Genetic structure and geographic variation in ecophysiological traits of Arbutus unedo L.

Abstract

The objectives of the thesis are: 1. To disentangle the colonization routes of A. unedo along the Mediterranean basin and Atlantic façade, with special interest in the estimation of the time to the most recent common ancestor (TMRCA) of the species. 2. To determine the concordance between the phylogeographic patterns observed in other Mediterranean plants (longitudinal/latitudinal clines of diversity, location of glacial refugia, and role of the straits as barriers to dispersal) and those that we observed in the strawberry tree. 3. To analyse if the kind of distribution (largely one-dimensional) of the strawberry tree led to the existence of an isolation by distance (IBD) pattern, either around the Mediterranean basin or along the Atlantic coast. 4. To stablish whether the contrasted climatic conditions throughout the distribution range of the species has resulted in gene flow restrictions and/or differentiation between regions 5. To contribute to elucidate the Lusitanian debate by determining the most likely origin for the Irish population of A. unedo. 6. To describe the response of structural and physiological traits in plants of strawberry tree subject to variations in nutrients and water availability, what will increase our ability to predict how this species will respond to ongoing global changes. 7. To elucidate whether the plants of different provenances of A. unedo differed in the level of plasticity in response to variations in nutrients and water availability, what may be important for determining how the distribution of this species could be affected in future scenarios. 8. To define the role of natural selection and local adaptation in determining the current genetic structure of A. unedo. 9. To estimate the additive genetic variance, heritability and evolvability in a set of morphological and physiological traits of the strawberry tree, in order to know how could affect to its evolution. Taking into account the results obtained, can formulate some remarkable and general conclusions: 1. The strawberry tree possess a considerable ability to disperse which leads to the existence of an isolation by distance patter. The species is able to migrate over thousands of kilometres and even crossing stretches of sea as Gibraltar and Sicily straits. Besides, this ability led to the existence of a clear pattern of isolation by distance and to the colonization of Ireland directly from Iberia. Surprisingly, this ability does not erase the genetic split between East and West Iberia, what must be explained due to habitat unsuitability rather than dispersal limitations. This ability to disperse could be useful in a scenario of future changes that creates favourable conditions for the species. 2. The low variation between provenances of A. unedo in response to variations in the nutrients and water availability, what makes that any population are particularly favoured or disfavoured because of modifications in the availability of these resources. This situation is consistent with the demonstrated stabilizing selection for the species, what could be partially explained by an event of canalization. The stabilizing selection and canalization reduce the variability in the responses, and, consequently, reduce the costs associated with changes. 3. Arbutus unedo has evolved a strategy of conservative use of resources (nutrients and water) when they are scarce, showing a response based on a high degree of plasticity on key traits together with a relevant degree of evolvability. Together, plasticity and evolvability, explain the current persistence of the species throughout its distribution range and will be determinant to cope with future changes of the environmental conditions. Therefore, it seems likely that the strawberry tree has a role in the future Mediterranean plant communities.