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Theses

The genetic background of bovine αs1- and αs2-casein phosphorylation

Abstract : Phosphorylation of caseins (CN) is a crucial post-translational modification allowing caseins to aggregate as micelles. The formation and stability of casein micelles are important for transporting abundant minerals to the neonate and manufacturing of dairy products. Therefore, it is of great interest to explore variation in degrees of phosphorylation of caseins and study to what extent genetic and other factors contribute to this variation. This thesis aimed to investigate the genetic background of bovine milk protein composition with a focus on phosphorylation of αs1- and αs2-CN. Thus, two studies were conducted to quantify phosphorylation levels of αs1- and αs2-CN: one in French Montbéliarde (FM) using liquid chromatography coupled with electrospray ionization mass spectrometry and the other in Dutch Holstein Friesian (DHF) using capillary zone electrophoresis. In FM, in addition to the known isoforms αs1-CN-8P and-9P and αs2-CN-10P to -13P, three new phosphorylation isoforms were detected, namely αs2-CN-9P, αs2-CN-14P, and αs2-CN-15P. Relative concentrations of the phosphorylation isoforms varied considerably among cows. Phenotypic correlations showed that isoforms phosphorylated at higher degrees (αs1-CN-9P and αs2-CN-12P to -14P) correlated negatively with isoforms phosphorylated at lower degrees (αs1-CN-8P, αs2-CN-10P, and -11P). Furthermore, it was shown that αs1- and αs2-CN phosphorylation profiles changed across parity and lactation, and exploitable genetic variation for the phosphorylation degrees of αs1- and αs2-CN (defined as the proportion of higher-degree isoforms in αs1- and αs2-CN, respectively) exists in FM. In DHF, three αs2-CN isoforms, namely αs2-CN-10P to -12P, and the phosphorylation degrees of αs1- and αs2-CN were quantified. High intra-herd heritabilities were estimated for individual αs2-CN phosphorylation isoforms and the phosphorylation degrees of αs1- and αs2-CN (ranging from 0.54 to 0.89). This suggests that genetic factors contribute substantially to observed differences in αs1- and αs2-CN phosphorylation profiles. The correlation between the phosphorylation degrees of αs1- and αs2-CN was 0.94. Additionally, a total of 10 regions, distributed across Bos taurus autosomes (BTA) 1, 2, 6, 9, 11, 14, 15, 18, 24 and 28, were detected to be associated with individual αs1- and αs2-CN phosphorylation isoforms and their phosphorylation degrees in DHF. Regions on BTA1, 6, 11 and 14 were associated with multiple traits studied. Two quantitative trait loci (QTL) regions were detected on BTA1: one affecting αs2-CN production, and the other affecting αs1-CN PD and αs2-CN PD. The QTL region on BTA6 affected only individual αs2-CN isoforms. The QTL region on BTA11 and 14 affected relative concentrations of αs2-CN-10P and αs2-CN-11P, αs1-CN PD and αs2-CN PD. Results suggested that effects of identified genomic regions on αs1-CN PD and αs2-CN PD are probably due to changes in milk synthesis and phosphorus secretion in milk. Finally, differences among studies due to factors such as analytical methods, trait definitions, and breed on genetic parameters and correlations are discussed using the two dataset from this thesis. It is concluded that differences in heritability estimates for αs1-CN-8P and -9P, αs2-CN-10P, -11P and -12P, and αs1-CN PD between FM and DHF were mainly due to genetic differences between breeds. As for αs2-CN PD, it was defined differently in FM and DHF due to analytical methods used. It is shown that both trait definitions successfully quantified the proportion of isoforms with higher degrees of phosphorylation because of similar estimated correlations using both definitions on the FM dataset. Additionally, it is hypothesized that a two-casein-kinase system is involved in the phosphorylation of αs1- and αs2-CN based on results in this thesis.
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Zih-Hua Fang. The genetic background of bovine αs1- and αs2-casein phosphorylation. Animal genetics. Institut agronomique, vétérinaire et forestier de France; Université de Wageningen, 2017. English. ⟨NNT : 2017IAVF0020⟩. ⟨tel-02879335⟩

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