Testing the evolutionary drivers of nitrogen-fixing symbioses in challenging soil environments

- While the importance of root nodular symbioses (RNS) in plants has long been recognized, the ecological and evolutionary factors maintaining RNS remain obscure. RNS is associated with environmental stressors such as aridity and nitrogen-poor soils; the ability to tolerate harsh environments may provide ecological opportunities for diversification, yet, nodulators are also diverse outside these environments. - We test several environmental determinants of increased survival and enhanced diversification of RNS species, using an explicitly phylogenetic approach for the first time. We assembled the largest phylogeny of the nitrogen-fixing clade to date and a comprehensive set of abiotic niche estimates and nodulation data. We used comparative phylogenetic tools to test environmental and diversification associations. - We found that RNS is associated with warm, arid, and nitrogen-poor habitats. However, RNS was gained long before lineages entered these habitats. RNS is associated with accelerated diversification, but diversification rates are heterogeneous among nodulators, and non-legume nodulators do not show elevated diversification. - Our findings undermine the interpretation that RNS directly drove the invasion of challenging habitats, and do not support a direct relationship between soil or climate and the diversity of nodulators. Still, RNS may have been an important exaptation allowing further niche evolution.