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Sequencing of the plastome in the leafless green mycoheterotroph Cymbidium macrorhizon helps us to understand an early stage of fully mycoheterotrophic plastome structure

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Abstract

To date, plastome studies of mycoheterotrophic orchids have focused on nongreen mycoheterotrophic or partially mycoheterotrophic species. Cymbidium macrorhizon is a fully mycoheterotrophic orchid that lacks leaves and roots, although its inflorescence rachis is pale green. It has degraded stomata, specific fungal partners, and high concentrations of heavy stable nitrogen and carbon isotopes. Therefore, the plastome of this species is expected to represent an early stage of a fully mycoheterotrophic plastome. In this study, we sequenced the plastomes of C. macrorhizon and closely related species (C. ensifolium, C. kanran, and C. lancifolium). Plastomes of the four Cymbidium species were almost identical structurally, but differed somewhat from those of previously studied species. The genes for the photosynthetic subunits of NADH dehydrogenase, ndhF and ndhH, were absent from all four newly sequenced plastomes, whereas only ndhJ was absent from C. ensifolium. In section Pachyrhizanthe (C. lancifolium and C. macrorhizon), ndhE, ndhI, and ndhJ were pseudogenized. With the exception of ndh and ycf, 64 protein-coding genes in C. macrorhizon were apparently functional. Most of them were highly conserved and under purifying selection. Therefore, no direct evidence is available to suggest that genes related to photosynthesis have lost their functions in C. macrorhizon. This discordance between molecular and physiological features for the trophic status of C. macrorhizon might result from a lag between photosynthetic function loss and relaxed purifying selection.

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Funding

This work was supported by research program  of the Korea National Arboretum (KNA 1-2-13, 14-2).

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Correspondence to Joo-Hwan Kim.

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The authors declare that they have no competing interests.

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Human and animal materials are not used in this paper.

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Handling editor: Eric Schranz.

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Online Resource 1. Alignment of 68 genes for phylogenetic study.

Online Resource 2. Log likelihoods of branch models for 64 coding genes.

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Kim, H.T., Shin, CH., Sun, H. et al. Sequencing of the plastome in the leafless green mycoheterotroph Cymbidium macrorhizon helps us to understand an early stage of fully mycoheterotrophic plastome structure. Plant Syst Evol 304, 245–258 (2018). https://doi.org/10.1007/s00606-017-1472-1

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  • DOI: https://doi.org/10.1007/s00606-017-1472-1

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