Search

Abstract: Ovine susceptibility to scrapie is largely controlled by polymorphisms in the PRNP gene. Beginning in 2003, breeding programmes based on the known association between this gene and the susceptibility/resistance status of this disease have been implemented in many European countries. This is not the case in Turkey, where the PRNP gene was not genotyped in native sheep breeds until recently. We sequenced the complete open reading frame of the PRNP gene in 100 sheep belonging to five native Turkish sheep breeds (Akkaraman=21; Morkaraman=20; Tuj=17; Hemsin=23; Karayaka=19). Based on the variability found at codons 136, 154 and 171 (also referred to as standard codons), we determined six alleles VRQ, TRQ, ARR, ARH, ARK and ARQ. The archetype, ARQ, was the most frequent variant in each of the five breeds (across-breed frequency=0.710), while the second most frequent was the resistance-associated ARR allele (0.140). The susceptible VRQ allele exhibited the lowest frequency (0.015). The AHQ allele was not found in any of the analysed breeds. Beyond variability in the standard codons, we found ten additional amino acid variants (M112T, G127S, G127V, M137T, L141F, H143R, N146S, Y172D, Q189L and V213E), one of which, V213E, has not been previously described to our knowledge. Overall, this report may serve as a complement of previous studies on the genetic background of native Turkish sheep regarding the resistance/susceptibility status to classical and atypical scrapie. [Copyright &y& Elsevier]

Genetic relationships among Turkish sheep breeds were analysed on the basis of 30 microsatellite markers. Phylogenetic analyses based on the estimation of genetic distances revealed the closest relationships for the Akkaraman, Morkaraman and Tuj breeds, which were clearly differentiated from the others in the dendrogram. Our pattern was completely confirmed by results from the Factorial Correspondence Analysis. All the results described analysing either population parameters or individuals revealed a clear separation between the fat-tailed group and the others. These results, based on nuclear DNA, are discussed along with those already reported for these breeds through the investigation of mitochondrial DNA, which had revealed the invaluable significance of the genetic background of these Turkish sheep.

The variability of mtDNA was analysed in local sheep breeds reared throughout Turkey, for which a fragment of the D-loop region and the complete cytochrome b were sequenced. Phylogenetic analyses performed independently for the D-loop and the Cyt b gene revealed three clearly separated clusters indicating three major maternal lineages, two of which had been previously described as types B and A. The new type, C, was present in all the breeds analysed and showed considerable mtDNA variability. Divergence time was obtained on the basis of Cyt b gene and was estimated to be around 160,000-170,000 years ago for lineages B and A, whereas the divergence of lineage C proved to have occurred earlier (between 450,000 and 750,000 years ago). These times greatly predate domestication and suggest that the origin of modern sheep breeds was more complex than previously thought and that at least three independent sheep domestication events occurred. Our results, together with archaeological information and the current wild sheep populations in the Near East region support the high importance of this area in the sheep domestication process. Finally, the evidence of a third maternal lineage has important implications regarding the history of modern sheep.