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WHOLE PLANT AND ECOPHYSIOLOGY

Crassulacean Acid Metabolism and Epiphytism Linked to Adaptive Radiations in the Orchidaceae^1,[OA]

Biochemistry and Molecular Biology MS 200, University of Nevada, Reno, Nevada 89557–0200 (K.S., J.C.C.); Smithsonian Tropical Research Institute, Balboa, Ancón, Republic of Panama (K.S., K.W.); and Botany and Plant Sciences, University of California, Riverside, California 92521 (L.S.S.)

Species of the large family Orchidaceae display a spectacular array of adaptations and rapid speciations that are linked to several innovative features, including specialized pollination syndromes, colonization of epiphytic habitats, and the presence of Crassulacean acid metabolism (CAM), a water-conserving photosynthetic pathway. To better understand the role of CAM and epiphytism in the evolutionary expansion of tropical orchids, we sampled leaf carbon isotopic composition of 1,103 species native to Panama and Costa Rica, performed character state reconstruction and phylogenetic trait analysis of CAM and epiphytism, and related strong CAM, present in 10% of species surveyed, to climatic variables and the evolution of epiphytism in tropical regions. Altitude was the most important predictor of photosynthetic pathway when all environmental variables were taken into account, with CAM being most prevalent at low altitudes. By creating integrated orchid trees to reconstruct ancestral character states, we found that C₃ photosynthesis is the ancestral state and that CAM has evolved at least 10 independent times with several reversals. A large CAM radiation event within the Epidendroideae, the most species-rich epiphytic clade of any known plant group, is linked to a Tertiary species radiation that originated 65 million years ago. Our study shows that parallel evolution of CAM is present among subfamilies of orchids, and correlated divergence between photosynthetic pathways and epiphytism can be explained by the prevalence of CAM in low-elevation epiphytes and rapid speciation of high-elevation epiphytes in the Neotropics, contributing to the astounding diversity in the Orchidaceae.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Katia Silvera (silverak{at}unr.nevada.edu).

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.132555

^* Corresponding author; e-mail silverak{at}unr.nevada.edu.

Received November 24, 2008; accepted January 28, 2009; published January 30, 2009.

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Peter V. Minorsky
Plant Physiol. 2009 149: 1621-1622. [Full Text]