title: Rampant gene loss in the underground orchid Rhizanthella gardneri highlights evolutionary constraints on plastid genomes.
abstract: Since the endosymbiotic origin of chloroplasts from cyanobacteria 2 billion years ago, the evolution of plastids has been characterized by massive loss of genes. Most plants and algae depend on photosynthesis for energy and have retained ∼110 genes in their chloroplast genome that encode components of the gene expression machinery and subunits of the photosystems. However, nonphotosynthetic parasitic plants have retained a reduced plastid genome, showing that plastids have other essential functions besides photosynthesis. We sequenced the complete plastid genome of the underground orchid, Rhizanthella gardneri. This remarkable parasitic subterranean orchid possesses the smallest organelle genome yet described in land plants. With only 20 proteins, 4 rRNAs, and 9 tRNAs encoded in 59,190 bp, it is the least gene-rich plastid genome known to date apart from the fragmented plastid genome of some dinoflagellates. Despite numerous differences, striking similarities with plastid genomes from unrelated parasitic plants identify a minimal set of protein-encoding and tRNA genes required to reside in plant plastids. This prime example of convergent evolution implies shared selective constraints on gene loss or transfer.
authors.affiliation: Australian Research Council Centre of Excellence in Plant Energy Biology, University of Western Australia, Perth, Australia. delannoy@evry.inra.fr
authors.forename: Etienne
authors.initials: E
authors.lastname: Delannoy
authors.forename: Sota
authors.initials: S
authors.lastname: Fujii
authors.forename: Catherine
authors.initials: C
authors.lastname: Colas des Francs-Small
authors.forename: Mark
authors.initials: M
authors.lastname: Brundrett
authors.forename: Ian
authors.initials: I
authors.lastname: Small
mesh_terms: D001499:Bayes Theorem; D002736:Chloroplasts / Q000235:genetics*; D016000:Cluster Analysis; D019143:Evolution, Molecular; D017353:Gene Deletion; D017343:Genes, Plant; D018745:Genome, Plant; D023281:Genomics; D029595:Orchidaceae / Q000235:genetics* / Q000502:physiology; D010788:Photosynthesis / Q000235:genetics; D010802:Phylogeny; D010944:Plants / Q000235:genetics; D014176:Protein Biosynthesis / Q000235:genetics; D012343:RNA, Transfer / Q000235:genetics
chemical_list: D012343:RNA, Transfer