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


Nitrogen-fixing mutualistic relationships with bacteria borne in root nodules are tightly distributed in only ten angiosperm families among four orders, comprising the so-called nitrogen-fixing clade. Despite its status as one of the most globally successful symbioses, the evolutionary and ecological origins of the relationship between nitrogen-fixing bacteria and nodulating plants remain obscure. Basic questions such as the number of origins of nodulation, the drivers that led to its emergence, and its fine-scale distribution across today’s landscape remain unanswered. While poor phylogenetic resolution and limited distributional data have hampered robust quantitative approaches to these questions, evidence to date suggests nitrogen-fixing strategies are indeed primarily associated with environmental factors such as aridity and nitrogen-poor soils despite the ecological diversity of the clade. Although a causative role for the origins and maintenance of the relationship remains to be tested, this evidence is consistent with theoretical predictions that rich soil environments should be associated with decreased investment in expensive symbiotic structures and with a decreasingly mutualistic nature of the plant-bacterium relationship. Here, we make initial progress on these questions by testing hypotheses that (1) nodular nitrogen-fixing strategies are most prevalent in challenging soil environments, and that (2) diversification rates of nitrogen-fixing plants are higher in poor, arid soils, reflecting the specificity of their global success to these environments. To test these hypotheses, we assemble the largest phylogenetic estimate yet for the nitrogen-fixing clade, reconciling GenBank sequences with a backbone informed by densely sampled phylogenomic data and containing more than 19,000 taxa. We synthesize this tree together with global datasets of soil and environmental parameters, densely sampled distributional data, and parametric and non-parametric macroevolutionary methods. See video here: https://drive.google.com/file/d/1lbLkc_ysqiYAbFW565oS8fyCAm5y2MI3/view?usp=sharing

Jul 29, 2020 12:00 AM
Botany 2020 Virtual
Carolina M. Siniscalchi
Carolina M. Siniscalchi
Post Doctoral Associate

I’m a plant systematist, with interests in plant evolution, systematics and taxonomy.