Lineage-Specific Reductions of Plastid Genomes in an Orchid Tribe with Partially and Fully Mycoheterotrophic Species.

There appears to be a clear trend that genes for the thylakoid NAD(P)H dehydrogenase ( ndh genes), the plastid-encoded polymerase (PEP; rpo genes), RuBisCO ( rbcL locus), photosystems I and II ( psa and psb genes, respectively), and the cytochrome b 6 f complex ( pet genes) are lost or pseudogenized soon after the transition to a heterotrophic and nonphotosynthetic lifeform, whereas the thylakoid ATP synthase complex ( atp genes) and some ribosomal protein genes ( rpl and rps genes) as well as tRNAs are lost later ( Barrett and Davis 2012 ; Barrett et al. 2014 ; Cusimano and Wicke 2016 ).

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A comparison of both visibly green and visibly nongreen mycoheterotrophs versus autotrophic orchids also indicates a weak trend for relaxed purifying selection ( k = 0.99), which is, however, not significant ( P = 0.118).

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