Plastome structure of 8 Calanthe s.l. species (Orchidaceae): comparative genomics, phylogenetic analysis

BACKGROUND

Calanthe (Epidendroideae, Orchidaceae) is a pantropical genus distributed in Asia and Africa. Its species are of great importance in terms of economic, ornamental and medicinal values. However, due to limited and confusing delimitation characters, the taxonomy of the Calanthe alliance (Calanthe, Cephalantheropsis, and Phaius) has not been sufficiently resolved. Additionally, the limited genomic information has shown incongruences in its systematics and phylogeny. In this study, we used illumina platform sequencing, performed a de novo assembly, and did a comparative analysis of 8 Calanthe group species' plastomes: 6 Calanthe and 2 Phaius species. Phylogenetic analyses were used to reconstruct the relationships of the species as well as with other species of the family Orchidaceae.

RESULTS

The complete plastomes of the Calanthe group species have a quadripartite structure with varied sizes ranging between 150,105bp-158,714bp, including a large single-copy region (LSC; 83,364bp- 87,450bp), a small single-copy region (SSC; 16,297bp -18,586bp), and a pair of inverted repeat regions (IRs; 25,222bp - 26,430bp). The overall GC content of these plastomes ranged between 36.6-36.9%. These plastomes encoded 131-134 differential genes, which included 85-88 protein-coding genes, 37-38 tRNA genes, and 8 rRNA genes. Comparative analysis showed no significant variations in terms of their sequences, gene content, gene order, sequence repeats and the GC content hence highly conserved. However, some genes were lost in C. delavayi (P. delavayi), including ndhC, ndhF, and ndhK genes. Compared to the coding regions, the non-coding regions had more sequence repeats hence important for species DNA barcoding. Phylogenetic analysis revealed a paraphyletic relationship in the Calanthe group, and confirmed the position of Phaius delavayi in the genus Calanthe as opposed to its previous placement in Phaius.

CONCLUSION

This study provides a report on the complete plastomes of 6 Calanthe and 2 Phaius species and elucidates the structural characteristics of the plastomes. It also highlights the power of plastome data to resolve phylogenetic relationships and clarifies taxonomic disputes among closely related species to improve our understanding of their systematics and evolution. Furthermore, it also provides valuable genetic resources and a basis for studying evolutionary relationships and population genetics among orchid species.

A new phylogenetic analysis sheds new light on the relationships in the Calanthe alliance (Orchidaceae) in China

The taxonomy of the Calanthe alliance (Epidendroideae, Orchidaceae), consisting of Calanthe, Cephalantheropsis, and Phaius, has been difficult for orchidologists to understand because of the presence of common morphological features. In this study, in addition to morphological and geographical analyses, maximum parsimony and Bayesian inference analyses were performed based on nucleotide sequences of the nuclear internal transcribed spacer and cpDNA genes of 88 taxa representing the major clades of the Calanthe alliance in China. The results indicated that Cephalantheropsis is monophyletic, while both Phaius and Calanthe are polyphyletic. In Phaius, a total of three species, P. flavus, P. columnaris, and P. takeoi, were segregated to form a new genus, Paraphaius. In Calanthe, subgenus Preptanthe and sect. Styloglossum were both categorized as distinct genera from Calanthe. Our results also confirm that Calanthe delavayi and C. calanthoides are members of Calanthe. Previous studies assigned C. delavayi to Phaius and C. calanthoides to Ghiesbrechtia. Five sections, namely, Alpinocalanthe, Puberula, Ghiesbrechtia, Tricarinata, and Calanthe, three of which are new taxa, were recognized in Calanthe. Therefore, we propose that the Calanthe alliance is composed of six genera: Calanthe, Cephalantheropsis, Paraphaius, Phaius, Preptanthe and Styloglossum.