Variations On A Theme: Repeated Evolution Of Succulent Life Forms In the Portulacineae (Caryophyllales)

Abstract

The succulent life form is a tried and true strategy for plants living in arid environments. It has evolved in many distantly related lineages comprising 12,500 species from 70 flowering plant families and has spawned remarkable radiations. Three major groups are generally recognized: (1) stem succulents (that is, leafless cactus-like growth forms), (2) leaf succulents, and (3) caudiciform and pachycaul succulents. All three lifeform groups are represented in the relatively small suborder Portulacineae. Here we suggest that this diversity provides a unique opportunity to evaluate early cactus evolution within a richer contextual framework. We briefly review what we know about the phylogenetic relationships within the suborder Portulacineae (that is, Basellaceae, Cactaceae, Didiereaceae, and Portulacaceae) and the morphology and ecology of all major Portulacineae lineages. We then outline what we believe to be key areas for future research on these understudied plants and discuss several hypothetical “pre-adaptations” and conditions in ancestral Portulacineae that may have promoted the repeated evolution of unusual succulent life forms.