Epsilon Archive for Student Projects

Abstract

Metabolic traits are of paramount importance in agricultural production, as this group includes most traits of economic interest in livestock improvement. Examples include growth rate, feed efficiency and fat deposition. An improved understanding of the genetic basis of these traits can both improve our understanding of the genes that have been under selection and identify genes and pathways to be included in future breeding programs.

A novel genetic mechanism has been found to regulate growth in chicken lines divergently selected for body weight. A network of four interacting genes explains nearly half of the difference in body weight at 8-weeks of age between the two lines. The central locus in this network is located on chromosome 7 and it has a role in releasing the genetic effects of three other loci in the network located on chromosome 3, 4 and 20. Interestingly, the release of the genetic effects is also reciprocal as the loci on chromosome 3, 4 and 20 jointly release the genetic effect on growth for the QTL on chromosome 7. The original report by Carlborg et al report results on body weight and fat deposition, the study does, however not report results on other phenotypes collected on the F2 individuals. This thesis presents results from analyses to evaluate the effects of the four QTL network on other measured traits in the F2 population and to see which traits that are useful in further epistatic analyses (CARLBORG et al. 2006). Furthermore the study also serves as a replication of the original study by analysing data on a larger number of added genetic markers in the QTL regions.

The four QTL network was shown to have significant effects on body weight at different ages, abdominal fat and body compositions. The effect of body composition is most likely the results of an increase in general body size as the effects were not significant after corrected for body weight in the analyses. The network do, however, appear to have an effect on abdominal fat deposition and breast weight even after correcting for body weight. When corrected for body weight at slaughter (10-weeks of age) there were no significant effects on shank weight. No effects could be shown for the gene pair 7 and 4, and for 7 and 20 for other traits than body weight. The regression analysis indicates that chromosome 3 in a chromosome 7-homozygous low-line (LL) background increases relative abdominal fat and decreases relative breast muscle going from LL to HH (Homozygous high-line). When abdominal fat is not corrected for body weight at slaughter, the increase in fat deposition is proportional with increased body weight. In a chromosome 7-HH background, absolute abdominal fat is increasing with increased body-weight but relative abdominal fat is not when chromosome 3 is going from LL to HH. Relative breast muscle is decreasing, while absolute breast muscle is proportional with an increase in body weight. These results suggest that there is a change in the chicken body composition when selected for higher body weight. Chickens tend to go from lean and muscular to fat and thin.

Understanding the genetic regulation of metabolic traits for which the lines differ is of crucial interest. Therefore we collected information from the literature on the descriptive statistics for these traits. Statistics were used to explore the sample size needed in an experiment to detect genetic effects of various sizes in the high- and low- lines. From these it was concluded that there is a lack of power to detect genetic effects on the network for most tested metabolic traits on the cross and that another experimental strategy is needed to explore this further. Next step in this study will be to introgress the 4-QTLs from the low line chickens into a high line background.