Time- and population-dependent genetic patterns underlie bovine milk somatic cell count

Area of Science:

• Animal Genetics and Genomics
• Dairy Science
• Immunology

Background:

• Bovine milk somatic cell count (SCC) is a key indicator for mastitis, a prevalent dairy cattle disease.
• Genetic factors influencing SCC are crucial for breeding mastitis-resistant cattle.
• Understanding genetic regulation across lactation stages is vital for accurate genetic improvement.

Purpose of the Study:

• To investigate if genetic regulation of bovine milk SCC changes throughout lactation.
• To identify quantitative trait loci (QTL) differentiating chronically mastitic from healthy cows.
• To pinpoint candidate genes involved in immune responses related to mastitis resistance.

Main Methods:

• Genotyped 471 cows using the Illumina BovineHD 777K BeadChip.
• Collected milk SCC at five key lactation stages (0-1, 3-5, 10-14, 90-110, 210-230 days in milk).
• Performed genome-wide association studies and calculated fixation indices (FST) to identify differentiating genomic regions and candidate genes.

Main Results:

• Identified 167 single nucleotide polymorphisms (SNPs) significantly associated with SCC across lactation.
• Discovered 27 novel QTL associated with clinical mastitis or SCC, with candidate genes involved in immune pathways (e.g., toll-like receptor signaling, complement pathway).
• Found 12 SNPs differentiating healthy, chronically mastitic, and average cows, with candidate genes related to cell signaling and immune response (e.g., JAKMIP1, MADCAM1).

Conclusions:

• Bovine mastitis is a complex trait influenced by multiple genes across the genome.
• Genetic regulation of SCC is dynamic throughout lactation, suggesting stage-specific selection may be beneficial.
• Identifying specific QTL and candidate genes provides targets for marker-assisted selection to enhance mastitis resistance in dairy cattle.