Young-Kee Kim1p, Jun-Ho Song2

1Advanced-Basic-Convergence Research Institute, Chungbuk National University, Cheongju, Korea; 2Department of Biology, Chungbuk National University, Cheongju, Korea

While plastome reduction in mycoheterotrophic plants is well documented, mitochondrial genome (mitogenome) evolution remains poorly understood—particularly within the Orchidaceae, where no complete mitogenomes from the subfamily Orchidoideae have been reported. Here, we present the complete mitogenome of Chamaegastrodia shikokiana, comprising 19 contigs with a total length of 412,787 bp. In contrast to its significantly reduced plastome, the C. shikokianamitogenome retains a nearly full set of protein-coding genes, lacking only four (nad1 exon1, rpl2, rpl16, and rps19). We identified 21 plastome-derived homologous regions, including photosynthesis-related and housekeeping genes. However, comparative analyses revealed no significant correlation between these transferred regions and elevated substitution rates or altered selection pressure. Although RELAX analysis indicated a mix of relaxed and intensified selection among plastome genes, these patterns were not associated with intracellular gene transfer (IGT). Our findings suggest that plastome-to-mitogenome gene transfer in C. shikokianaoccurs independently of functional constraints or adaptive significance. This study provides a crucial reference for filling the phylogenetic gap in Orchidaceae and offers new insights into organellar co-evolution in mycoheterotrophs.