SCALES AND DRIVERS OF LOCAL ADAPTATION IN BRASSICA NIGRA (BRASSICACEAE) POPULATIONS

Premise of the Study: Although there is much empirical evidence for local adaptation in plant populations, spatial scales and drivers are still poorly understood. We used the annual species Brassica nigra to (1) test scales of differentiation and adaptation among coastal wetland and inland river-valley populations and (2) analyze herbivory as a potential driver of local plant adaptation. Methods: In a common garden experiment, we compared seven populations collected at different geographic scales in both habitat types. To evaluate adaptation to herbivory, we removed the aphid Brevicoryne brassicae from half of the plants. In a reciprocal transplant experiment, we tested local adaptation in two coastal and two river-valley populations. Natural colonization by dominant herbivore species was recorded. Key Results: In the common garden, the river-valley populations showed a higher performance than the coastal ones, whereas large-scale differentiation within habitats was small. Such a differentiation among plant populations was also found in spontaneous infestation by several herbivore groups but not in the plant response to aphid removal. In the reciprocal transplant experiment at natural sites, both plant populations performed better in their home habitat, indicating local adaptation to environmental differences between coastal and river-valley sites. A lower aphid infestation on local plants suggests a contribution of herbivores to local plant adaptation and illustrates the need for reciprocal transplant experiments to evaluate this contribution. Conclusions: Our study demonstrates that adaptive differentiation among habitats may be stronger at relatively small scales than large-scale adaptation within these habitats. It provides new insights into the role of herbivory in driving local plant adaptation.