Plastid Genome Evolution in the Subtribe Calypsoinae (Epidendroideae, Orchidaceae)

Calypsoinae is a small subtribe in Orchidaceae (Epidendroideae) characterized by diverse trophic strategies and morphological characters. Calypsoinae includes 13 genera, four of which are leafless and mycoheterotrophic. Mycoheterotrophic species in the leafless genus Corallorhiza are well suited to studies of plastome evolution. However, the lack of plastome sequences for other genera in Calypsoinae limits the scope of comparative and phylogenetic analyses, in particular our understanding of plastome evolution. To understand plastid genome evolution in Calypsoinae, we newly sequenced the plastomes of 12 species in the subtribe, including representatives of three mycoheterotrophic genera as well as five autotrophic genera. We detected two parallel photosynthetic losses in Corallorhiza. Evolutionary analyses indicated that the transition to obligate mycoheterotrophy leads to the relaxation of selection in a highly gene-specific pattern.