Abstract
Breeding sheep populations for scrapie resistance could result in a loss of genetic variability. In this study, the effect on genetic variability of selection for increasing the ARR allele frequency was estimated in the Latxa breed. Two sources of information were used, pedigree and genetic polymorphisms (fifteen microsatellites). The results based on the genealogical information were conditioned by a low pedigree completeness level that revealed the interest of also using the information provided by the molecular markers. The overall results suggest that no great negative effect on genetic variability can be expected in the short time in the population analysed by selection of only ARR/ARR males. The estimated average relationship of ARR/ARR males with reproductive females was similar to that of all available males whatever its genotype: 0.010 vs. 0.012 for a genealogical relationship and 0.257 vs. 0.296 for molecular coancestry, respectively. However, selection of only ARR/ARR males implied important losses in founder animals (87 percent) and low frequency alleles (30 percent) in the ram population. The evaluation of mild selection strategies against scrapie susceptibility based on the use of some ARR heterozygous males was difficult because the genetic relationships estimated among animals differed when pedigree or molecular information was used, and the use of more molecular markers should be evaluated.
Highlights
Genetic control programmes of scrapie susceptibility are underway in several countries founded on the strong association of scrapie with the prion protein (PrP) genotype
The genetic variability results obtained from pedigree information are not comparable among populations because, as pointed out by Huby et al [19], populations are usually different in total size, collective organisation for selection or conservation, development of artificial insemination, paternity control, etc
Some results are available for comparison from the few studies focussing on sheep that have been carried out to analyse the genetic variability based on pedigree information [15, 18, 19, 23, 26]
Summary
Genetic control programmes of scrapie susceptibility are underway in several countries founded on the strong association of scrapie with the prion protein (PrP) genotype. [7, 25]), in the European Union, the breeding programmes are still aimed at increasing the frequency of the ARR allele while reducing the prevalence of those alleles (especially the VRQ) which have been shown to contribute to susceptibility to scrapie [11]. The selection of animals for increasing the ARR allele frequency could result in a loss of genetic variability and a reduction in the effective size of sheep populations implying that genetic drift will increase, as well as inbreeding. These consequences are especially dangerous in breeds facing extinction, and important in populations undergoing selection because of the loss of the heritable part of the genetic variance. It is of interest to evaluate the magnitude of this effect to take it into account to design the selection strategy
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