(Orobanchaceae 21), and Cytinus ruber Fritsch (Cytinaceae 56), overall genome size has been greatly reduced, and key photosynthesis genes have most often become either pseudogenized or deleted altogether. (Orobanchaceae 76 17), Lathraea clandestina L. Barton (Orobanchaceae 22 77), Conopholis americana (L.) Wallr. For example, in holoparasites like Epifagus virginiana (L.) W. Plastid genomic studies of holoparasitic plants, i.e., those that are obligate, nonphotosynthetic, direct parasites on other plants, have revealed plastomes that are highly reduced in size and gene content as a result of relaxed negative (purifying) selection on genes involved in photosynthesis, as well as some serving other functions. Among land plants, the process of reduction has continued even further in numerous lineages engaged in heterotrophic nutritional strategies: parasitic plants and mycoheterotrophs. This process includes numerous gene transfers to the nucleus and a general increase in intergenomic integration of plastid function (reviewed in 2 1). Plastid genome evolution has involved continual gene loss over plant evolutionary history, from cyanobacterial plastid ancestors to angiosperms ( 2 36). The independent, unique processes of plastome modification among mycoheterotrophic lineages illustrate the urgency of their conservation. Comparing Corallorhiza with other heterotrophs allows some emergent evolutionary patterns to be inferred, but these remain as hypotheses to be tested, especially at lower taxonomic levels, and in lineages illustrating transitions from autotrophy to heterotrophy. Conclusions: The Corallorhiza plastome is not drastically reduced in overall size ( ∼6% reduction relative to that of photosynthetic Oncidium), but displays a pattern congruent with a loss of photosynthetic function.Intact photosynthesis genes (excluding atp genes) together displayed elevated nonsynonymous changes, while housekeeping genes did not. “Housekeeping” genes were intact, despite the loss of a single tRNA. Members of all major photosynthesis complexes, except ATP-synthase genes, were affected. Key results: Corallorhiza yielded a plastome of 137505 bp, with several photosynthesis-related genes either lost or pseudogenized.Patterns of nonsynonymous/synonymous mutations were also assessed, and comparisons were made between Corallorhiza and other heterotrophic plant lineages. vreelandii, a mycoheterotrophic orchid, to investigate the extent of plastome degradation. Methods: Short-read sequencing was used to generate a complete plastome sequence for Corallorhiza striata var.

The goal of this study was to focus on a mycoheterotrophic orchid hypothesized to be in the “early” stages of plastome degradation, to provide perspective on this process. The few mycoheterotrophic angiosperm plastomes sequenced to date display drastic patterns of degradation/reduction relative to those of photosynthetic relatives. Premise of the study: Plastid genomes of nonphotosynthetic, mycoheterotrophic plants represent apt systems in which to study effects of relaxed evolutionary constraints.

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