Rapid evolution of an adaptive cyanogenesis cline in introduced North American white clover (Trifolium repens L.)

White clover is polymorphic for cyanogenesis (HCN production after tissue damage), and this herbivore defense polymorphism has served as a classic model for studying adaptive variation. The cyanogenic phenotype requires two interacting biochemical components; the presence/absence of each component is controlled by a simple Mendelian gene (Ac/ac and Li/li). Climate-associated cyanogenesis clines occur in both native (Eurasian) and introduced populations worldwide, with cyanogenic plants predominating in warmer locations. Moreover, previous studies have suggested that epistatic selection may act within populations to maintain cyanogenic (AcLi) plants and acyanogenic plants that lack both components (acli plants) at the expense of plants possessing a single component (Acli and acLi plants). Here we examine the roles of selection, gene flow, and demography in the evolution of a latitudinal cyanogenesis cline in introduced North American populations. Using 1,200 plants sampled across a 1,650...