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Rediscovery of Pogostemon dielsianus (Lamiaceae, Lamioideae), a rare endemic species from southwestern China, after one century
expand article infoGuo-Xiong Hu, Ting Su, Ming-Tai An, Xiao-Yu Wang
‡ Guizhou University, Guiyang, China
Open Access

Abstract

Pogostemon dielsianus (Lamiaceae) was described in 1913 based on a single gathering from northwestern Yunnan of China collected in 1905, and thereafter no further collections were observed until 2019. We rediscovered the rare endemic species in Lushui County, Yunnan. Molecular phylogenetic analyses based on four cpDNA markers (rbcL, rps16, psbA-trnH, and trnL-trnF) and the nuclear ribosomal internal transcribed spacer (ITS) region confirmed its infrageneric placement within subg. Pogostemon. Based on observations of the rediscovered population of P. dielsianus, we updated its morphological description, provided an illustration, and discussed its distribution. Under IUCN criteria, the species was categorized as “Critically Endangered (CR)”.

Keywords

Critically endangered, Nujiang Canyon, Pogostemon elsholtzioides, Pogostemon griffithii, subg. Pogostemon, Yunnan

Introduction

Pogostemon Desf. is the largest genus of tribe Pogostemoneae of subfamily Lamioideae in Lamiaceae (Bendiksby et al. 2011; Li et al. 2016). After combining with Dysophylla Blume based on the molecular phylogenetic analyses (Bendiksby et al. 2011; Yao et al. 2015, 2016), Pogostemon, as currently circumscribed in a broad sense, contains approximately 80 species. On the basis of molecular and morphological evidence, Pogostemon was divided into two subgenera: subg. Pogostemon and subg. Dysophyllus (Bl.) Bhatti & Ingr. ex G. Yao, Y.F. Deng & X.J. Ge (Yao et al. 2016). The genus can be easily distinguished from other Lamiaceae genera by possessing moniliform hairs at the middle of the staminal filaments. Pogostemon is distributed mainly in tropical and subtropical regions of Asia with a few species in tropical Africa, Northern Australia, Japan and the Korea Peninsula (Bhatti and Ingrouille 1997; Harley et al. 2004; Yao et al. 2015).

In China, 27 species and two varieties were recorded, of which 10 species and one variety are endemic (Yao et al. 2015; Yao and Ge 2018). Pogostemon dielsianus was described in 1913 based on a gathering (G. Forrest 875) with two specimens deposited at E and K respectively from Fugong County, northwest Yunnan, China and was not collected again since over the following 100 years. When conducting a taxonomic revision of Chinese Pogostemon, Yao et al. (2015) noted that only the type specimens of P. dielsianus were examined, and the species was unable to be included in the subsequent molecular phylogenetic analysis (Yao et al. 2016).

During a scientific field trip in Nujiang Canyon, northwestern Yunnan of China in November 2019, a population of Pogostemon was discovered in thickets near a tributary of Nujiang River (also known as Salween River). After scrutiny of the data available (Wu and Huang 1977; Li and Hedge 1994; Bhatti and Ingrouille 1997; Yao et al. 2015), we rediscovered Pogostemon dielsianus after 106 years. This finding allowed us to update its morphological description, discuss its geographic distribution, assess its conservation status, and infer its phylogenetic position within Pogostemon.

Materials and methods

Taxon sampling, DNA extraction, amplification and sequencing

Following the latest phylogenetic study (Yao et al. 2016), a total of 28 species (including Pogostemon dielsianus) were sampled from both subgenera of Pogostemon to explore the phylogenetic position of P. dielsianus (Table 1). In addition, three species of its sister genus Anisomeles R. Brown were selected as outgroups based on previous studies (Li et al. 2016; Yao et al. 2016). Except for the newly generated sequences of P. dielsianus, all other data were downloaded from GenBank.

Table 1.

Voucher information and GenBank accession numbers for taxa used in this study. *indicates the new sequences, and “–” indicates missing data.

Taxa Voucher GenBank accession numbers
nrITS rbcL rps16 trnH-psbA trnL-F
Anisomeles heyneana Benth. HQ911589 HQ911659
A. indica (L.) Kuntze G. Yao 369 (IBSC) KR608726 KR608471 KR608595 KR608530 KR608658
A. malabarica (L.) R. Br. ex Sims Fagerlind & Klackenberg 343 (S) MH456886 FJ854013 FJ854260
Pogostemon benghalensis (Burm. f.) Kuntze R. G. Troth 677 (US) HQ911592 KR608568 HQ911663
P. amaranthoides Benth. J. Chen 668 (KUN) KR608745 KR608490 KR608614 KR608549 KR608677
P. aquaticus (C.H. Wright) Press Bidgood et al. 3387 (K) KR608767 KR608527 KR608655 KR608592 KR608717
P. auricularius (L.) Hassk. G. Yao 362 (IBSC) KR608761 KR608513 KR608638 KR608575 KR608700
P. barbatus Bhatti & Ingr. G. Yao 274 (IBSC) KR608762 KR608514 KR608639 KR608576 KR608701
P. brachystachyus Benth. G. Yao 358 (IBSC) KR608775 KR608517 KR608642 KR608579 KR608704
P. cablin (Blanco) Benth. G. Yao 291 (IBSC) KR608757 KR608503 KR608627 KR608562 KR608690
P. chinensis C.Y. Wu & Y.C. Huang G. Yao 445 (IBSC) KR608742 KR608512 KR608637 KR608573 KR608699
P. dielsianus Dunn Hu et al 636 (GACP) MW194872* MW194874* MW194875* MW194873* MW194876*
P. elsholtzioides Benth. Syn. s.n. (US sheet no. 262106) KR608633 KR608569 KR608720
P. formosanus Oliver R. Q. Gao & S. H. Lai 710 (PE) KR608779 KR608500 KR608624 KR608559 KR608687
P. fraternus Miq. Syn. 7655 (KUN) KR608781 KR608648 KR608585 KR608710
P. glaber Benth. G. Yao 364 (IBSC) KR608740 KR608496 KR608620 KR608555 KR608683
P. heyneanus Benth. G. Yao 297 (IBSC) KR608751 KR608492 KR608616 KR608551 KR608679
P. hispidocalyx C.Y. Wu & Y.C. Huang Expedition to QTP 9446 (KUN) KR608780 KR608644 KR608581 KR608706
P. linearis (Benth.) Kuntze G. Yao 348 (IBSC) KR608764 KR608521 KR608649 KR608586 KR608711
P. litigiosus Doan ex Suddee & A. J. Paton V. D. Nong 31712077 (IBSC) KR608776 KR608519 KR608645 KR608582 KR608707
P. macgregorii W. W. Sm. K.Iwatsuki et al. 9659 (A) KR608778
P. paniculatus (Willd.) Benth. Middleton et al. 1532 (K) KR608574 KR608721
P. paniculatus (Willd.) Benth. J. Klackenberg & R. Lundin 565 (S) FJ854071
P. parviflorus Benth. G. Yao 365 (IBSC) KR608749 KR608501 KR608625 KR608560 KR608688
P. petelotii Doan ex G. Yao, Y.F. Deng & X.J. Ge T. Sorensen et al. 6313 (KUN) KR608772 KR608529 KR608657 KR608594 KR608719
P. plectranthoides Desf. G. Yao 449 (IBSC) KR608758 KR608510 KR608635 KR608571 KR608697
P. quadrifolius (Benth.) F. Muell. F. G. Dickason 8194(A) KR608773 KR608518 KR608643 KR608580 KR608705
P. rogersii N E. Br. Phillips 3855 (K) KR608782 KR608647 KR608584 KR608709
P. sampsonii (Hance) Press G. Yao 273 (IBSC) KR608769 KR608524 KR608652 KR608589 KR608714
P. septentrionalis C.Y. Wu & Y.C. Huang G. Yao 264 (IBSC) KR608747 KR608497 KR608621 KR608556 KR608684
P. stellatus (Lour.) Kuntze B. Z. Xiao 4826 (K) KR608768 KR608523 KR608651 KR608588 KR608713
P. xanthiifolius C.Y. Wu & Y.C. Huang H. T. Tsai 59-10586 (KUN) KR608746 KR608493 KR608617 KR608552 KR608680
P. yatabeanus (Makino) Press G. Yao 285 (IBSC) KR608766 KR608526 KR608654 KR608591 KR608716

Total genomic DNA of Pogostemon dielsianus was extracted from silica gel-dried leaf material following the modified CTAB method of Doyle and Doyle (1987). The nuclear ribosomal internal transcribed spacer (ITS) region was amplified using primers ITS5 and ITS4 (White et al. 1990). Four chloroplast DNA markers were employed to make phylogenetic analyses and the rbcL was amplified with primers of Z1F and 51R (Soltis et al. 1992), the rps16 with rps-LamF and rps-LamR (Bendiksby et al. 2011), the psbA-trnH with psbAF and trnHR (Sang et al. 1997), and the trnL-trnF with trn-c and trn-f (Taberlet et al. 1991). All makers were amplified and sequenced with the same conditions following Hu et al. (2020).

Sequence alignment and phylogenetic analyses

Sequences were checked and assembled employing Sequencher v.4.1.4 (Gene Codes, Ann Arbor, Michigan, USA) and then aligned Mafft-win v7.221 (Katoh and Standley 2013) by default. The final alignments were manually adjusted in PhyDE v.0.9971 (Müller et al. 2010). Nuclear dataset (ITS) and plastid matrix (consisting of rbcL, rps16, psbA-trnH, and trnL-trnF) were analyzed separately using maximum likelihood (ML) and Bayesian inference (BI). ML analyses were performed using RAxML-HPC2 on XSEDE v.8.2.12 (Stamatakis 2014) under the GTRCAT model on the CIPRES science gateway portal (http://www.phylo.org/) (Miller et al. 2010). Except for setting the bootstrap iterations (-# | -N) to 1000, other parameters followed default. BI analysis was performed in MrBayes v3.2.6 (Ronquist et al. 2012) as implemented in PhyloSuite (Zhang et al. 2020) with the ModelFinder used to select the best model (Kalyaanamoorthy et al. 2017). Under the Akaike information criterion (AIC), the GTR+F+G4 model was selected for nrDNA dataset and the GTR+F+I+G4 for cpDNA matrix. In each analysis, four Markov chain Monte Carlo (MCMC) chains were run simultaneously for 20 million generations, starting with one random tree and sampling one tree every 1000th generation. Convergence of runs was reached when the average standard deviation of split frequencies (ASDSF) fell below 0.01. After discarding the first 25% of the resulting trees as burn-in, the remaining trees were used to assess posterior probabilities (PP) in a majority-rule consensus tree.

Results and discussion

Phylogenetic position of Pogostemon dielsianus

Both nrDNA and cpDNA analyses supported the monophylies of Pogostemon and its two subgenera (subg. Pogostemon and subg. Dysophyllus). Although Pogostemon dielsianus fell into the subg. Pogostemon in both trees, its phylogenetic position was not entirely consistent (Figs 1, 2). In nrDNA topology, P. dielsianus seemed to be sister to P. glaber Benth., then together sister to the clade consisting of P. chinensis C.Y. Wu & Y.C. Huang + P. septentrionalis C.Y. Wu & Y.C. Huang + P. amaranthoides Benth. (Fig. 1). However, in cpDNA tree, P. dielsianus, instead of grouping with P. glaber, was sister to P. amaranthoides, then together sister to P. chinensis (Fig. 2).

Figure 1. 

Cladogram of Pogostemon based on ML analysis of internal transcribed spacers (ITS) matrix. Pogostemon dielsianus is highlighted in red. Bootstrap values of ML are given above the branches with posterior probabilities (PP) of BI below. Bootstrap values <40% and PP< 0.6 are indicated by a dash.

Figure 2. 

Cladogram of Pogostemon based on ML analysis of the combined cpDNA (rbcL, rps16, psbA-trnH, and trnL-trnF) dataset. Pogostemon dielsianus is highlighted in red. Bootstrap values of ML are given above the branches with posterior probabilities (PP) of BI below. Bootstrap values <40% and PP< 0.6 are indicated by a dash.

Morphologically, Pogostemon dielsianus is similar to P. elsholtzioides Benth. and P. griffithii Prain in having lanceolate leaves (Yao et al. 2015). For the two similar species, only four cpDNA sequences of P. elsholtzioides are available in GenBank. In the cpDNA topology, instead of grouping with the morphologically similar species (P. dielsianus), P. elsholtzioides was sister to the clade consisting of P. glaber and P. xanthiiphyllus C.Y. Wu & Y.C. Huang. However, due to the unavailability of nrDNA sequences of P. elsholtzioides, the conclusion that P. dielsianus is not closely related to P. elsholtzioides solely on the basis of cpDNA result, cannot be drawn at present. In fact, discordances stemming from nuclear and plastid genomes are common in Lamiaceae, which may be attributed to ancient hybridization with chloroplast capture (Drew and Sytsma 2013; Xiang et al. 2013; Drew et al. 2014; Deng et al. 2015; Hu et al. 2018). Therefore, further studies, especially including nrDNA sequences of P. elsholtzioides and P. griffithii, are needed to clarify the true phylogenetic position of P. dielsianus within subg. Pogostemon.

Taxonomic treatment

Pogostemon dielsianus Dunn in Notes Bot. Gard. Edinburgh 8: 159. 1913.

Figs 3, 4

Lectotype

(designated by Bhatti and Ingrouille in Bull. Nat. Hist. Mus. Lond. (Bot.) 27: 99. 1997). China. Yunnan: Fugong, Valley of the Salween, between Shih-chi-ti and Shia-ku-ti, Salween-Irrawaddy Divide, 26°20'N, 1524–1829 m, November 1905, G. Forrest 875 (E [barcode E00087126, image!]; isolectotype: K [barcode K000249619, image!]).

Figure 3. 

Pogostemon dielsianus A floral branches B bract and bracteole C flower D dissected calyx E dissected corolla showing stamens F stamen G pistil. Drawn by Xiao-Yu Wang based on Hu et al. 636 (GACP).

Revised description

Perennial shrubs, up to 3 m tall. Stems solid, gray, ground diameter to 3.5 cm, branches terete or angular, slightly dilated at nodes, the initial branches green, densely strigose-pubescent, 2–3-year-old branches yellow-brown, subglabrous. Leaves opposite; petiole 0.5–2.5 cm long; blade linear-lanceolate to lanceolate, 8–14 × 2–4 cm, papery, both sides densely strigose-puberulent when young, the mature gradually subglabrous, base cuneate, margin serrate, apex acuminate, lateral veins 3–6 pairs. Spikes 3.5–7 cm long, 8–12 mm wide, terminal and axillary, subcontinuous, basally somewhat lax, with more than two lateral branches, densely appressed pubescent except for corolla, pedunculate, 0.5–2 cm long; cymes sessile, 8–14-flowered, flowers sessile. Bracts 4–6.5 mm, bracteoles 1.8–2.3 mm. Calyx tubular, 3.5–4.5 mm long, 5-veined; teeth 5, triangular, 1/5–1/4 as long as the calyx tube. Corolla rose, 2-liped, 7–9 mm long, glabrous outside; tube cylindric, dilated at throat, ca. 2× as long as calyx; upper lip 3-lobed, lobes triangular, subequal, 1.1–1.3 × 0.9–1.1 mm; lower lip entire, ca. 0.9 × 0.7 mm. Stamens 4, exserted from corolla; filaments 5.5–7 mm long, exserted portion ca. 3.5 mm. Style 6.3–8.5 mm long; stigma bifid, lobes subequal, 1.1–1.3 mm. Disc ca. 0.7 mm long. Nutlets 4, ca. 1.5 × 0.8 mm, lanceolate.

Figure 4. 

Habitat and morphology of Pogostemon dielsianus A habitat B habit C leaf, adaxial view D leaf, abaxial view E inflorescence F floral branches G flower H Stamens and style. Photographed by G.X. Hu.

Distribution and habitat

The type locality of Pogostemon dielsianus was recorded in Fugong County, northwestern Yunnan of China, which is the only historical known site until our new discovery. As coordinate information of the collection is incomplete due to the lack of longitude data, the precise situation of type specimen is unclear. Based on the latitude provided in the original record, the type specimen is more likely to be collected in the north of Lushui County, a neighboring county of Fugong (Fig. 5). Although the recently collected population was also discovered in north Lushui County, distribution of the two populations does not overlap because they are located on different sides of Nujiang River (Fig. 5). In accordance with type specimen record, P. dielsianus grows amongst thickets on dry rocky hillsides with elevations ranging from 1524–1829 m. The finding that the newly recorded population grows on the riverside indicates that P. dielsianus is more likely to occur in humid areas of dry hillsides. Actually, a similar habitat can also be found elsewhere in Nujiang Canyon. Potential populations of this species, therefore, may be discovered through further field investigation in this region.

Figure 5. 

Current distribution of Pogostemon dielsianus.

Phenology

Flowering and fruiting from November to December.

Conservation status and preliminary IUCN assessment

Pogostemon dielsianus is historically known from only two specimens collected from the type locality (Fugong, Yunnan, China) in 1905, and it has not been recollected for the past 114 years until our expedition to Nujiang Canyon in 2019. In the newly recorded locality (Lushui, Yunnan, China), only about 10 mature individuals have been discovered. Due to the lack of exact geographical information of the type locality, it is difficult to confirm the number of individuals there. Based on current investigations and historical records, we inferred that mature individuals of this species may be fewer than 250, and no subpopulation contains more than 50 mature individuals. Therefore, under the IUCN criteria C2a(i) (IUCN 2012), we propose that P. dielsianus should be classified as “Critically Endangered (CR)”.

Additional specimens examined

China. Yunnan: Lushui County, Daxingdi Town, Tuanjie Village, Luchuluo, amongst a thicket near the Luchuluo River, elevation 1786 m, 26°7.14'N, 98°53.78'E, 24 November 2019, Hu et al. 636 (GACP!, IBSC!, KUN!).

Notes

Pogostemon dielsianus is morphologically similar to P. elsholtzioides and P. griffithii in having lanceolate leaves. However, P. dielsianus can be easily distinguished from P. elsholtzioides and P. griffithii by its longer and tubular calyx, smaller ratio of the length of calyx teeth and calyx tube and longer corolla, filament and style (Table 2). In addition, the geographical distribution of these three species is also different in that P. dielsianus is endemic to NW Yunnan, China, P. elsholtzioides is widely distributed in the Himalayan regions (Bhutan, India, and SE Xizang, China), and P. griffithii is endemic to Myanmar (Bhatti and Ingrouille 1997; Yao et al. 2015; Yao and Ge 2018).

Table 2.

Morphological comparison between Pogostemon dielsianus and its morphologically similar species.

Character Pogostemon dielsianus Pogostemon elsholtzioides Pogostemon griffithii
Calyx tubular, 3.5–4.5 mm long campanulate, 3–3.5 mm long campanulate, ca. 3.5 mm long
Ratio of the length of calyx teeth and calyx tube 1/5–1/4 1/3–1/2 1/2–1
Corolla length 7–9 mm ca. 4.5 mm ca. 5 mm
Filament length 6.2–7 mm 4.5–5 mm 4.7–5.2 mm
Style length 6.3–8.8 mm ca. 5.5 mm ca. 5.5 mm
Nutlet lanceolate lanceolate oblong
Distribution China (NW Yunnan) Bhutan, India, China (SE Xizang) Myanmar

In the protologue, Dunn (1913) did not designate a type for the name Pogostemon dielsianus Dunn. Bhatti and Ingrouille (1997) indicated the specimen deposited in E and K as holotype and isotype, respectively. In fact, they effectively chose the lectotype for the name and the term“holotype” and “isotype” can be corrected as “lectotype” and “isolectotype” according to Article 9.10 of the International Code of Nomenclature for algae, fungi, and plants (Shenzhen Code) (Turland et al. 2018).

Acknowledgements

We thank Wei Zhang, Yan-Bing Yang and Jin-Zhu Shi for help with field work, Ling-Bin Yan for drawing the geographic distribution map, and two reviewers and the academic editor for their helpful suggestions and comments on how to improve the manuscript. This work was supported by the National Natural Science Foundation of China (32060048 and 31600164).

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