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

Zusammenfassung

This thesis is arranged into five chapters. In the first and the second chapter we employed DNA markers to unravel the phylogeography and the molecular structure and diversity of Arbutus unedo. The third and fourth chapters focused in analysing the ecophysiological response of the species to variations in nutrients and water availability. Finally, in the fifth and last chapter we compared the ecophysiological and molecular variance by using the QST-FST approach. Specifically, our objectives were: 1. To disentangle the colonization routes of A. unedo along the Mediterranean basin and the 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 wether the largely one-dimensional distribution 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. Carried out in Chapter II, by using amplified fragment length polymorphism (AFLP) markers. 4. To stablish whether the contrasted climatic conditions found throughout the distribution range of the species have resulted in gene flow restrictions and/or differentiation between regions. 5. To contribute to the Lusitanian debate by determining the most likely origin for the Irish population of A. unedo. 6. To describe differences in the plasticity of structural and physiological traits in plants of strawberry tree subjected to variations in nutrients and water availability, what will increase our ability to predict how this species will respond to ongoing global changes. Performed in Chapter III (nutrients) and IV (water), by using the phenotypic plasticity index. 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 environmental scenarios. 8. To model how nutrient levels affected photosynthetic responses of A. unedo, given that plant responses to light are influenced by nutrient availability. 9. To define the role of natural selection and local adaptation in determining the current genetic structure of A. unedo. Performed in Chapter V by using the QST-FST approach. 10. 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 this could affect to its evolution. Throughout the five chapters of this thesis we analysed the strawberry tree from the point of view of molecular ecology and ecophysiology, as well as from the integration of both perspectives in the QST-FST approach. Considering the results obtained we have reached the following conclusions (C.), which respond to the general objectives: C. 1. The strawberry tree showed a partition in two chloroplastidial clades, one Atlantic and another Mediterranean, as result of possible vicariance events during the Quaternary glaciations of the last 700 ky but before the last glacial maximum (20 ky ago). North Africa-Atlantic Iberia seems the likely refugium for the Atlantic clade, whereas the European rim and/or large islands of Mediterranean would be the refugia for the Mediterranean clade. Both clades underwent a subsequent westward expansion, the Atlantic also moved northwards. Conclusion based on results from chapter I. C. 2. The strawberry tree showed occasional connections between Europe and North Africa through the straits of Gibraltar and Sicily, but North Africa still retains a distinctive genetic composition. The species showed the East-West genetic divide found for other trees in the Iberian Peninsula, consistent with the refugia-within-refugia model. In comparison the Eastern Mediterranean showed lower genetic diversity and may have been more recently colonized, in contrasts with the pattern commonly seen in other plants. Conclusion based on results from chapter I. C. 3. Arbutus unedo showed an isolation by distance pattern that is stronger along the Atlantic than around the Mediterranean basin, and could be interpreted as evidence of a stepping-stone colonization along the Atlantic façade. The latter would also explain the gradual decrease in within-population genetic diversity detected in NW Iberia and, specially, Ireland. Conclusion based on results from chapter II. C. 4. The genetic structure derived from neutral genomic markers shows a clear divide of the strawberry tree in two groups,one Atlantic and another circum-Mediterranean. This separation fits the pattern observed in other widespread trees and could be attributed to the existence of separated and independent Last Glacial Maximum refugees. Conclusion based on results from chapter II. C. 5. Our results discard the existence of a continuous range of A. unedo along the maritime fringe of France. Instead, the species possibly arrived to Ireland on post-glacial times from N Iberia. Conclusion based on results from chapter I and II. C. 6. The response to low nutrient availability of A. unedo was more marked for structural traits (reduction and redistribution of biomass) than for physiological traits, indicating that the strategy is to optimize resource allocation but maintain the physiology stable. However, in response to water shortage, besides the high plasticity in structural traits that slows down the growth, we also observed notable plasticity in some physiological traits, which results in a high capacity to reduce water use and dissipate excess of radiation. We also conclude that it is not possible to generalize about the plasticity of traits of different nature as the level of plasticity depends on the environmental factors that induce variability. Conclusion based on results from chapter III (nutrients) and IV (water). C. 7. We observed similar levels of plasticity for Arbutus unedo provenances in response to variations both in nutrients and water availability. Therefore, none of the provenances seems to be more able of colonizing or more susceptible to become extinct than others as a consequence of changes in the availability of these resources. Conclusion based on results from chapter III (nutrients) and IV (water). C. 8. We observed that the supply of nutrients improved the performance of A. unedo under high radiation conditions, what could be particularly advantageous under the typical high irradiance of the Mediterranean summer, given the predicted increase in nutrient deposition. Conclusion based on results from chapter III. C. 9. Arbutus unedo showed lower QST than FST in many traits, which means that stabilizing selection may select for a norm of reaction that produces a similar phenotype even from diverse genotypes from different localities. Conclusion based on results from chapter V. C. 10. The evolvability that we found in key morpho-structural traits predicts that Arbutus unedo retains genetic variation that will allow its genetic adaptation to future changeable environments. Conclusion based on results from chapter V. Taking into account these conclusions, we can formulate some remarkable and general conclusions (G.C.): G. C. 1. The strawberry tree possess a considerable ability to disperse, as it seems to have been able to migrate over thousands of kilometres and crossing stretches of sea as the Mediterranean straits. Surprisingly, this ability does not erase the genetic split between East and West Iberia. This ability to disperse could be useful in a scenario of future changes that creates favourable conditions for this species. Conclusion based on C. 1, C. 2, C.4, C. 5. G. C. 2. The low variation between provenances of A. unedo in response to variations in the nutrients and water availability 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. Conclusion based on C. 6, C. 9. G. C. 3. Arbutus unedo has evolved a conservative strategy in the use of nutrients and water when they are scarce. This strategy based on a high degree of plasticity on key traits along with a relevant degree of evolvability. Both, 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 will continue to play an important role in the Mediterranean plant communities. Conclusion based on C. 7, C. 10.