Molecular phylogenetic study of the tribe Tropidieae (Orchidaceae, Epidendroideae) with taxonomic and evolutionary implications

The orchid tribe Tropidieae comprises three genera, <i>Tropidia</i>, <i>Corymborkis</i> and <i>Kalimantanorchis</i>. There are three fully mycoheterotrophic species within Tropidieae: <i>Tropidia saprophytica</i>, <i>T. connata</i> and <i>Kalimantanorchis nagamasui</i>. A previous phylogenetic study of <i>K. nagamasui</i>, based only on plastid <i>matK</i> data, placed <i>K. nagamasui</i> outside the clade of <i>Tropidia</i> and <i>Corymborkis</i> without support. In this study, we performed phylogenetic analyses using a nuclear ribosomal DNA spacer (ITS1-5.8S-ITS2), a low-copy nuclear coding gene (<i>Xdh</i>) and a mitochondrial intron (<i>nad1b-c</i> intron) to study the phylogenetic relationships within Tropidieae. We included six photosynthetic and all three fully mycoheterotrophic Tropidieae species. The resulting phylogenetic trees placed these fully mycoheterotrophic species inside the <i>Tropidia</i> clade with high support. In our trees, these three species do not form a monophyletic group together, because the photosynthetic <i>T. graminea</i> is nested amongst them. Our results also suggest that the loss of photosynthetic ability occurred at least twice in <i>Tropidia</i>.