High resolution linkage disequilibrium mapping of QTL affecting milk production traits in Israel Holstein dairy cattle

Original objectives: To create BAC contigs covering two QTL containing chromosomal regions (QTLR) and obtain BAC end sequence information as a platform for SNP identification. Use the SNPs to search for marker-QTL linkage disequilibrium (LD) in the test populations (US and Israel Holstein cattle). Identify candidate genes, test for association with dairy cattle production and functional traits, and confirm any associations in a secondary test population. Revisions in the course of the project: The selective recombinant genotyping (SRG) methodology which we implemented to provide moderate resolution QTL mapping turned out to be less effective than expected, due to problems introduced by incomplete marker informativity. This required a no-cost one-year extension of the project. Aside from this, the project was implemented essentially as envisaged, but only with respect to a single QTLR and single population association-test. Background to the topic. Dairy cattle breeders are looking to marker-assisted selection (MAS) as a means of identifying genetically superior sires and dams. MAS based on population-wide LD can be many times more effective than MAS based on within-family linkage mapping. In this proposal we developed a protocol leading from family based QTL mapping to population-wide LD between markers and the QTL Major conclusions, solutions, achievements. The critical importance of marker informativity for application of the SRG design in outcrossing random mating populations was identified, and an alternative Fractioned Pool Design (FPD) based on selective DNA pooling was developed. We demonstrated the feasibility of constructing a BAC contig across a targeted chromosomal region flanking the marker RM188 on bovine chromosome BTA4, which was shown in previous work to contain a QTL affecting milk production traits. BAC end sequences were obtained and successfully screened for SNPs. LD studies of these SNPs in the Israel population, and of an independent set of SNPs taken across the entire proximal region of BTA4 in the USA population, showed a much lower degree of LD than previously reported in the literature. Only at distances in the sub-cM level did an appreciable fraction of SNP marker-pairs show levels of LD useful for MAS. In contrast, studies in the Israel population using microsatellite markers, presented an equivalent degree of LD at a 1-5 separation distance. SNP LD appeared to reflect historical population size of Bostaurus (Ne=5000- 10,000), while microsatellite LD appeared to be in proportion to more recent effective population size of the Holstein breed (Ne=50-100). An appreciable fraction of the observed LD was due to Family admixture structure of the Holstein population. The SNPs MEOX2/IF2G (found within the gene SETMAR at 23,000 bp from RM188) and SNP23 were significantly associated with PTA protein, Cheese dollars and Net Merit Protein in the Davis bull resource population, and were also associated with protein and casein percentages in the Davis cow resource population. Implications. These studies document a major difference in degree of LD presented by SNPs as compared to microsatellites, and raise questions as to the source of this difference and its implications for QTL mapping and MAS. The study lends significant support to the targeted approach to fine map a previously identified QTL. Using high density genotyping with SNP discovered in flanking genes to the QTL, we have identified important markers associated with milk protein percentage that can be tested in markers assisted selection programs.