Aridity shapes cyanogenesis cline evolution in white clover (Trifolium repens L.)

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 beneficia...