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.