Bianca Moioli, Luigi De Grossi, Roberto Steri, Silvia D’Andrea, Fabio Pilla
The marker-assisted selection exploits anonymous genetic markers that have been associated with measurable differences on complex traits; because it is based on the Linkage Disequilibrium between the polymorphic markers and the polymorphisms which code for the trait, its success is limited to the population in which the association has been assessed. The identification of the gene with effect on the target and the detection of the functional mutations will allow selection in independent populations, while encouraging studies on gene expression. The results of a genome-wide scan performed with the Illumina Ovine SNP50K Beadchip, on 100 sheep, 50 of which positive at paratuberculosis serological assessment, identified two candidate genes of immunity response, the PCP4 and the CD109, located in proximity of the markers with different allele frequency in positive and negative sheep. The coding region of the two genes was directly sequenced: three missense mutations were detected: two in the PCP4 gene and one in the second exon of the CD109 gene. The PCP4 mutations had a very low frequency (.12 and .07) so making hazardous to hypothesize their direct effect on immune response. On the contrary, the mutation detected in the CD109 gene showed a strong linkage disequilibrium with the anonymous marker. Direct sequencing of the DNA of sheep of different populations showed that disequilibrium was maintained. Allele frequency at the hypothesized marker associated to immune response, calculated for other breeds of sheep, showed that the marker allele potentially associated to disease resistance is more frequent in the local breeds and in breeds that have not been submitted to selection programs.