Genomic analyses of migratory divides in the willow warbler

Abstract: In many species of birds juveniles migrate independently of experienced adults and thus have to rely on innate information about migratory routes and wintering area. This information is encoded as a set of inherited migratory directions and a timing schedule that provides information on when and how far to migrate. Apart from these remarkable behavioral adaptations, migration also requires several morphological, physiological and immunological adaptations. Although many migratory traits have been demonstrated to have strong heritable basis, virtually nothing is known about the specific genes underlying them. The lack of known migration genes limits a deeper understanding of the mechanistic processes and evolution of migration. In this thesis, I use the willow warbler Phylloscopus trochilus to explore the molecular genetics of migration. The willow warbler occurs with two subspecies in Europe. Ringing recoveries and stable isotopes have demonstrated that the subspecies use different migratory routes and wintering areas. An extremely low genetic differentiation and otherwise similar phenotypes, suggest that most of the genetic differences are likely to be involved in adaptations to the different migratory strategies of the subspecies. Here I use several different high throughput molecular techniques to identify differences between the subspecies. I find that genetic differences between the willow warbler subspecies are clustered in three divergent regions on different chromosomes, which each span several million base pairs and are comprised of numerous coding genes. The regions show restricted recombination between the warbler subspecies and could be maintained by inversions. A region on chromosome 3 is not specifically associated with the subspecies, but appears to be involved in adaptations to high altitude and latitude environments. The two other chromosome regions contain subspecies-specific variation. In a region on chromosome 1, genetic differentiation peaks close to the gene FOXO1, which is important in the formation of fat cells and gluconeogenesis. In the other region, which is located on chromosome 5, there is an enrichment of gene functions associated with fatty acid genes. The functions of these genes suggest that the two chromosome regions could be associated with adaptations to fuelling. Future studies on genetics of migration would benefit from assembled genomes that could be used as a basis for several high-resolution molecular techniques that could detect more localized differences and provide higher resolution of divergent chromosome regions. There is also a need to associate particular genes or chromosome regions with particular phenotypes. This would be aided by efficient and precise high-throughput phenotyping for example by new tracking technologies or controlled behavioral experiments in laboratories.