Fig. 2.

Pie charts indicate the likelihood of a host node being in either a nonsymbiotic state (orange), a “regular” N₂-fixing state (green), or in a stable fixing state (purple), where the symbiosis has a high persistence. Represented are the legumes from their ancestral node about 62 million years ago. After the evolution of symbiotic nitrogen fixation (transition from orange to green), repeated evolution of stable fixing (transition from green to purple) is found across the legumes. Particularly within the Papilionoideae, legume species are very likely to currently be stable fixers, meaning that they show a continued capacity to form the symbiosis over evolutionary time. (Scale bar: 10 million years.)