Coexistence of sympatric taxa of Chamaecrista section Xerocalyx: Addressing the interplay between morphology and biotic interactions

Animal-plant interactions may prevent gene flow and promote divergent selection among closely related plants, ultimately leading to formation of new species. This may be the case for Chamaecrista sect. Xerocalyx, in which two or more of the 24 varieties often are encountered in the same area, with marked morphological and phenological differences among them. Over a broad geographical range, however, the morphological gaps among varieties disappear, and they cannot be distinguished clearly.;Several biotic interactions contribute to the fitness of Chamaecrista species. The flowers are pollinated by bees that remove pollen as a reward. Leaves, flowers, fruits, and seeds are attacked by herbivores that can directly and indirectly reduce reproductive success. In addition, all Xerocalyx species have extrafloral nectaries on the leaves. The nectar produced by these structures attracts ants that may attack herbivores and improve plant fitness. Thus, dissimilarities in the morphology of co-occurring Chamaecrista can potentially reduce plant-plant competition for pollinators and mutualistic ants, reduce the number of shared herbivores, and/or reflect diverging strategies for resource acquisition and defense against herbivory.;Hand pollination experiments demonstrated that production of hybrid seeds among syntopic varieties of C. desvauxii, a species within section Xerocalyx, was severely limited by slower pollen tube growth in foreign styles, differences in style length between pollen donor and pollen receiver, and abortion of developing fruits and seeds by the mother plant. In addition to the existence of reproductive isolation mechanisms, cooccurring varieties were also clearly distinguishable based on morphological traits, including the sizes of flowers, leaves, and extrafloral nectaries. Variation in nectary size modifies the role of ants as a defensive mechanism. In the field, the variety bearing the largest nectary also had the higher amount of nectar production and the highest levels of visitation by ants. Removal of the extrafloral nectaries resulted in decreased fruit and seed set, but only for that variety. However, the benefit of attracting ants was greatly reduced when the ecological context was modified by the exclusion of seed predators. Overall, these results provide support for separating the varieties into different species, and suggest a role for interactions with mutualists and herbivores in shaping morphological traits of sympatric taxa in this group.;To understand the patterns of taxa co-occurrence in this group, a novel approach was used to assess local morphological dissimilarity across a wide geographic scale using collection data. Interactions between co-occurring varieties may result in a pattern of morphological divergence within sympatric communities. Both vegetative and reproductive traits were more dissimilar between pairs of sympatric individuals of different varieties than between pairs of allopatric individuals. Based on permutation tests, this pattern is more likely to stem from competitive exclusion and ecological sorting than from character divergence following competitive interactions.;This work provides new insight into the patterns and processes of coexistence in phenotypically continuous taxa, from local to broad geographic scales. In particular, it shows how widespread patterns of locally differentiated assemblages of taxa can be formed and maintained in groups that are phenotypically continuous over their full range. Chamaecrista section Xerocalyx is now well positioned as a valuable system to further investigate the evolution of morphological diversification, and how trait divergence influences coexistence.