Effects on inbreeding of different strategies aimed at eliminating scrapie sensitivity alleles in rare sheep breeds in The Netherlands

Abstract

The Dutch scrapie eradication programme aims at the exclusive use of homozygous ARR/ARR breeding rams by the end of 2004. As a consequence, breeds with a small population size and a low frequency of the ARR allele may suffer unacceptable losses of genetic diversity and high inbreeding levels. We simulated three breeding strategies to assess their efficacy in eliminating scrapie sensitive alleles and their effect on inbreeding levels. Under mild selection, both homozygous and heterozygous ARR rams were used indiscriminately. Under moderate selection, homozygous ARR rams were used preferably but they were supplemented with heterozygous rams when necessary. Under severe selection, only homozygous rams were used. Severe selection mimics then the proposed eradication programme. Simulations were carried out with allelic frequencies and population parameters of existent rare breeds in The Netherlands. With severe selection all simulated breeds showed unacceptably high inbreeding rates (> 0·5% per year). For some breeds, moderate selection resulted in acceptable inbreeding rates, while for other breeds only mild selection resulted in acceptable rates. The frequency of the ARR allele after 5 years of selection was only slightly lower with moderate selection than with severe selection (0·8% lower on average), but it was clearly lower with mild selection. Based on these simulations, we propose a selection programme where with low frequencies of the ARR allele, mild selection is used initially. Once the ARR frequencies rise to a particular value, the switch can be made to moderate selection. The population size (e.g. below 750, 750 to 3750 and above 3750 ewes) determines the frequency of the ARR allele at which the switch can be made (33%, 25% and 10%, respectively). With even higher ARR frequencies (above 70%, 50% and 33%, respectively) the regime can be changed to severe selection.

Prion protein genetic polymorphisms and breeding strategies in Portuguese breeds of sheep

“…This problem was studied by Windig et al (2004) who have concluded that severe selection for ARR leads to unacceptably high rates of inbreeding, especially in rare breeds with low ARR frequencies. Nevertheless, if the selection program is designed for a longer period of time, the negative impact of inbreeding and possible founder effects can be minimized, as discussed by Gama et al (1992) in the framework of introgression of a desirable transgene from one founder into a breeding population, which is similar to selection for fixation of a desirable genetic marker from one or a few founders, as was the case here for some of the breeds.…”

Genetic predisposition of some Bulgarian sheep breeds to the scrapie disease

“…It is very likely that this is a result of the synergism between two factors: the unplanned breeding of animals Virus Genes (2011) 43:153-159 157 with ARR allele in the parent flocks and the small populations of the breeds. That is why, in these breeds and Replyan sheep it is appropriate to apply the Dutch model of selection, which includes mild (using homozygous and heterozygous ARR rams for breeding, without preferences), moderate (using preferentially homozygous ARR rams) and severe selection (using only homozygous ARR rams) [48]. The prevalence of the ARR allele in Local Stara Zagora sheep (41.25%) allows the simultaneous elimination of homozygous VRQ.…”

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

“…The third risk identified was that of breeds being taken through a genetic bottleneck due to the eradication programme (Windig et al, 2004), resulting in loss of genetic variation. The breeds most at risk are those already classified rare and those breeds with a low proportion of the ARR haplotype, which will potentially be made rare by the programme.…”

“…The approach to achieving this aim varies between countries. For example the Dutch scrapie eradication plan only allows matings using homozygous ARR rams (Windig et al, 2004), whereas the UK National Scrapie Plans allow additional options for matings using heterozygous rams carrying ARR, ARH, AHQ or ARQ (Defra, 2004). Ultimately both approaches will eradicate the VRQ haplotype and increase the frequency of the ARR haplotype, although the Dutch approach is more likely to drive the ARR to fixation in a shorter time period, with one consequence of this being a greater threat to their genetic base.…”

“…However, these studies have been limited in scope and available data, although extending information to other traits and breeds is currently underway. The third risk (lost variation) of selecting exclusively on the PrP genotype is the potential genetic bottle neck caused by limiting selection for breeding to animals of particular genotypes (Windig et al, 2004). This mainly applies to breeds that are already at risk because of their small population size, and to breeds likely to be put at risk by the programme because they have a low frequency of the favourable genotype.…”