have been genotyped. A comparison of ram lambs born in 2002 with those in 2006 shows evident changes in PrP genotype frequencies which are predicted to be associated with a reduction in disease risk. Various concerns have been raised regarding possible unintended consequences of widespread selection on PrP genotype, including impacts on other performance traits and possible effects on inbreeding and genetic diversity. To date, these concerns appear to be unfounded, as no consistent associations have been found with performance traits, nor are there likely to be any detectable impacts on inbreeding in mainstream breeds. Currently, semen banks have been implemented in Great Britain to store samples from animals of all common PrP genotypes, should these genotypes be required in the future. Various strategies to minimise future disease risks are discussed in the paper.

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Section: Resultsmentioning

confidence: 99%

Progress and limits of PrP gene selection policy

Dawson¹,

Moore

Bishop

2008

Vet. Res.

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“…The combined use of genealogical and molecular information in this scenario of conservation can be recommended. The implementation of a germplasm bank in order to manage the risks of novel ovine PrP polymorphisms and losses of genetic potential is also recommendable (Roughsedge et al, 2006).…”

Section: Discussionmentioning

confidence: 99%

Genetic diversity loss due to selection for scrapie resistance in the rare Spanish Xalda sheep breed

Álvarez¹,

Royo²,

Gutiérrez³

et al. 2007

Livestock Science

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The effect of selection for scrapie resistance on genetic variability in the rare Xalda sheep breed was studied. Pedigree information comprised 1851 animals (1444 alive) at the moment of sampling. A total of 304 reproductive (or selected for reproduction) Xalda individuals were sampled and genotyped for 14 microsatellites. Genetic variability was assessed using: gene diversity (1 − average kinship, GD), mean average relatedness (AR) and self-coancestry (c i ) at the genealogical level; and expected heterozygosity (H e ), molecular mean kinship (Mk), molecular self-coancestry (s i ) and rarefacted average number of alleles per locus (A) at the molecular level. Two breeding strategies were evaluated: a) use of only young rams with genotype ARR/ARR and young ewes with low to moderate risk (risk groups R1 to R3); b) breeding without selection for PrP genotypes. The major cause of losses of genetic variability in the Xalda breed is the drift that occurs when a new group of reproductive individuals is selected. The loss of genetic variability is small in females compared to the males, where it is considerable. However, losses at the molecular level for young females with respect to adult females were above 5%. Young male individuals also retained most of the genetic variability assessed in adult rams. Selection against susceptibility to scrapie produced additional losses of more than 2% for H e and A when rYF individuals are considered. As regards males, the situation becomes critical because of the scant number of available ARR/ARR young rams. The consequences for the management of the Xalda breed are discussed.

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“…However, this strategy might not be advisable since, in addition to the concurrent reduction in genetic progress for performance traits and the high genotyping costs involved, fixation of the ARR allele is risky given the uncertainty surrounding scrapie [4,5]. A safeguard is to maintain the alternative alleles in an appropriately designed gene bank [14,23]. In general, strategies leading to faster increases in the frequency of the ARR allele also led to slower rates of gain in performance.…”

Section: Discussionmentioning

confidence: 99%

Predicting the consequences of selecting on PrP genotypes on PrP frequencies, performance and inbreeding in commercial meat sheep populations

et al. 2007

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-Selection programmes based on prion protein (PrP) genotypes are being implemented for increasing resistance to scrapie. Commercial meat sheep populations participating in sire-referencing schemes were simulated to investigate the effect of selection on PrP genotypes on ARR and VRQ allele frequencies, inbreeding and genetic gain in a performance trait under selection. PrP selection strategies modelled included selection against the VRQ allele and in favour of the ARR allele. Assuming realistic initial PrP frequencies, selection against the VRQ allele had a minimal impact on performance and inbreeding. However, when selection was also in favour of the ARR allele and the frequency of this allele was relatively low, there was a loss of up to three to four years of genetic gain over the 15 years of selection. Most loss in gain occurred during the first five years. In general, the rate of inbreeding was reduced when applying PrP selection. Since animals were first selected on their PrP genotype before being selected on the performance trait, the intensity of selection on performance was weaker under PrP selection (compared with no PrP selection). Eradication of the VRQ allele or fixation of the ARR allele within 15 years of selection was possible only with PrP selection targeting all breeding animals. sire referencing / scrapie / prion / PrP selection / inbreeding

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Determining the relationship between restorative potential and size of a gene bank to alleviate the risks inherent in a scrapie eradication breeding programme

Cited by 15 publications

References 20 publications

Progress and limits of PrP gene selection policy

Progress and limits of PrP gene selection policy

Genetic diversity loss due to selection for scrapie resistance in the rare Spanish Xalda sheep breed

Predicting the consequences of selecting on PrP genotypes on PrP frequencies, performance and inbreeding in commercial meat sheep populations

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