Abstract
Variability of phenotypes is an ubiquitous phenomenon in animals and plants that is considered to be the basis on which organisms undergo evolutionary changes. The present thesis addresses the variability and genetic basis of limb bone length phenotypes in different populations and strains. I included mice from four outbred populations (three from the subspecies M. m. domesticus and one from the subspecies M. m. musculus), as well as the commercially available outbred stock CD1 (Charles River) called cumulatively the "outbred group". The inbred strains are represented by PWD (a wild derived inbred strain from M. m. musculus) and C57BL/6J (a classical inbred strain originally derived from M. m. domesticus) cumulatively called the "inbred group". Further I used individuals from the first-generation offspring of mice captured in a natural hybrid zone between the two Mus musculus subspecies. To reduce environmental influences on the variability, all animals were kept under the same environmental conditions before their analysis. The first two chapters investigate mainly questions of developmental architecture. Chapter one deals with degrees of fluctuating asymmetry (FA). This is an indicator of developmental stability, reflected in the possibility of an organism to ensure the same phenotypic expression under the same genetic and environmental conditions on both sides of the body. I found that the first bone of the forelimb, the humerus, shows generally the highest level of FA. The lowest variance of bone length measures was found in inbred strains, but at least one of them (PWD) showed at the same time the highest level of FA. For the hybrid group I found the level of FA was not affected by the degree of hybridization and that they showed the highest level of stability. My results suggest a potential benefit of heterozygosity in the hybrid group, whereas the larger deviation observed in the inbred group might be a consequence of higher homozygosity. The second chapter examines covariation between bone lengths, which are considered to reflect levels of morphological integration. Close to natural populations are assumed to show greater level of integration due to a stronger influence of stabilizing selection which should affect the covariance structure. I found a higher degree of integration in the hybrid animals which was significant compared to the inbred and outbred groups. Outbred populations and inbred strains did not have larger differences except significantly higher integration in one population that belongs to the outbred stock (CD1). Moreover, different levels of integration could be noticed among populations of the hybrid group. Influence of size was also found as an important factor in shaping the overall integration in each observed group. In 8 addition, I investigated whether stronger connections could be found between developmentally related bones, which was supported in outbred and hybrid groups. The third chapter constitutes an approach to map genetic factors that generate limb variation and considers genetic variation that can affect multiple traits. In this part of the study, I used only mice from the hybrid zone, since these were previously shown to be suitable to conduct a genome wide association study with them. Based on the results from the second chapter which showed high phenotypic correlations, I asked whether traits that are developmentally and functionally related could have common genetic variants underlying these complex structures. Accordingly, special interest is given to genomic regions that underly individual bone length, as well as correlated variation between the bones. Overall, these traits revealed high heritability explained by genotyped markers, as well as a polygenic genetic architecture. Candidate genes previously described in limb and bone formation were identified together with genetic variants that were not previously reported in QTL studies of this phenotype. Further I found genetic regions (loci) associated with different bones, as well as high genetic correlations between the bones that share developmental mechanisms, i.e. serially homologous structures. Most interestingly, none of the genes known to be required for embryonic development of the limbs showed up as a factor shaping the adult development.
