Interaction between floral rewards and floral symmetry shapes diversification dynamics in Amazonian trees

Floral zygomorphy, or monosymmetry, is thought to have a positive effect on the diversification rates of angiosperms, but its true impact is still an open topic. Given the controversy surrounding this matter, our study evaluates whether rewards such as nectar and pollen produced by floral structures evolve in correlation with floral symmetry and its effects on diversification. Using the family Lecythidaceae, we characterized the floral structures that produce different rewards for pollinators and assessed whether these rewards evolve correlated with the varying levels of floral symmetry observed in the subfamily Lecythidoideae. We also used trait-dependent diversification to assess whether this trait correlation affects lineage diversification rates. Floral rewards are produced by structures that are morphological modifications of the androecium, resulting in unusual floral nectaries and leading to a gradual sterilization of the male function in the flowers. The kinds of rewards evolve in a correlated fashion with the varying levels of floral symmetry observed in the subfamily, with nectar production being most likely associated with strongly monosymmetric floral forms. Lastly, our results suggest that this combination of trait states, nectar, and monosymmetry, is loosely associated with increased diversification in Lecythidoideae. We propose that these patterns might be related to the degree of specialization of pollinators attracted by the different types of flowers and rewards, and to the complex interaction between biotic and abiotic factors; however, this is still open to debate. The evaluation of correlated floral traits, especially symmetry and nectar production, is essential for a better understanding of the diversification dynamics of angiosperm lineages.