Effect of selection for scrapie resistance on genetic diversity in a rare and locally adapted sheep breed: The case of Sambucana.

In Italy, since 2005 a breeding plan to increase scrapie resistance has been adopted. The impact of this selection on genetic diversity was assessed on Sambucana, an autochtonous sheep breed reared in southern Piedmont, by analysing the evolution of allele frequencies at different levels: PRNP (prion protein) gene, microsatellite loci on OAR13 (where PRNR maps), and microsatellite loci on other chromosomes, not subjected to selection for scrapie resistance. A total of 147 young rams, 80 born in 2004 and 67 in 2008–2009 were analysed. Evidence of diversity loss was observed for PRNP gene as a consequence of the directional selection. Diversity was affected in the immediate vicinity of PRNP but the effect on more distant loci on the same chromosome was trivial. With regard to neutral markers, lack of heterozygosis with no changeover of allele frequencies was observed suggesting an increase of inbreeding. Mating policies would be sufficient to solve these problems. A selection scheme based on genotyping rams and eliminating carriers of both susceptible and high susceptible alleles is the best way to improve natural resistance to scrapie with low costs and minimal problems in the current conservation programmes targeting rare breeds.