Interactions between environmental factors drive selection on cyanogenesis in Trifolium repens

Phenotypic and genetic clines frequently evolve due to varying selection along environmental gradients. However, the specific environmental factors that impose differential selection are often multivariate and difficult to tease apart. We addressed this complexity using a factorial manipulation of watering, soil nutrients and (simulated) herbivory in controlled conditions to better understand the agents of selection driving the evolution of clines in a polymorphic chemical antiherbivore defence, hydrogen cyanide (HCN), of the plant white clover Trifolium repens. We found the presence or absence of the two metabolic components required for HCN production (cyanogenic glucosides and linamarase) to be more prominent determinants of vegetative growth and sexual reproduction in T. repens than HCN itself. We also found that the ability to produce one or both of cyanogenic glucosides or linamarase resulted in a growth advantage under drought and simulated herbivory that outweighs the metabolic cost of their production. These results support the view that the metabolic components underlying HCN play important roles beyond defence by increasing plant tolerance to stress. The growth advantage under drought, however, was diminished in the absence of nutrient addition, consistent with multivariate interactions as drivers of selection. This study provides novel insight into how a cosmopolitan plant has adapted to environmental gradients, and more generally, highlights the importance of considering interactions between multiple environmental factors when studying the evolution of phenotypic and genetic clines.