Micro- and macroevolutionary adaptation through repeated loss of a complete metabolic pathway.

There is growing evidence for the convergent evolution of physically linked gene clusters encoding chemical defense pathways. Metabolic clusters are proposed to evolve because they ensure co-inheritance of all required genes where the defense is favored, and prevent inheritance of toxic partial pathways where it is not. This hypothesis rests on the assumption that clusters evolve in species where selection favors intraspecific polymorphism for the defense; however, they have not been examined in polymorphic species. We examined metabolic cluster evolution in relation to an adaptive polymorphism for cyanogenic glucoside (CNglc) production in clover. Using 163 accessions, we performed CNglc assays, BAC sequencing, Southern hybridizations and molecular evolutionary analyses. We find that the CNglc pathway forms a 138-kb cluster in white clover, and that the adaptive polymorphism occurs through presence/absence of the complete cluster. Component genes are orthologous to those in the distantly related legume Lotus japonicus. These findings provide empirical support for the co-inheritance hypothesis, and they indicate that adaptive CNglc variation in white clover evolves through recurrent deletions of the entire pathway. They further indicate that the shared ancestor of many important legume crops was likely cyanogenic and that this defense was lost repeatedly over the last 50 Myr.