title: Investigating the path of plastid genome degradation in an early-transitional clade of heterotrophic orchids, and implications for heterotrophic angiosperms.
abstract: Parasitic organisms exemplify morphological and genomic reduction. Some heterotrophic, parasitic plants harbor drastically reduced and degraded plastid genomes resulting from relaxed selective pressure on photosynthetic function. However, few studies have addressed the initial stages of plastome degradation in groups containing both photosynthetic and nonphotosynthetic species. Corallorhiza is a genus of leafless, heterotrophic orchids that contains both green, photosynthetic species and nongreen, putatively nonphotosynthetic species, and represents an ideal system in which to assess the beginning of the transition to a "minimal plastome." Complete plastomes were generated for nine taxa of Corallorhiza using Illumina paired-end sequencing of genomic DNA to assess the degree of degradation among taxa, and for comparison with a general model of degradation among angiosperms. Quantification of total chlorophyll suggests that nongreen Corallorhiza still produce chlorophyll, but at 10-fold lower concentrations than green congeners. Complete plastomes and partial nuclear rDNA cistrons yielded a fully resolved tree for Corallorhiza, with at least two independent losses of photosynthesis, evidenced by gene deletions and pseudogenes in Co. striata and nongreen Co. maculata. All Corallorhiza show some evidence of degradation in genes of the NAD(P)H dehydrogenase complex. Among genes with open reading frames, photosynthesis-related genes displayed evidence of neutral evolution in nongreen Corallorhiza, whereas genes of the ATP synthase complex displayed some evidence of positive selection in these same groups, though for reasons unknown. Corallorhiza spans the early stages of a general model of plastome degradation and has added critical insight for understanding the process of plastome evolution in heterotrophic angiosperms.
authors.affiliation: Department of Biological Sciences, California State University, Los Angeles cbarret5@calstatela.edu.
authors.forename: Craig F
authors.initials: CF
authors.lastname: Barrett
authors.affiliation: Department of Evolution, Ecology, and Organismal Biology and the Museum of Biological Diversity, Ohio State University.
authors.forename: John V
authors.initials: JV
authors.lastname: Freudenstein
authors.affiliation: Department of Biological Sciences, California State University, Los Angeles.
authors.forename: Jeff
authors.initials: J
authors.lastname: Li
authors.affiliation: Division of Biological Sciences, University of Missouri.
authors.forename: Dustin R
authors.initials: DR
authors.lastname: Mayfield-Jones
authors.affiliation: Department of Biological Sciences, California State University, Los Angeles.
authors.forename: Leticia
authors.initials: L
authors.lastname: Perez
authors.affiliation: Division of Biological Sciences, University of Missouri.
authors.forename: J Chris
authors.initials: JC
authors.lastname: Pires
authors.affiliation: Department of Biological Sciences, California State University, Los Angeles.
authors.forename: Cristian
authors.initials: C
authors.lastname: Santos
mesh_terms: D002734:Chlorophyll / Q000378:metabolism; D019143:Evolution, Molecular; D017343:Genes, Plant; D054627:Genome, Plastid; D052836:Heterotrophic Processes / Q000235:genetics; D008969:Molecular Sequence Data; D029595:Orchidaceae / Q000235:genetics* / Q000378:metabolism; D010802:Phylogeny; D017422:Sequence Analysis, DNA
chemical_list: D002734:Chlorophyll
keywords: Orchidaceae; chlorophyll; chloroplast; parasite; photosynthesis; pseudogene