The dynamics of hereditary scrapie resistance in Estonian sheep breeds

Abstract

A total of 2,411 sheep from the Estonian Whitehead (N = 1301) and Estonian Blackhead (N = 1110) breeds were genotyped for markers of scrapie risk in the period 2005–2017. Sanger sequencing was used to identify nucleotide substitutions in the PRNP gene at codons 136, 141, 154 and 171 to determine corresponding amino acids in prion protein. Sheep were divided into 13 groups according to their birth year to assess the temporal changes on the genetic profile of the studied population. Seven different alleles and 16 different genotypes (12 for Estonian Blackhead and 16 for Estonian Whitehead) were identified. In the birth year group 2007 (two years after the introduction of a scrapie eradication programme) a statistically significant change in the allele ALRR frequencies was found. The selective breeding for allele ALRR (the scrapie resistant marker) increased in frequency from 0.40 to 0.70 during the period 2006–2017. Significant declines in allele frequencies ALRQ (0.40–0.20) and ALRH (0.13–0.03) were found. The frequency of the most susceptible to scrapie allele (VLRQ) was low (< 0.03), and the decrease of the frequency during the program was insignificant. Both sheep breeds showed statistically significant changes in allele, genotype and risk group frequencies between the birth year groups before the introduction of the scrapie eradication programme and at its end. The effect on genetic profile in terms of changed allele frequencies was statistically more significant in the Estonian Whitehead sheep (p < 0.001) than in the Estonian Blackhead sheep (p < 0.05).

Keywords

• scrapie risk
• scrapie eradication program
• Estonian sheep breeds
• PRNP allele frequency