The effect of selection on casein genetic polymorphisms and haplotypes in Italian Holstein cattle

Abstract

Milk protein genes are known to be highly polymorphic. Several studies have shown the influence of milk proteins genetic variants and casein haplotypes on milk nutritional and technological properties. From 1990, the analysis of caseins polymorphism gained new attention due to the concern about possible negative effects of CSN2 Ã A 1 on human health. As a consequence, the CSN2 Ã A 2 variant gained interest and milk produced by with A 2 A 2 b-casein cows is now available in different countries. Aim of the present paper was to analyse how casein variants' and haplotypes' frequencies changed in the Italian Holstein breed, due to the health 'claims' about CSN2 Ã A 2 , and the possible effects on milk technological properties. Data were compared also with Italian Jersey cattle. A total of 223,655 Holstein and of 622 Jersey were genotyped using Illumina beadchips and data of 62 SNP in the casein cluster were analysed to reconstruct casein genotypes and haplotypes. The results demonstrate that although the selection towards b-casein A 2 is not so effective in Italy, an increase of the frequency of this allele to the disadvantage of A 1 and B alleles, included in the most favourable genotypes and haplotypes for cheese making, is occurring and should not be forced. Indeed, although a selection for the favourable j-casein B allele, and against the unfavourable E allele, is occurring and is limiting the general loss of haplotypes associated with good technological properties, the sharp decrease of the favourable B-B and I-B b-j-haplotypes, is an alert of the risk of losing useful biodiversity. HIGHLIGHTS After the concern about the b-caseinA1 variant on human health, a selection favoring the A2 allele was carried out in different countries. In Italy milk is mostly destined to cheese-making and even without a direct selection for theb-casein A2 caseins allelic frequencies are changing. Genotyping data should be used to monitor and maybe contrast the reduction of variants associated with milk favorable technological properties.

The Polymorphism in Various Milk Protein Genes in Polish Holstein-Friesian Dairy Cattle

“…Out of three casein alpha S1 (CSN1S1) genetic variants, only one (CSN1S1_3) was found to be polymorphic, although the frequency of B*B* genotype (c.*31GG) was proven to dominate at the level of 99%. Chessa et al [ 8 ] investigated the polymorphism in CSN1S1 (in the present study marked as CSN1S1_3) in Italian cattle. They reported similar frequency of allele B as in obtained in the present study: in Holstein-Friesian (HF), population frequency was at the level of 0.998, and in Italian Holstein, the population was at 0.997.…”

“…For CSN2_6, guanine is present in allele B, while the rest contain cytosine. Chessa et al [ 8 ] also investigated data coming from microarrays that contained several CSN2 SNPs that were also present in the present study. They reported similar frequencies of allele B* in HF and Italian Holstein populations for CSN2_3 (0.609 and 0.608 compared to 0.619 in the present study), CSN2_5 (0.998 and 0.998 comparing to 0.999), and CSN2_7 (0.997 and 0.999 comparing to 0.999).…”

“…Currently, the genomic assessment of cattle breeding value that is carried out in many countries, preceded by genotyping of animals with the use of Illumina microarrays, allows obtaining additional information on the variability of SNP located in genes, e.g., determining milk protein variants. Studying the results coming from microarrays allows tracking changes in SNP frequencies in a given population [ 8 ] as well as for the further analysis of the relationship between selected polymorphism and the level of dairy cows’ characteristics (e.g., the level of bioactive milk components). Variant frequencies may differ according to breed and even herd.…”

“…Variant frequencies may differ according to breed and even herd. This may suggest that the variability of different milk protein SNPs should be monitored in dairy cattle herds [ 7 , 8 ]. Through the years, a selection aiming at improving milk performance might have changed genotype frequencies of different SNPs, rendering some of them monomorphic.…”

“…Through the years, a selection aiming at improving milk performance might have changed genotype frequencies of different SNPs, rendering some of them monomorphic. Chessa et al [ 8 ] reported that selection favored one of the alleles for CSN2. They pointed that the direct selection aiming at improving production is shifting genetic variants frequencies, increasing the frequency of A2 and decreasing A1 variant.…”