Comprehensive species sampling reveals pervasive pseudogenization and recurrent complete loss of plastid <i>ndh</i> genes in the genus <i>Cymbidium</i> (Orchidaceae)

Plastome evolution in plants with diverse nutritional strategies provide unique insights into genomic level adaptive processes. The genus Cymbidium, characterized by its heterogeneous life forms and transitions toward mycoheterotrophy, serves as an exemplary natural system for investigating chronospatial dynamics between ecological adaptation and plastomic evolutionary trajectories. In this study, we conducted comparative analyses of plastome evolution using nuclear ribosomal DNA and plastome phylogenies derived from 56 Cymbidium species, representing about 90% of all recognized species and encompassing both epiphytic and terrestrial species. We revealed massive ndh gene degradation events in Cymbidium across both epiphytic and terrestrial lineages, which related to their specific habitat adaptation. The fully mycoheterotrophic C. macrorhizon maintains a relatively intact plastome, suggesting a temporal lag in plastomic degradation following heterotrophic transition.