Abstract
The rationale of this work is based on recent evidences suggesting that: 1) both qualitative and quantitative β-lactoglobulin (β-LG) polymorphism may be found in bovine milk; 2) quantitative polymorphisms are often the result of expression gradients in multiple copies of a gene; 3) the β-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 β-LG polymorphism checking phenotypic evidence for copy-number variants of β-LG in sheep. To the purpose, 206 milk samples were collected, during a small-scale survey within sheep farms breeding Southern Italian breeds. PAGIF screening of the samples revealed that approximately 50% individuals exhibited β-LG polymorphism and 4 different quantitative patterns, which were characterized in detail by a proteomic approach relying on combined chromatographic and mass spectrometric techniques. The expected figures based on the expression gradient models were compared with well-established α-globin gene arrangements in sheep. The different phenotypes suggest the presence of both duplicate and triplicate BLG haplotypes. The occurrence of a triplicate haplotype was supported by population data. The current study supports the helpfulness of up-to-date proteomics for inferring copy number polymorphisms through the characterization of the phenotypic expression.
Highlights
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 often the result of expression gradients in multiple copies of a gene; 3) the -LG gene is duplicated in the dog and bovine genome; 4) mammary genes are highly conserved across Mammalia
PAGIF screening of the samples revealed that approximately 50% individuals exhibited -LG polymorphism and 4 different quantitative patterns, which were characterized in detail by a proteomic approach relying on combined chromatographic and mass spectrometric techniques
Once the analytical identification of the two molecular species was obtained, the theoretical tools for understanding the phenomenon had to be decided on to formulate a hypothesis for the presence of copy number variations (CNVs) at the BLG locus
Summary
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 often the result of expression gradients in multiple copies of a gene; 3) the -LG gene is duplicated in the dog and bovine genome; 4) mammary genes are highly conserved across Mammalia. It has been recently evidenced that the occurrence of several, differently expressed α-lactalbumin (α-LA) gene products in water buffalo are related to the presence of two tandemly repeated sequences 15 ; preliminary results suggest that an analogous duplication should occur both in sheep and cattle 16,17. With the completion of the Bos taurus genome assembly, bovine milk and lactation data can be linked to other mammalian genomes for the first time, providing additional insight into the molecular evolution of the milk composition and lactating machinery 14 Both copy number and sequence variation contribute to the diversity of milk protein composition across species. The presence of the BLG* pseudogene as the result of a recent gene conversion event was confirmed in bovine, sheep and goat
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