Adaptive Evolution of Plastic Foraging Responses in a Clonal Plant

Stoloniferous rosette plants may show horizontal and vertical foraging responses, such as changes in branching frequency, stolon internode length, leaf length, and height growth of stolons. To study whether such plastic foraging responses constitute an adaptation to heterogeneity in competition, we studied genetic variation in and fitness consequences of plastic foraging responses in the stoloniferous lake-shore plant Ranunculus reptans. Because plastic foraging responses are likely to have been more strongly selected for in heterogeneous environments, we used 15 genotypes from competitive, heterogeneous microhabitats and 15 from competition-free, homogeneous microhabitats from Lake Constance (central Europe). We planted vegetatively propagated rosettes of the 30 genotypes (totaling 236 rosettes) into a greenhouse environment with spatially heterogeneous competition. Four replicates of each genotype were planted into the competition-free halves of experimental trays, and four other replicates into the halves with the naturally co-occurring grass Agrostis stolonifera. We found significant variation among genotypes in vertical and horizontal foraging responses. In line with the hypothesis of adaptive plasticity, genotypes from the competitive heterogeneous microhabitats more strongly increased the vertical angle of the first stolon internode (126%) and the specific stolon length (166%) in response to competition than genotypes from the competition-free homogeneous microhabitats. Moreover, we found that genotypes that were more plastic in the vertical angle of the first stolon internode, stolon height, and specific internode length produced more rosettes and flowers than less plastic genotypes (all selection gradients for plasticity . 0.316). Our findings strongly suggest that plastic foraging responses constitute an adaptation to environmental heterogeneity, at least in the stoloniferous R. reptans.