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MBE Advance Access originally published online on March 22, 2007
Molecular Biology and Evolution 2007 24(6):1366-1379; doi:10.1093/molbev/msm059
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© The Author 2007. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

Research Articles

Chloroplast Genome (cpDNA) of Cycas taitungensis and 56 cp Protein-Coding Genes of Gnetum parvifolium: Insights into cpDNA Evolution and Phylogeny of Extant Seed Plants

Chung-Shien Wu*,{dagger}, Ya-Nan Wang{dagger}, Shu-Mei Liu* and Shu-Miaw Chaw*

* Research Center for Biodiversity, Academia Sinica, Taipei, Taiwan
{dagger} School of Forestry and Resource Conservation, National Taiwan University, Taipei, Taiwan

E-mail: smchaw{at}sinica.edu.tw.

Accepted for publication March 15, 2007.

Phylogenetic relationships among the 5 groups of extant seed plants are presently unsettled. To reexamine this long-standing debate, we determine the complete chloroplast genome (cpDNA) of Cycas taitungensis and 56 protein-coding genes encoded in the cpDNA of Gnetum parvifolium. The cpDNA of Cycas is a circular molecule of 163,403 bp with 2 typical large inverted repeats (IRs) of 25,074 bp each. We inferred phylogenetic relationships among major seed plant lineages using concatenated 56 protein-coding genes in 37 land plants. Phylogenies, generated by the use of 3 independent methods, provide concordant and robust support for the monophylies of extant seed plants, gymnosperms, and angiosperms. Within the modern gymnosperms are 2 highly supported sister clades: Cycas–Ginkgo and Gnetum–Pinus. This result agrees with both the "gnetifer" and "gnepines" hypotheses. The sister relationships in Cycas–Ginkgo and Gnetum–Pinus clades are further reinforced by cpDNA structural evidence. Branch lengths of Cycas–Ginkgo and Gnetum were consistently the shortest and the longest, respectively, in all separate analyses. However, the Gnetum relative rate test revealed this tendency only for the 3rd codon positions and the transversional sites of the first 2 codon positions. A {Psi}tufA located between psbE and petL genes is here first detected in Anthoceros (a hornwort), cycads, and Ginkgo. We demonstrate that the {Psi}tufA is a footprint descended from the chloroplast tufA of green algae. The duplication of ycf2 genes and their shift into IRs should have taken place at least in the common ancestor of seed plants more than 300 MYA, and the tRNAPro-GGG gene was lost from the angiosperm lineage at least 150 MYA. Additionally, from cpDNA structural comparison, we propose an alternative model for the loss of large IR regions in black pine. More cpDNA data from non-Pinaceae conifers are necessary to justify whether the gnetifer or gnepines hypothesis is valid and to generate solid structural evidence for the monophyly of extant gymnosperms.

Key Words: Cycas taitungensis • Gnetum parvifolium • Ginkgo • chloroplast genome • gymnosperms • seed plants • phylogeny • evolution • substitution rate

William Martin, Associate Editor


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