Clines are the spatial manifestation of an ecological tradeoff, and are frequently used to understand how divergent selection pressures create adaptive divergence between populations. In white clover, clines in the polymorphic trait cyanogenesis, the ability to produce hydrogen cyanide after tissue damage, have formed recurrently in white clover populations worldwide, with a greater frequency of cyanogenic plants in warmer climates. Previous studies from the native range (Eurasia) have posited that cyanogenesis clines are maintained by an ecological tradeoff whereby cyanogenic plants are better defended against herbivores, but allocate fewer resources to growth and reproduction and are selected against in areas with lower herbivore pressure. However, our recent studies from the introduced range (North America and New Zealand) suggest that there may be alternative selective pressures that target not only cyanogenesis per se, but also the two individual biochemical components required for the cyanogenic phenotype (cyanogenic glucosides and their hydrolyzing enzyme). These studies suggest that plants with cyanogenic glucosides may have an alternative function in nitrogen storage leading to a greater ability to tolerate stress. In this study, we investigate the tradeoffs that maintain a cyanogenesis cline across central U.S. populations using experimental manipulations in greenhouse common garden and field experiments.
Results/Conclusions
While we find little evidence for differential herbivory on cyanogenic adult plants or seedlings in any experiment, in greenhouse experiments we find acyanogenic plants do flower earlier and more prolifically under standard greenhouse conditions. However, plants that possess cyanogenic glucosides flower earlier and more prolifically in water-stress treatments. Additionally, cyanogenic plants flower more often than acyanogenic plants in water-stress and fertilizer-stress treatment. These results indicate that fitness tradeoffs maintaining the cyanogenesis polymorphism in central U.S. populations directly correspond to water and nutrient stress rather than herbivore pressure. Further, while the cyanogenesis polymorphism is a target of selection, there also is selection acting on the cyanogenic glucosides polymorphism alone. These results suggest that cyanogenic glucosides and hydrogen cyanide may have multiple functions within a plant and that these functions have impacts on fitness that may or may not be in the same direction. We conclude that relationships between selective pressures involved in ecological tradeoffs may vary both in space and in time, and will affect spatial patterns of phenotypic and genetic diversity.