Data from: Aridity shapes cyanogenesis cline evolution in white clover (Trifolium repens L.)
Kooyers, Nicholas J.; Gage, Lily R.; Al-Lozi, Amal; Olsen, Kenneth M. (2014), Data from: Aridity shapes cyanogenesis cline evolution in white clover (Trifolium repens L.), Dryad, Dataset, https://doi.org/10.5061/dryad.j7q43
Adaptive differentiation between populations is often proposed to be the product of multiple interacting selective pressures, although empirical support for this is scarce. In white clover, populations show adaptive differentiation in frequencies of cyanogenesis, the ability to produce HCN after tissue damage. This polymorphism arises through independently segregating polymorphisms for the presence/absence of two required cyanogenic components, cyanogenic glucosides and their hydrolyzing enzyme. White clover populations worldwide have evolved a series of recurrent, climate-associated clines, with higher frequencies of cyanogenic plants in warmer locations. These clines have traditionally been hypothesized to reflect a fitness tradeoff between chemical defense in herbivore-rich areas (warmer climates) and energetic costs of producing cyanogenic components in areas of low herbivore pressure (cooler climates). Recent observational studies suggest that cyanogenic components may be beneficial in water-stressed environments. We investigated fitness tradeoffs associated with temperature-induced water-stress in the cyanogenesis system using manipulative experiments in growth chambers and population surveys across a longitudinal precipitation gradient in the central U.S. We find that plants producing cyanogenic glucosides have higher relative fitness in treatments simulating a moderate, persistent drought stress. In water-neutral treatments, there are energetic costs to producing cyanogenic components, but only in treatments with nutrient stress. These fitness tradeoffs are consistent with cyanogenesis frequencies in natural populations, where we find clinal variation in the proportion of plants producing cyanogenic glucosides along the precipitation gradient. These results suggest that multiple selective pressures interact to maintain this adaptive polymorphism, and that modeling adaptation will require knowledge of environment-specific fitness effects.