Extensive losses of photosynthesis genes in the plastome of a mycoheterotrophic orchid, Cyrtosia septentrionalis (Vanilloideae: Orchidaceae)

Cyrtosia septentrionalis is an achlorophyllous mycoheterotrophic orchid in the subfamily Vanilloideae (Orchidaceae). This article reports C. septentrionalis's complete plastome sequence and compare it with other orchid plastomes with a same mycoheterotrophic nutritional mode. The C. septentrionalis plastome has decreased to 96,859 bp in length, but it still maintains a quadripartite structure. The C. septentrionalis plastome contains 38 protein-coding genes, 25 tRNA genes, and four ribosomal RNA genes. Most genes related to photosynthesis have been lost, whereas the majority of housekeeping genes remain; this pattern corresponds to the end of stage 3 gene degradation. The inverted repeat regions of the C. septentrionalis plastome have decreased to 10,414 bp and mainly contain the gene ycf2. A block consisting of four rrn genes and rps7 and rps12 has shifted to a small single-copy region. As a result, the small single-copy region was found to be expanded, despite the loss of all ndh genes in the region. Three inversion mutations are required to explain the C. septentrionalis plastome's current gene order. The species is endangered, and these results have implications for its conservation.