Evolutionary and genetic basis of morphological variation in Populus nigra (European black poplar)

Changes in precipitation over the next century may impact the distribution of species, particularly in southern Europe, where droughts are predicted to increase in frequency. In forest trees, intraspecific variation in leaf size, branching architecture, and growth rate among populations are considered adaptive and likely related to climatic differences between sites. A previous common garden study of Populus nigra L. showed morphological variation to be highly heritable and significantly differentiated among populations, indicating phenotypic differences may be adaptive.This project studied the evolutionary processes that have contributed to the morphological differentiation observed in P. nigra. Examining scales ranging from landscape-level patterns of variation to cellular differences within developing leaves identified historic and developmental processes contributing to the phenotypic differences in this species. Both isolation by distance, where migrants do not move equally across the landscape, and isolation by adaptation, where genetically divergence varies with morphological differences, have influenced differentiation among populations of P. nigra in western Europe. These patterns broadly correspond to the recolonization routes following the most recent glacial event, indicating that historic vicariance and not just adaptive divergence influence phenotypic variation. Identification of quantitative trait loci (QTL) for insect herbivory in a common garden study of hybrid poplar indicated that leaf morphology might also be influenced by insect preference.Among individuals, differences in leaf size corresponded to variation in cell number and not cell size, indicating natural selection may have influenced the regulation of cell division. Further, variation in gene regulation across the developing leaf identified differences across the leaf lamina. Finally, simulations of demographic, genetic, and adaptive processes among populations revealed that a lack of correspondence between the optimal phenotype of colonists and the optimal phenotype and newly colonized populations significantly affects levels of phenotypic differentiation among populations. In addition, changes in phenotypic optima, as may occur due to climate change, impacted the level of genetic variance, and thus the future adaptive potential of populations. Together, these results provide insight into the evolution of phenotype in P. nigra, and contribute information for management efforts in the context of a changing climate.