Heat Stress in Desert Sheep and Goats: Signatures of Natural Selection and Initiation of GWAS

Barki sheep and goats are indigenous to the Coastal Zone of the Western Desert in Egypt, and thus well adapted to desert conditions. In an initiative to characterize the genomic regions involved in adaptation to heat stress, 68 Barki sheep and 59 Barki goats, were genotyped using Illumina 50k SNP chips. The genotyped animals represented heat tolerant and susceptible phenotypes that had been selected from 240 sheep and 147 goats.  Tolerance levels to heat stress were defined by a physiological index composed of observed changes in thermal, respiratory and heat production parameters after exposure to physical exercise under high temperature. Signatures of   natural selection were examined based on the extent of haplotype homozygosity (iHS).  Several candidate regions were detected across the genome. Similarity of these small ruminant genomes allowed finding evidence of common selection between goats and sheep under the same geographic region. Barki share a consensus selection signature that is located in a conserved syntenic segment between 35 and 43 Mb on chromosome 10 in sheep and chromosome 12 in goats. The F_ST analysis of European Texel and Romney sheep vs. Barki sheep, demonstrated substantial genomic differences, whereas Barki goats did not show explicit differences when compared to the Mediterranean LaMancha goat. Association of stress tolerance traits with candidate autosomal regions was also examined. The results present an example of consensus signatures of natural selection and initiate GWAS studies for tolerance to heat stress. This may provide signals to identify genes of adaptation to the studied desert conditions.