title: Mechanistic model of evolutionary rate variation en route to a nonphotosynthetic lifestyle in plants.
abstract: Because novel environmental conditions alter the selection pressure on genes or entire subgenomes, adaptive and nonadaptive changes will leave a measurable signature in the genomes, shaping their molecular evolution. We present herein a model of the trajectory of plastid genome evolution under progressively relaxed functional constraints during the transition from autotrophy to a nonphotosynthetic parasitic lifestyle. We show that relaxed purifying selection in all plastid genes is linked to obligate parasitism, characterized by the parasite's dependence on a host to fulfill its life cycle, rather than the loss of photosynthesis. Evolutionary rates and selection pressure coevolve with macrostructural and microstructural changes, the extent of functional reduction, and the establishment of the obligate parasitic lifestyle. Inferred bursts of gene losses coincide with periods of relaxed selection, which are followed by phases of intensified selection and rate deceleration in the retained functional complexes. Our findings suggest that the transition to obligate parasitism relaxes functional constraints on plastid genes in a stepwise manner. During the functional reduction process, the elevation of evolutionary rates reaches several new rate equilibria, possibly relating to the modified protein turnover rates in heterotrophic plastids.
authors.affiliation: Department of Botany and Biodiversity Research, University of Vienna, A-1030 Vienna, Austria; Institute for Evolution and Biodiversity, University of Muenster, 48149 Muenster, Germany; susann.wicke@uni-muenster.de.
authors.forename: Susann
authors.identifier: 0000-0001-5785-9500
authors.initials: S
authors.lastname: Wicke
authors.affiliation: Institute for Evolution and Biodiversity, University of Muenster, 48149 Muenster, Germany;
authors.forename: Kai F
authors.initials: KF
authors.lastname: Müller
authors.affiliation: Department of Biology, Institute of Molecular Evolutionary Genetics, Pennsylvania State University, University Park, PA 16802;
authors.forename: Claude W
authors.initials: CW
authors.lastname: dePamphilis
authors.affiliation: Nees Institute for Biodiversity of Plants, University of Bonn, 53115 Bonn, Germany;
authors.forename: Dietmar
authors.initials: D
authors.lastname: Quandt
authors.affiliation: Department of Plant Biodiversity, Technical University of Munich, 85354 Freising, Germany.
authors.forename: Sidonie
authors.initials: S
authors.lastname: Bellot
authors.affiliation: Department of Botany and Biodiversity Research, University of Vienna, A-1030 Vienna, Austria;
authors.forename: Gerald M
authors.identifier: 0000-0003-2811-3317
authors.initials: GM
authors.lastname: Schneeweiss
mesh_terms: D019143:Evolution, Molecular; D054627:Genome, Plastid; D010272:Parasitic Diseases / Q000235:genetics; D010788:Photosynthesis; D010802:Phylogeny; D010944:Plants / Q000235:genetics*; D018087:Plastids / Q000378:metabolism; D012641:Selection, Genetic
keywords: Orobanchaceae; evolutionary rates; parasitism; plastid genomes; relaxed selection