Abstract Detail

Systematics Section

Horn, James W. [1], van Ee, Benjamin W. [2], Riina, Ricarda [3], Morawetz, Jeffery J. [4], Berry, Paul E. [5], Steinmann, Victor Werner [6], Wurdack, Kenneth [7].

Are growth forms and photosynthetic pathways correlates of diversification in Euphorbia (Euphorbiaceae)?

A fundamental goal in understanding the evolutionary biology of “giant“ plant genera is developing and testing competing hypotheses of mechanisms engendering these spectacular radiations. With over 2100 recognized species, Euphorbia is the second largest genus of angiosperms, and perhaps the most varied with respect to growth form. The NSF-funded Euphorbia Planetary Biodiversity Inventory (PBI) provides the collaborative infrastructure that enables macroevolutionary questions in this giant, globally distributed genus to begin to be addressed. The cyathium—a unique specialized pseudanthial inflorescence type and synapomorphy for Euphorbia—has frequently been invoked as being the primary correlate of species diversification in the genus. However, the extreme evolutionary lability in growth forms, in addition to the multiple, independent origins of alternative photosynthetic pathways (C4 and CAM) could be more important explanatory factors. Using a robustly resolved phylogeny of tribe Euphorbieae (using 9 loci and ~16 kb per taxon), we test the significance of growth form and photosynthetic pathway type on diversification in Euphorbia. To assess the photosynthetic pathway for each species, d13C values were obtained for 174 of the 176 species included in the study. Obligate CAM plants independently evolved at least 10 times within Euphorbia, and are represented in each of the 4 major subgeneric clades. Two of these four clades (subg. Rhizanthium and Euphorbia) have major radiations of cactiform succulents correlated with the origin of obligate CAM photosynthesis; collectively they account for nearly 650 species. There appears to be only a single origin of C4 photosynthesis in the core clade of species formerly recognized as the genus Chamaesyce, which constitutes a radiation of nearly 300 species. Although a woody, non-succulent growth form optimizes as the ancestral state along all nodes of the backbone of the phylogeny, the independent evolution of (C3) herbs in subgenera Esula and Chamaesyce gave rise to radiations totaling about 600 species.

Broader Impacts:

Related Links:
http://www.euphorbiaceae.org/

1 - Smithsonian Institution, Department of Botany, MRC–166, P.O. Box 37012, Washington, DC, 20013, USA
2 - University of Michigan, Herbarium and Department of Ecology and Evolutionary Biology, 3600 Varsity Dr., Ann Arbor, MI, 48108, USA
3 - Real Jardin Botanico de Madrid, Plaza de Murillo, 2. E-28014, Madrid, Spain
4 - Rancho Santa Ana Botanic Garden, 1500 N. College Ave., Claremont, CA, 91711, USA
5 - University of Michigan, Herbarium & Dept. Ecology and Evolutionary Biology, 3600 Varisty Drive, Ann Arbor, MI, 48108, USA
6 - Instituto De Ecología A.C., Avenida Lázaro Cárdenas 253, Apartado Postal 386, 61600 Pátzcuaro, Michoacán, Mexico
7 - Smithsonian Institution, Department of Botany, MRC-166, National Museum of Natural History, PO Box 37012, Washington, DC, 20013-7012, USA

Keywords:
Euphorbia
C4 photosynthesis
morphology
Crassulacean acid metabolism (CAM)
diversification.

Presentation Type: Oral Paper:Papers for Sections
Session: 9
Location: Ballroom B/Convention Center
Date: Monday, August 2nd, 2010
Time: 2:45 PM
Number: 9007
Abstract ID:626