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CSN1S2 gene encodes the αS2-casein, one of the phosphoproteins (αS1, β, αS2 and κ) secreted in ruminants milk in form of stable calcium–phosphate micelles. CSN1S2⁎B variant contains the amino acid substitutions p.Asp75Tyr and p.Ile105Val in the mature protein. Recently, we have shown that two adjacent SNPs, [FN_601350: g.153G>T and 155C>T], located at exon 10, are causers of the p.Asp75Tyr substitution, while the non-synonymous mutation SNP [FN_601350: g.504A>G], in exon 11, led to codon exchange of p.Ile105Val. PCR-RFLP and RT-PCR analyses were used to detect and characterize the single nucleotide polymorphisms involved in the B variant of the CSN1S2 gene and to assess their effects on milk yield and composition traits. Four hundred and thirty samples from different breeds (Berrinchon du cher, Castellana, Ile de France and Merino) were analyzed and population genetic parameters were estimated for each locus and breed. Both polymorphic sites of the SNPs g.[153G>T and 155C>T] are conserved across analyzed breeds. Haplotype allele frequencies from the SNPs g.[153G>T and 155C>T]—GC, GT, TC and TT—varied among the breeds, being GC the most frequent allele in all populations (>0.5). Allele G from SNP g.504A>G had the minor allele frequency in all breeds analyzed (ranged from 0.225 to 0.333). The value of genetic differentiation (FST=0.0419) across breeds and loci indicates that there was no genetic divergence among the analyzed breeds. The haplotype frequencies, the deduced amino acid haplotypes and the occurrence of linkage disequilibrium were estimated in a large population (377 samples) of Merino sheep breed. The eight possible haplotypes of these two loci were identified, and substantial linkage disequilibrium was found. GCA was the haplotype most frequent (>70%) and Asp75–Ile105 the most frequent aminoacid haplotype (0.713). Recombinants haplotypes were observed in low frequencies (≤0.1). Association study between the ovine CSN1S2⁎B polymorphisms and the milk traits were estimated in 669 lactations from 218 Merino ewes. The CSN1S2 p.75 and CSN1S2 p.105 polymorphisms affected the non-fat solids and lactose percentages in milk, respectively. The CSN1S2 Tyr75Tyr ewes had the highest levels of non fat solids and the ewes carrying CSN1S2 Val105Val genotype had the smallest lactose percentage. Similar effects of the single markers have been observed for homozygous Tyr75–Val105 ewes on lactose percentage. This work provides the basis to study in further sheep breeds the CSN1S2⁎B polymorphisms and their effects on milk traits. |
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