Molecular Ecology and Evolution
Barrett, Craig F. , Freudenstein, John .
Molecular evolution of rbcL in the mycoheterotrophic Coralroot orchids (Corallorhiza: Orchidaceae: Epidendroideae).
The photosynthesis gene rbcL (RuBisCO Large Subunit) has been the focus of numerous plant phylogenetic studies, and of studies on molecular evolution in parasitic plants. However, there has been a complete lack of investigation of photosynthesis gene molecular evolution in fully mycoheterotrophic plants. These plants invade pre-existing mutualistic associations between ectomycorrhizal trees and fungi, from which they obtain fixed carbon and nutrients. We sequenced the rbcL gene for several members of the mycoheterotrophic orchid genus Corallorhiza, which contains both green (photosynthetic) and non-green (putatively non-photosynthetic) species, to test hypotheses regarding phylogenetic relationships and molecular evolutionary processes. We hypothesized that some Corallorhiza lineages would show evidence of pseudogene formation due to a relaxation of purifying selection on rbcL's carboxylase function. Phylogenetic analysis of rbcL + nuclear Internal Transcribed Spacer (ITS) gave generally high jackknife support for relationships among species. Furthermore, rbcL showed substantial variation in nucleotide sequence within the C. striata and C. maculata complexes, demonstrating its potential utility for multilocus phylogeographic study. We found evidence of pseudogene formation in all lineages of the C. striata complex, and in some lineages of the C. maculata complex (those from the USA but not Mexico). This evidence includes: premature stop codons, insertions/deletions causing reading frame shifts, decreased dS/dN ratios, replacement substitutions not observed in other photosynthetic species, evolutionary rate heterogeneity, and high likelihood of neutral evolution among specific branches of the rbcL phylogeny. Taken together, these findings support our hypothesis that pseudogene formation has occurred within the C. striata and C. maculata lineages. The evolution of rbcL in Corallorhiza may serve as an exemplary system to study the effects of relaxed evolutionary constraints on photosynthesis genes for the > 400 documented fully mycoheterotrophic plant species.
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1 - Ohio State University, Department of Evolution, Ecology, and Organismal Biology, OSU Herbarium (OS), Museum of Biological Diversity, 1315 Kinnear Rd., Columbus, Ohio, 43212, USA
2 - Ohio State University, Department of Evolution, Ecology, and Organismal Biology, University Herbarium, 1315 Kinnear Rd., Columbus, Ohio, 43212, USA
Presentation Type: Oral Paper:Papers for Topics
Location: Boulevard B/Hilton
Date: Wednesday, July 11th, 2007
Time: 11:45 AM