Adaptive cyanogenesis clines evolve recurrently through geographical sorting of existing gene deletions.

Identifying the genetic basis of parallel phenotypic evolution provides insight into the process of adaptation and evolutionary constraint. White clover (Trifolium repens) has evolved climate-associated adaptive clines in cyanogenesis (the ability to produce hydrogen cyanide upon tissue damage) in several world regions where it has been introduced. Gene-deletion polymorphisms at the CYP79D15 and Li loci underlie the presence/absence of the cyanogenic phenotype. Both loci have undergone multiple independent gene-deletion events, which are identifiable through molecular signatures in flanking regions. To investigate whether cyanogenesis clines in introduced populations have evolved through the sorting of standing genetic variation or de novo gene deletions, we examined cyanogenesis gene-flanking regions in three world regions. In comparison with native Eurasian populations, we find no evidence for novel gene deletion events in any introduced region, which suggests that these adaptive clines have evolved through the geographical sorting of pre-existing genetic variation.

Searching for the bull's eye: agents and targets of selection vary among geographically disparate cyanogenesis clines in white clover (Trifolium repens L.).

The recurrent evolution of adaptive clines within a species can be used to elucidate the selective factors and genetic responses that underlie adaptation. White clover is polymorphic for cyanogenesis (HCN release with tissue damage), and climate-associated cyanogenesis clines have evolved throughout the native and introduced species range. This polymorphism arises through two independently segregating Mendelian polymorphisms for the presence/absence of two required components: cyanogenic glucosides and their hydrolyzing enzyme linamarase. Cyanogenesis is commonly thought to function in herbivore defense; however, the individual cyanogenic components may also serve other physiological functions. To test whether cyanogenesis clines have evolved in response to the same selective pressures acting on the same genetic targets, we examined cyanogenesis cline shape and its environmental correlates in three world regions: southern New Zealand, the central United States and the US Pacific Northwest. For some regional comparisons, cline shapes are remarkably similar despite large differences in the spatial scales over which clines occur (40-1600 km). However, we also find evidence for major differences in both the agents and targets of selection among the sampled clines. Variation in cyanogenesis frequency is best predicted using a combination of minimum winter temperature and aridity variables. Together, our results provide evidence that recurrent adaptive clines do not necessarily reflect shared adaptive processes.