Growth form evolution in Adenia (Passifloraceae) and a model of the evolution of succulence

Abstract

The architecture of a plant is intimately tied to its fitness. Knowledge of the processes and patterns of growth form evolution can therefore contribute to a richer understanding of plant evolution. The genus Adenia (Passifloraceae) of ca. 100 species is an Old World lineage in which growth form radiated. I constructed a molecular phylogeny of the group, analyzed the stem and tuber anatomy of over half the species, and investigated patterns of growth form evolution in a phylogenetic context. I also described four new species and a new combination. Predictions based on evolutionary developmental models of growth form evolution were tested in Adenia, and one of them, the homeotic switch hypothesis, was tested throughout the eudicots. The switch hypothesis claims that the storage tissue of tubers and stems results from a common developmental origin. Phylogenetic analyses revealed that growth form transitions were frequent, and anatomical studies revealed traits that are associated with each growth form; moreover, traits are shared between tubers and succulent stems as predicted by the switch hypothesis. As expected, tuberous plants and succulents are also closely related across the eudicots. The switch hypothesis is substantiated in Adenia and the eudicots as a whole.