Marte Sodeland defended her thesis on December 9th 2010

Ane Gro Siri Skjelfjord

Genomic architecture and complex traits in Norwegian Red cattle

The predominant cattle breed in Norway, Norwegian Red cattle (NRF), is an admixed breed formed from local Norwegian breeds and imported animals from other Nordic breeds. The extensive phenotypic records for NRF represent a unique resource for studying genetic factors affecting complex traits of importance for animal production. Phenotypic records of traits related to milk production, meat production and fertility, as well as health traits such as veterinary treated clinical mastitis, are now kept for 96% of Norwegian cattle. Records of veterinary treated clinical mastitis have been kept for most NRF animals for the last three decades.

Mastitis is inflammation of the mammary gland and is the most widespread disease affecting dairy cattle world-wide. Consequences of this disease include animal suffering, reduced milk quality, unwanted use of antibiotics and a more costly production. Main objective of the work described in this doctoral thesis has been to study the genomic architecture of the admixed NRF breed in order to understand the genetics underlying complex traits in cattle, particularly susceptibility to mastitis.

The study was initiated with the genotyping of 2,589 NRF bulls for single-nucleotide polymorphisms (SNPs) using the Bovine Affymetrix 25k MIP SNP array. Construction of linkage maps provided a powerful resource for quality assessment of the bovine genome assembly Btau_4.0 and for investigations of recombination rates and linkage disequilibrium (LD) across the NRF genome. Differences between recent and historic recombination rates were used to identify genomic loci subjected to strong artificial selection in the observed pedigree. Reduced LD was found in NRF compared with other breeds included in the study. The high LD generally reported in cattle facilitates association mapping studies for detection of quantitative trait loci (QTLs) affecting complex traits. In order to detect QTLs affecting susceptibility to mastitis genome-wide association studies with over 17,000 SNPs were performed for occurrence of clinical mastitis (CM) in seven lactational time periods and for lactation average somatic cell score (SCS). Although there is a genetic correlation between CM and SCS, no consistencies were found between SNPs significantly associated with CM and those associated with lactation average SCS. Combined linkage and linkage disequilibrium analysis confirmed quantitative trait loci for CM on bovine chromosomes 2, 6, 14 and 20, with the highest test score for CM being found for a SNP at 90.67Mb on chromosome 6. In addition to the QTL for CM on chromosome 6, a QTL affecting milk protein yield (PY) has been found to coincide with the casein genes around 88Mb on this chromosome. Fine mapping gave highest test scores for PY in and around the casein genes CSN2 and CSN1S2 (at 88.33Mb and 88.41Mb), while highest test scores for CM were found within the region 89 to 91Mb. It has been suggested that a haplotype encompassing the casein genes, with a favorable effect on PY and an unfavorable effect on CM, was introduced into the NRF population through importation of a Holstein- Fresian bull (1606 Frasse) in the 1970s. High-throughput re-sequencing allowed for molecular characterization of the long range haplotype from 1606 Frasse and revealed plausible causal polymorphisms in the promoter region of the gene CSN1S2 and in a known regulatory motif in the 5'-flanking UTR of CSN1S2

Updated: 13.12.10
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