Anatomical variation in Cactaceae and relatives: Trait lability and evolutionary innovation

Abstract

The cacti have undergone extensive specialization in their evolutionary history, providing an excellent system in which to address large-scale questions of morphological and physiological adaptation. Recent molecular phylogenetic studies suggest that (1) Pereskia, the leafy genus long interpreted as the sister group of all other cacti, is likely paraphyletic, and (2) Cactaceae are nested within a paraphyletic Portulacaceae as a member of the "ACPT" clade (Anacampseroteae, Cactaceae, Portulaca, and Talinum). We collected new data on the vegetative anatomy of the ACPT clade and relatives to evaluate whether patterns in the distributions of traits may provide insight into early events in the evolutionary transition to the cactus life form. Many traits had high levels of homoplasy and were mostly equivocal with regard to infraclade relationships of ACPT, although several characters do lend further support to a paraphyletic Pereskia. These include a thick stem cuticle, prominent stem mucilage cells, and hypodermal calcium oxalate druses, all of which are likely to be important traits for stem water storage and photosynthesis. We hypothesize that high lability of many putative "precursor" traits may have been critical in generating the organismal context necessary for the evolution of an efficient and integrated photosynthetic stem.

Decline of Leaf Hydraulic Conductance with Dehydration: Relationship to Leaf Size and Venation Architecture

“…It is important to recognize, however, that there can be other possible routes to leaf drought tolerance than a low hydraulic vulnerability and thus being able to maintain hydraulic and photosynthetic function during drought. Thus, some species can achieve substantial drought tolerance via a low cuticular conductance and water storage, with an extreme development of this mechanism in succulent plants (Ogburn and Edwards, 2009). In this study, Hedera canariensis showed this drought tolerance mechanism (Sack et al, 2003c;Metcalfe, 2005), possibly explaining its relatively high K leaf vulnerability for a species that can tolerate drought.…”

Character lability in deep-sea bamboo corals (Octocorallia, Isididae, Keratoisidinae)

“…Trait lability has been reported in a number of land organisms such as plants (Kimball & Crawford 2004, Ogburn & Edwards 2009), mosses (Shaw & Allen 2000), mammals (Sánchez-Villagra & Williams 1998), wasps (Jousselin et al 2004) and amphibians (e.g. salamanders, Chippindale et al 2004, Mueller et al 2004.…”

Increased Water Storage Capacity in Cactus Wood: A Study in the Tribe Cereeae (Cactoideae, Cactaceae)

“…Cactaceae is nested within Portulacaceae, according to recent molecular phylogenies, (Applequist and Wallace 2001, Edwards and others 2005, Nyffeler 2007). However, besides stem and leaf succulence, the majority of Caryophyllales lack these anatomical synapomorphies (Nyffeler and others 2008;Ogburn and Edwards 2009). All that one can say is that the clade containing Cactaceae, Portulacaceae, and Didiereaceae share a trend toward more succulence, albeit sometimes using different strategies.…”