A proteomic approach to investigate the qualitative and quantitative polymorphism of β-lactoglobulin in ovine milk: Inference on gene copy-number variations.

The rationale of this work is based on recent evidences suggesting that: 1) both qualitative and quantitative -lactoglobulin (-LG) polymorphism may be found in bovine milk; 2) quantitative polymorphisms are of- ten the result of expression gradients in multiple cop- ies of a gene; 3) the -LG gene is duplicated in the dog and bovine genome; 4) mammary genes are highly conserved across Mammalia. Thus, an investigation was conducted on ovine -LG polymorphism checking phe- notypic evidence for copy-number variants of -LG in sheep. To the purpose, 206 milk samples were collected, during a small-scale survey within sheep farms breed- ing Southern Italian breeds. PAGIF screening of the samples revealed that approximately 50% individuals exhibited beta-LG polymorphism and 4 different quan- titative patterns, which were characterized in detail by a proteomic approach relying on combined chro- matographic and mass spectrometric techniques. The expected figures based on the expression gradient mod- els were compared with well-established α-globin gene arrangements in sheep. The different phenotypes sug- gest the presence of both duplicate and triplicate BLG haplotypes. The occurrence of a triplicate haplotype was supported by population data. The current study sup- ports the helpfulness of up-to-date proteomics for in- ferring copy number polymorphisms through the char- acterization of the phenotypic expression.