The Evolutionary Significance of Leaf Nodulation: Evidence from Ardisia and Its Relatives (Primulaceae: Myrsinoideae)

Abstract

Simple Summary Leaf nodule symbiosis, a unique bacterial association occurring in the phyllosphere microbiome, has been shown to influence host growth, immune responses, and secondary metabolism, suggesting its potential role in promoting evolutionary radiations. Nevertheless, direct macroevolutionary evidence linking this symbiosis to lineage diversification has remained scarce. In this study, we used Ardisia, a lineage characterized by typical leaf nodulation, to test this hypothesis. We reconstructed a robust phylogenetic framework based on plastid genomes, nrDNA, and genome-wide single nucleotide polymorphism (SNP) datasets. We resolved several longstanding phylogenetic uncertainties within Ardisia and its allies, including the placement of Sadiria, Tapeinosperma, Amblyanthus, and Amblyanthopsis, and revealed a middle Miocene rapid radiation in Ardisia and its allies. Importantly, we demonstrate that the origin of leaf nodule symbiosis coincided temporally with this radiation (~11–8 Ma). Diversification analyses further show that leaf-nodulated lineages exhibit significantly elevated speciation rates, suggesting that leaf nodule symbiosis may represent a key evolutionary innovation driving diversification.