Convergent evolution of red pigmentation in extrafloral nectaries: global patterns and mechanisms

Convergent traits are a signature of adaptation, where shared selective pressures may lead to the replicated evolution of traits with similar function. Here, we present an integrative investigation of a widespread yet understudied feature of an iconic, convergent mutualistic plant trait - red pigmentation in extrafloral nectaries (EFNs). We surveyed 625 species to describe the distribution of red coloration in EFNs across plant morphology, biogeography, and phylogeny for the first time and propose several adaptive hypotheses for the drivers of their convergence across plants. We then test several of these hypotheses by integrating our global-scale biogeographic and phylogenetic analysis with phylogenetically paired common garden bioassays and a museum collections survey. We find that across the plant phylogeny, cold wet regions and deserts are hotspots of red EFN coloration, and multiple lines of evidence point to a fungal defense role of red pigments in EFNs. Ultimately, our findings highlight red EFNs as a highly convergent phenotype across plants and point to distinct selective pressures underlying the distribution of EFN coloration and EFNs themselves.