From the ground up: the evolution of structurally dependent plants in Hylocereeae (Cactaceae)

Abstract

Structurally dependent plants have evolved six times independently in Cactaceae, where the tribe Hylocereeae is one of the two largest lineages in terms of number of species. To understand the acquisition of different ecological strategies of growth and its relation with the structural evolution of the stems, we have studied the morpho-anatomy of sixty-nine species from all genera of the Hylocereeae. Trait evolution analyses for thirty-six structural attributes, as well as a trait correlation analysis through Bayesian methods were performed. The results allowed us to explain the evolutionary relevance of the patterns of structural changes and their strong associations with the growth form. Terrestrial and epiphytic taxa have contrasting structural characteristics related to the mechanical support as the number of ribs, the appearance of spines, the type hypodermis and cortex, the sclerenchyma in cortical vascular bundles and pith, as well as the patterns of wood development, whereas Aporocactus, Selenicereus and Weberocereus seem to be transitional between terrestrial and epiphytic genera, by showing structural features from both growth forms.