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

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.

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.

Phylogenetic incongruence in Cymbidium orchids

Cymbidium, which includes approximately 80 species, is one of the most ornamental and cultivated orchid genera. However, a lack of markers and sparse sampling have posed great challenges to resolving the phylogenetic relationships within the genus. In the present study, we reconstructed the phylogenetic relationships by utilizing one nuclear DNA (nrITS) and seven plastid genes (rbcL, trnS, trnG, matK, trnL, psbA, and atpI) from 70 species (varieties) in Cymbidium. We also examined the occurrence of phylogenetic conflict between nuclear (nrITS) and plastid loci and investigated how phylogenetic conflict bears on taxonomic classification within the genus. We found that phylogenetic conflict and low support values may be explained by hybridization and a lack of informative characteristics. Our results do not support previous classification of the subgenera and sections within Cymbidium. Discordance between gene trees and network analysis indicate that reticulate evolution occurred in the genus Cymbidium. Overall, our study indicates that Cymbidium has undergone a complex evolution.

Orchid diversity in China: Recent discoveries

•The project, Orchid Diversity Survey in China, has been initiated by Department of Wildlife Conservation of National Forestry and Grassland Administration (DWC-NFGA) from 2019 to 2022.•During botanical investigation, approximately 30 new species and 10 newly recorded species have been discovered.

Plastome structure and adaptive evolution of Calanthe s.l. species

Calanthe s.l. is the most diverse group in the tribe Collabieae (Orchidaceae), which are pantropical in distribution. Illumina sequencing followed by de novo assembly was used in this study, and the plastid genetic information of Calanthe s.l. was used to investigate the adaptive evolution of this taxon. Herein, the complete plastome of five Calanthe s.l. species (Calanthe davidii, Styloglossum lyroglossa, Preptanthe rubens, Cephalantheropsis obcordata, and Phaius tankervilliae) were determined, and the two other published plastome sequences of Calanthe s.l. were added for comparative analyses to examine the evolutionary pattern of the plastome in the alliance. The seven plastomes ranged from 150,181 bp (C. delavayi) to 159,014 bp (C. davidii) in length and were all mapped as circular structures. Except for the three ndh genes (ndhC, ndhF, and ndhK) lost in C. delavayi, the remaining six species contain identical gene orders and numbers (115 gene). Nucleotide diversity was detected across the plastomes, and we screened 14 mutational hotspot regions, including 12 non-coding regions and two gene regions. For the adaptive evolution investigation, three species showed positive selected genes compared with others, C. obcordata (cemA), S. lyroglossa (infA, ycf1 and ycf2) and C. delavayi (nad6 and ndhB). Six genes were under site-specific positive selection in Calanthe s.l., namely, accD, ndhB, ndhD, rpoC2, ycf1, and ycf2, most of which are involved in photosynthesis. These results, including the new plastomes, provide resources for the comparative plastome, breeding, and plastid genetic engineering of orchids and flowering plants.

Calanthe sieboldopsis (Orchidaceae, Epidendroideae, Collabieae), a new species from Luoxiao Mountains, eastern China

Calanthe sieboldopsis, a new species, is here described and illustrated from Luoxiao Mountains, Jiangxi Province, eastern China. It is morphologically similar to C. sieboldii Decne. ex Regel, but differs from the latter in having smaller flowers, longer spurs, rectangular mid-lobes with emarginate apex (vs. elliptic mid-lobes with mucronate apex), disc with 3 ridges and the proximal ends of the lateral 2 ridges enlarged with light reddish spots and minute white hairs (vs. disc with 5 ridges and 2 rows of white short hairs at base) and pollinia equal in size (vs. unequal in size). A preliminary risk-of-extinction assessment, according to the IUCN Red List Categories and Criteria, is given for the new species.

Development of polymorphic microsatellite markers by using de novo transcriptome assembly of Calanthe masuca and C. sinica (Orchidaceae)

BACKGROUND

Calanthe masuca and C. sinica are two genetically closely related species in Orchidaceae. C. masuca is widely distributed in Asia, whereas C. sinica is restricted to Yunnan and Guangxi Provinces in southwest China. Both play important roles in horticulture and are under the pressure of population decline. Understanding their genetic background can greatly help us develop effective conservation strategies for these species. Simple sequence repeats (SSRs) are useful for genetic diversity analysis, presumably providing key information for the study and preservation of the wild populations of the two species we are interested in.

RESULTS

In this study, we performed RNA-seq analysis on the leaves of C. masuca and C. sinica, obtaining 40,916 and 71,618 unigenes for each species, respectively. In total, 2,019/3,865 primer pairs were successfully designed from 3,764/7,189 putative SSRs, among which 197 polymorphic SSRs were screened out according to orthologous gene pairs. After mononucleotide exclusion, a subset of 129 SSR primers were analysed, and 13 of them were found to have high polymorphism levels. Further analysis demonstrated that they were feasible and effective against C. masuca and C. sinica as well as transferable to another species in Calanthe. Molecular evolutionary analysis revealed functional pathways commonly enriched in unigenes with similar evolutionary rates in the two species, as well as pathways specific to each species, implicating species-specific adaptation. The divergence time between the two closely related species was tentatively determined to be 3.42 ± 1.86 Mya.

CONCLUSIONS

We completed and analysed the transcriptomes of C. masuca and C. sinica, assembling large numbers of unigenes and generating effective polymorphic SSR markers. This is the first report of the development of expressed sequence tag (EST)-SSR markers for Calanthe. In addition, our study could enable further genetic diversity analysis and functional and comparative genomic studies on Calanthe.

Diversity of fungi associated with roots of Calanthe orchid species in Korea

While symbiotic fungi play a key role in the growth of endangered Calanthe orchid species, the relationship between fungal diversity and Calanthe species remains unclear. Here, we surveyed root associated fungal diversity of six Calanthe orchid species by sequencing the internal transcribed spacer (ITS) region using 454 pyrosequencing. Our results revealed that Paraboeremia and Coprinopsis are dominant fungal genera among Calanthe species. In terms of overall relative abundance, Paraboeremia was the most common fungal genus associated with Calanthe roots, followed by Coprinopsis. Overall fungal diversity showed a significant degree of variation depending on both location and Calanthe species. In terms of number of different fungal genera detected within Calanthe species, C. discolor had the most diverse fungal community, with 10 fungal genera detected. This study will contribute toward a better understanding of those fungi that are required for successful cultivation and conservation of Korean Calanthe species.

DNA Barcoding Evaluation and Its Taxonomic Implications in the Recently Evolved Genus Oberonia Lindl. (Orchidaceae) in China

The orchid genus Oberonia Lindl., is a taxonomically complex genus characterized by recent species radiations and many closely related species. All Oberonia species are under conservation as listed in the CITES and the IUCN Red List Categories and Criteria. Given its difficulties in taxonomy and conservation status, Oberonia is an excellent model for developing DNA barcodes. Three analytical methods and five DNA barcoding regions (rbcL, matK, trnH-psbA, ITS, and ITS2) were evaluated on 127 individuals representing 40 species and 1 variety of Oberonia from China. All the three plastid candidates tested (rbcL, matK, and trnH-psbA) have a lower discriminatory power than the nuclear regions (ITS and ITS2), and ITS had the highest resolution rate (82.14%). Two to four combinations of these gene sets were not better than the ITS alone, but when considering modes of inheritance, rbcL+ITS and matK+ITS were the best barcodes for identifying Oberonia species. Furthermore, the present barcoding system has many new insights in the current Oberonia taxonomy, such as correcting species identification, resolving taxonomic uncertainties, and the underlying presence of new or cryptic species in a genus with a complex speciation history.