Fatty Acid Composition of Bovine Milk: Results From a Genome-Wide Scan and Opportunities for Selection

The Dutch Milk Genomics Initiative aims at the identification of genes that contribute to natural genetic variation in quality traits of bovine milk, among which milk-fat composition. For this purpose, we have collected milk and blood samples of 2000 Dutch Holstein Friesian cows.

We have shown that there is considerable genetic variation between dairy cows in milkfat composition, which implies that there are opportunities to change the fat composition of bovine milk by selective breeding. We have also shown that the K232A polymorphism in the DGAT1 gene and the A293V polymorphism in the SCD1 gene have significant effects on bovine milk-fat composition. However, not all genetic variation in milk-fat composition can be attributed to these two polymorphisms. Therefore, we performed a genome-wide scan to identify quantitative trait loci (QTL) that contribute to the genetic variation in short, medium and long chain milk fatty acids.

A total of 849 cows, representing 7 paternal half-sib families, and their 7 sires were genotyped for 1341 molecular markers (single nucleotide polymorphism, SNP) across all 29 Bos taurus autosomes (BTA), and a genetic map was constructed. A total of 44 traits were evaluated: individual fatty acids, groups of fatty acids, and unsaturation indices. QTL were identified on 6 chromosomes: BTA 6, 14, 15, 16, 19, and 26. The QTL on BTA 14 affected many different fatty acids and is most likely caused by the K232A polymorphism in the DGAT1 gene. Similarly, the QTL on BTA 26 affected many different fat traits, in particular cis9-unstaturation, and is most likely caused by the A293V polymorphism in the SCD1 gene. Preliminary calculations show that the genetic variation that is present in the Dutch dairy cattle population enables selection for a different milk-fat composition, e.g. lower in amount of saturated fatty acids and higher in amount of unsaturated fatty acids.

We are currently performing an association analysis to narrow the genomic regions of identified QTL and, ultimately, to enable identification of new candidate-genes for milk-fat synthesis and metabolism. For this we have genotyped all 2000 cows for about 60.000 SNPs, and association analyses are in progress.