Plant performance under thermal stress : strategies and trade-offs, and their potential role in determining species distribution

The distribution of species is often restricted to a particular climatic zone with distribution boundaries following thermal isoclines. What hinders species from expanding their geographic distribution? To answer this question it is important to consider the traits of climate adaptation and their genetic architecture. For this, we used an integrative approach, combining common garden experiments and the analysis of the transcriptome based on RNA-sequencing to explore both phenotypic and genetic aspects of the thermal stress response. Study organism was the plant species Arabidopsis lyrata ssp. lyrata (Brassicaceae). Chapter 1 addressed the question of whether populations differed in thermal stress resistance and tolerance along two latitudinal clines and tested for the existence of trade-offs. A main result was that frost tolerance traded off against plant size in the absence of thermal stress. This genetically-based trade-off could be involved in constraining the evolution at the northern species border if selection favors fast development to large size at the cost of less frost tolerance. Chapter 2 focused on the acclimation process, its impact on thermal stress resistance, the cost and the underlying changes in gene expression. Acclimation increased thermal stress resistance independent of acclimation temperature. Furthermore, the plastic acclimation response did not seem to be costly. But a negative correlation between basal and induced resistance was found, which may constrain the evolution of acclimation. Chapter 3 explored genetic divergence in thermal resistance and tolerance and fitness related traits in a single population of a heterogeneous sand dune landscape. Overall, the work indicates high lability in the evolution of thermal stress-resistance strategies. Few constraints seem to exist, with the most important probably being one between frost tolerance and fast reproductive development.