Taxonomic and phylogenetic study of the subgenus Tricercandra in the genus Chloranthus (Chloranthaceae)

The genus Chloranthus consists of 17 taxa with the two subgenera Chloranthus and Tricercandra. These two subgenera can be distinguished by the morphology of the androecium. In South Korea, C. japonicus and C. fortunei of the subgenus Tricercandra are distributed. Although C. fortunei is typically not branched, a variant that branches from the stem has recently been discovered. Here, we conducted morphological and molecular phylogenetic analyses to elucidate the interspecific relationships within the subgenus Tricercandra and to evaluate the taxonomic position of the variant of C. fortunei. Morphological analyses revealed distinct differences among C. angustifolius, C. japonicus, C. fortunei, and the variant of C. fortunei. However, distinguishing between C. holostegius and C. nervosus was challenging. A principal component analysis also showed that while C. angustifolius, C. japonicus, C. fortunei, and the corresponding variant clustered independently, C. holostegius and C. nervosus overlapped, rendering them indistinguishable. Molecular phylogenetic studies using nuclear ribosomal internal transcribed spacer (nrITS) regions and three plastid regions, rbcL, rpl20-rps12, and trnL-F, revealed that the subgenus Tricercandra was monophyletic. Within the subgenus Tricercandra, C. angustifolius formed a sister clade with C. fortunei in the combined three plastid gene tree, while C. japonicus displayed different phylogenetic relationships between the nrITS and plastid gene trees. In addition, neither C. holostegius nor C. nervosus was monophyletic, consistent with the morphological analyses. The variant of C. fortunei formed an independent subclade on the nrITS tree but was not distinguished from other C. fortunei on the plastid gene tree. Our study elucidated clear species delimitation among C. angustifolius, C. japonicus, and C. fortunei and suggested key morphological characters useful for distinguishing them. Furthermore, our findings highlighted the ambiguous species delimitation and phylogenetic uncertainty between C. holostegius and C. nervosus. While we demonstrated morphological differences between the typical C. fortunei and its variant, the limited molecular phylogenetic evidence implies a need for further study.