A search for major genes affecting meat quality in pigs / Søk etter gener med stor effekt på kjøttkvalitet hos gris
The main purpose of quantitative trait loci (QTL) analysis in livestock populations is to identify genes controlling important production, health and quality traits. A genome scan using anonymous molecular markers has proved to be an efficient way of detecting QTL. One drawback of this method is poor mapping resolution, which makes the process from genome scan to identifying the actual gene causing the QTL effect difficult and time consuming. Therefore, the candidate gene approach might be a useful supplement for investigating genes in the actual QTL region. Candidate genes for the trait of interest can also be selected solely on the biological or physiological function. In this thesis both genome mapping and candidate gene approaches have been used in the search for major genes affecting meat quality in pigs. The resource population used in all studies, except from the expression study, was a commercial slaughterpig cross in Norway (25% Yorkshire, 25% Duroc, 50% Norwegian Landrace). The meat quality traits recorded in the 305 F2-animals were: intramuscular fat (IMF), pH, drip loss, protein traits, fatty acid composition, and sensory traits (smell, taste, texture). Additionally, seven commercial breeding lines from the US were used in two of the association studies.
Two biologically interesting candidate genes for meat quality traits were chosen for further investigations; Peroxisome Proliferator Activated Receptor γ (PPARγ) and Major Insulin Sensitive Glucose Transport gene (GLUT4). PPARγ is shown to be a key regulator of adipocyte differentiation and adipocyte gene expression in several species. Since PPARγ was not previously characterised in pigs, sequencing of two cDNA isoforms of the gene was initially performed. An expression study was also completed in order to investigate the expression pattern in pigs from different breeds and at different ages. PPARγ was highly abundant in adipose tissues and considerable differences in expression levels were found between Norwegian Landrace and Duroc breeds. Porcine PPARγ was mapped to porcine chromosome 13 both by physical and genetic mapping. This location is in accordance with the porcine and human comparative maps of the region. A polymorphism (Met59Val) was detected in porcine PPARγ and used in an association study, investigating the relationship between PPARγ and meat quality traits. Significant associations were found between PPARγ and six different fatty acids, along with a taint trait in the commercial slaughterpig cross in Norway.
Another biological candidate gene investigated in this thesis was the porcine Major Insulin Sensitive Glucose Transport gene (known as either GLUT4 or SLC2A4). In several species glucose transport in skeletal muscle fibres is previously shown to be at least partially, if not entirely, dependent upon the total amount of GLUT4 glucose transporters protein. The GLUT4 gene was mapped to the porcine chromosome 12 by physical and genetic mapping. A study was performed between polymorphism within the gene and meat quality traits recorded both in a commercial slaughterpig cross in Norway and seven breeding lines in the US. The results did not provide strong support to the hypothesis that variation in porcine GLUT4 is associated with variation in meat quality traits.
Moreover, a primary scan for QTL affecting meat quality traits in a commercial slaughterpig cross in Norway was completed for chromosomes 4, 6, and 7. Suggestive evidence for QTL was found for some meat quality traits on all the three chromosomes. The most confident result was obtained for IMF on chromosome 6 (P<0.001). This QTL has been mapped to approximately the same region in several other studies made up as crosses between other breeds. A 95% confidence interval of the QTL position in our study was initially found to be 14 cM. Thus, to narrow this position, nine markers were added to the QTL region, and a modified interval mapping method was used to do further analyses. Heart fatty acid-binding protein (H-FABP) and melanocortin 5 receptor (MC5R) were included in the map as positional candidate genes for the QTL. The most likely marker interval in this follow up study was slightly telomeric of the QTL position in the initial study. The confidence interval of the QTL position was estimated to be as narrow as 1.1 cM. Moreover, the fine mapping data excluded H-FABP and MC5R as causative positional candidate genes for the QTL.
