Ecology and evolution of local adaptation in Arabidopsis thaliana

In this thesis, I investigate the ecological genetics of local adaptation in the annual herb Arabidopsis thaliana with the aim to identify key traits, agents of selection, and genomic regions contributing to adaptive differentiation. The thesis focuses on adaptive differentiation in timing of germination and seed dormancy between populations located close to the southern and northern range margins in Europe. I aimed to: (1) determine how much a particular genomic region contributes to local adaptation between an Italian and a Swedish population, (2) characterize selection on timing of germination in a Mediterranean climate, (3) quantify variation in seed dormancy and identify potential agents of selection on seed dormancy within Italy and Fennoscandia, and (4) examine the genetic basis of the seed dormancy cycle and the adaptive value of seed banks. Differences in a genomic region at the end of chromosome 5 explained a large proportion of differences in fitness, and in germination and flowering time between the Italian and Swedish population. A field experiment at the site of the Italian population, indicated strong stabilizing selection on timing of germination with an optimum coinciding with the time of germination in the local population. Seed dormancy of Italian populations was stronger than that of Fennoscandian populations, but also varied considerably within regions, indicating that this trait has considerable evolutionary flexibility. In Fennoscandia, variation in seed dormancy was related to climatic conditions in summer, suggesting that differences are at least partly adaptive. The seed dormancy cycle in the soil differed between the focal Italian and Swedish population, and matched seasonal changes in conditions for seedling establishment at their sites of origin. Differences in the genomic region at the end of chromosome 5 could explain a large proportion of the difference in the seed dormancy cycle. Mortality of seeds was much higher in Italy than in Sweden, indicating that the importance of the seed bank for population dynamics differs between the two sites. Overall, the results suggest that differences in a genomic region on chromosome 5 and in early life stages can play a key role in local adaptation in A. thaliana.