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Eutrema nanum (Brassicaceae), a new species from Chola Shan, Southwest China
expand article infoGuoqian Hao, Ihsan A. Ali Al-Shehbaz§, Lei Zhang|, Xinyi Guo|, Hao Bi|, Xu Songbai, Jianquan Liu|
‡ Mount Emei Scenic Area Management Committee, Leshan, China
§ Missouri Botanical Garden, St. Louis, United States of America
| Sichuan University, Chengdu, China
Open Access

Abstract

Eutrema nanum, a new high-elevation (4500–4600 m) species from Chola Shan, Sichuan (Southwest China), is described and illustrated. It is similar morphologically to E. nepalense but is readily distinguished by having oblong to elliptic or obovate to spatulate (vs. suborbicular to broadly ovate) leaves, glabrous (vs. puberulent) sepals and ovate to oblong fruit 4–7 × 2–3 mm with flattened valves (vs. ovoid to subglobose fruit 2–3 × 1.8–2 mm with rounded valves). The genetic differences amongst E. nanum, E. nepalense and other close relatives are further confirmed by phylogenetic analyses using ITS and cpDNA sequence variations. The new combination E. sinense is proposed.

Keywords

Cruciferae, Eutrema nanum, molecular phylogeny, Sichuan, Eutrema sinense

Introduction

The boundaries of Eutrema R.Br. (Brassicaceae or Cruciferae) have recently been expanded to include 38 species, several of which were previously placed in the six genera Taphrospermum C.A.Mey., Thellungiella O.E.Schulz, Neomartinella Pilg., Platycraspedum O.E.Schulz, Chalcanthus Boiss. and Pegaeophyton Hayek & Hand.-Mazz. (Al-Shehbaz and Warwick 2005; Hao et al. 2017a). The taxonomic knowledge of this genus is still incomplete because numerous collections from the high-elevation regions in Southwest China were often overlooked and many areas remain poorly explored. We reported two new species during recent field investigations and molecular analyses (Hao et al. 2016, 2017b). Here we report the third one, Eutrema nanum, found in Chola Shan at a high elevation of 4500–4600 m in Sichuan Province, Southwest China. This new species is morphlogically similar to E. nepalense (Al-Shehbaz, Kats Arai & H.Ohba) Al-Shehbaz, G.Q.Hao & J.Quan Liu but, as shown below, it is readily distinguished by several aspects of leaves and fruit. The phylogenetic studies on both species and their other relatives were also conducted herein and the results support the recognition of this novelty. In addition, one of six species which were used to determine the systematic position of E. nanum was found to need a taxonomic combination and a new name Eutrema sinense (Hemsl.) G.Q.Hao, J.Quan Liu & Al-Shehbaz is therefore proposed herein.

Material and methods

We examined morphological traits of Eutrema nanum and several relative species. We followed Hu et al. (2015) and Hao et al. (2017a) in examining the genetic differences between this novelty (two accessions) and the morphologically similar E. nepalense (one accession). In order to determine the systematic position of E. nanum, we futher included six species (E. scapiflorum (Hook.f. & Thomson) Al-Shehbaz, G.Q.Hao & J.Quan Liu, E. sinense (Hemsl.) G.Q.Hao, J.Quan Liu & Al-Shehbaz, E. hookeri Al-Shehbaz & Warwick, E. fontanum (Maxim.) Al-Shehbaz & Warwick, E. verticillatum (Jeffrey & W.W.Sm.) Al-Shehbaz & Warwick and E. deltoideum (Hook.f. & Thomson) O.E.Schulz) in our analyses. All six species were shown to be close relatives to E. nepalense in our previous study (Hao et al. 2017a) and two (E. scapiflorum and E. sinense) were previously placed in the genus Pegaeophyton. The related E. integrifolium Bunge (see Hao et al. 2017a) was selected as the outgroup. The collection information of the sampled species is listed in Table 1 and Figure 3 and the voucher specimens were deposited in the Sichuan University Herbarium (SZ).

The sources of materials used for molecular analyses of Himalayan Eutrema (all vouchers at SZ).

Taxon Voucher Source Coordinate
E. nanum Liu & Hao 14091 Chola Shan, Sichuan, China 31°55'N, 98°54'E
E. nanum Liu 17124 Chola Shan, Sichuan, China 31°55'N, 98°54E
E. nepalense Long et al. 605 Sikkim, India 27°36'N, 88°12'E
E. sinense Liu 13114 Biluo Snow Mountain, Yunnan, China 27°59'N, 98°47'E
E. scapiflorum Liu & Hao 13074 Yarla Shampo Mountain, Tibet, China 28°51'N, 91°59E
E. fontanum Liu & Hao 13144 Zhuodala Mountain, Sichuan, China 31°24'N, 99°56'E
E. hookeri Liu 17108a Mila Mountain, Tibet, China 29°49'N, 92°90'E
E. verticillatum Liu & Hao 14094 Maila Mountain, Sichuan, China 30°58'N, 98°58'E
E. deltoideum Liu 13024 Lasa, Tibet, China 29°42'N, 91°09'E
E. integrifolium Liu & Hao 13049 Tianshan Mountain, Xinjiang, China 43°12'N, 84°49'E

We extracted the total DNA and amplified and sequenced four DNA markers, the nuclear internal transcribed spacer (ITS) and three chloroplast DNA (cpDNA) regions (psbA-trnH, rbcL, matK), following Hu et al. (2015) and Hao et al. (2017a). The sequences firstly reported here were placed in GenBank under the accession numbers (MH702367, MH793597, MH793598, MH793599). We aligned all sequences using Clustal X (Thompson et al. 1997) and refined them manually. We concatenated three cpDNA sequences into a single matrix for Maximum Parsimony (MP) and Maximum Likelihood (ML) analyses. We coded indels using the simple code method by GapCoder (Young and Healy 2003). We constructed phylogenetic relationships based on two datasets (ITS and cpDNAs) using MP analyses by PAUP* 4.10b (Swofford 2003) and ML analyses using RAxML 7.2.6 (Stamatakis 2006). MP analyses employed a heuristic search with 10,000 replicates and TBR branch swapping and bootstrap values (Felsenstein 1985) were estimated with 1000 replicates and 100 random-addition-sequence replicates per bootstrap replicate. ML analyses were performed with raxmlHPC -f a -s sequence. phy -n boot2 -m GTRGAMMA -x 1234 -# 1000 -n outname. The most suitable GTRGAMMA models were used and bootstrap analyses were estimated with 1000 replicates.

Taxonomy

Eutrema nanum G.Q.Hao, J.Quan Liu & Al-Shehbaz, sp. nov.

小山嵛菜

Type

China. Sichuan: Chola Shan, 31°55'32"N, 98°54'35"E, 4500 m elev., 16 August 2014, Liu & Hao 14091 (Holotype, SZ). Figures 1, 2.

Description

Herbs perennial, 3–6 cm tall, glabrous or puberulent; caudex slender, ca. 3–5 mm long. Leaves basal, rosulate, 20–25 per caudex; petiole 13–20 mm long, slender at base, glabrous or with few trichomes; blade oblong, elliptic, obovate, spatulate, 6–10 × 3–4 mm, fleshy, glabrous or abaxially pubescent with trichomes, 0.3–0.6 mm long, base subattenuate, to cuneate, margin entire, apex obtuse to subrounded. Pedicels slender, 18–23 mm long at anthesis, not elongated in fruit, not persistent. Flowers 5–8 per plant; sepals ovate to oblong, 1–1.5 mm long; petals white, broadly obovate to spatulate, blade 2–3 ×1–2 mm, persistent to fruit maturity, claw-like base 0.5–1 mm long. Ovules 2–4 per ovary. Fruit latiseptate, dehiscent, ovate to oblong, somewhat curved, 4–7 × 2–3 mm; valves nearly flat, extending along part of fruit length; gynophore 0.1–0.3 mm long; replum 0.3–0.4 mm wide; style 0.6–1 mm long. Seeds broadly ovate, brown, plump, 2–4 per fruit, 1.4–2 × 0.6–1 mm.

Eutrema nanum is morpholgically most similar to E. nepalense, from which it is readily distinguished by having oblong, elliptic, obovate to spatulate leaves, glabrous sepals and ovate to oblong larger fruit 4–7 × 2–3 mm with flattened, glabrous valves. In contrast, E. nepalense (https://www.gbif.org) has suborbicular to broadly ovate leaves, puberulent sepals and ovoid to subglobose smaller fruit 2–3 × 1.8–2 mm with rounded, puberulent valves. Eutrema nanum was only found with around 100 individuals along a stream in a valley about 2 kilometres from the Chola Shan peak, whereas E. nepalense occurs across Himalyas Mountains in Bhtan, China, Nepal and India.

Figure 1. 

Eutrema nanum. G.Q. Hao, J.Quan. Liu & Al-Shehbaz. A Plant B Fruit C Leaf trichomes.

Figure 2. 

Holotype of Eutrema nanum.

Phenology

Flowering: June–August. Fruiting: August–September.

Distribution and habitat

Eutrema nanum is currently known only from Chola Shan, part of Hengduan Mountains in West Sichuan, China (Fig. 3). It grows under rocks by streams close to glaciers, damp or gravelly scree, wet sand at a very high elevation of 4500–4600 m.

Figure 3. 

Geographical distribution of the sampled populations of Eutrema nanum and related species.

Paratype. China. Sichuan: Chola Shan, 31°55'32"N, 98°54'35"E, 4500 m elev., Liu 17124 (SZ).

Eutrema sinense (Hemsl.) G.Q.Hao, J.Quan Liu & Al-Shehbaz, comb. nov. Based on Braya sinensis Hemsl., J. Linn. Soc., Bot. 30: 303. 1892.

Syn.: Eutrema robustum (O.E.Schulz) Al-Shehbaz, G.Q.Hao & J.Quan Liu, Bot. J. Linn. Soc. 184: 2019. 2017. Basionym: Pegaeophyton sinense var. robustum O.E.Schulz, Notizbl. Bot. Gart. Gerlin-Dahlem 9: 477. 1926.

The earliest available epithet of this taxon at the species rank is “sinensis” and it should have been been transferred to Eutrema by Hao et al. (2017a) instead of using the varietal epithet “robustum.”

Genetic differences between Eutrema nanum, E. nepalense and other relatives

Sequence data from Eutrema nanum and E. nepalense reveals that one nucleotide substitution in ITS, two in rbcL, 18 in matK and eight substitutions and three indels in psbA-trnH distinguish them very well (Table 2).

Diagnosing sites of the aligned ITS and three cpDNA sequences between Eutrema nanum and E. nepalense.

Species ITS rbcL matK
508 82 337 165 276 333 342 391 449 483 495 497 549
Eutrema nanum C C T C T T T T T T C T T
Eutrema nepalense T A C T C G C A G C T C A
matK psbA-trnH
601 603 633 638 657 28 40 48 92 114 115 138- 212 228 235
Eutrema nanum C C T T G T C G - G C - 2 nt C C
Eutrema nepalense T T C A A A G A 6 nt T A 74 nt - T A

Based on sequence variations of ITS and cpDNAs (Table 3), phylogenetic analyses suggested that Eutrema nanum is mostly related to E. nepalense, E. sinense and E. scapiflorum. However, phylogenetic relationships of these four species are incongurent between ITS and plastid DNA tree (Fig. 4). In the ML analyses of ITS sequence data, E. nanum and E. nepalense formed a single cluster sister to E. sinense and together are sister to E. scapiflorum with high support values (>80%) (Fig. 4A). By contrast, in the ML analyses of cpDNAs sequences, the phylogenetic relationships were maintined between E. nanum and sister E. sinense and together as sister to E. scapiflorum, but E. nepalense fell outside that relationship and was separated from them by E. hookeri with medium support (>50%) (Fig. 4B). MP analyses produced almost the same tree topologies with similar bootstrap support values.

Tree statistics for analyses of the datasets.

Data set ITS* psbA-trnH rbcL matK Combined cpDNA*
No. of sequences 30 30 30 30 30
Aligned length used in analyses 698 455 506 779 1786
No. of variable characters 141 58 17 66 169
No. of parsimony-informative characters 56 15 7 22 47
Tree length (steps) 78 70 10 71 195
Consistency (CI) 0.833333 0.900000 1.000000 0.873239 0.892308
Retention index (RI) 0.803030 0.708333 1.000000 0.790698 0.764045
Rescaled consistency index (RC) 0.669192 0.637500 1.000000 0.690468 0.681763
Figure 4. 

The Maximum Likelihood tree based on analysis of ITS (A) and Three cpDNA regions (B). Numbers above branches are maximum likelihood bootstrap support values and maximum parsimony bootstrap support values. ‘-’ represents <50%.

Discussion

Both Eutrema nanum and E. nepalense are small plants similar in flower traits and seed size. However, as discussed above, they are quite different morphologically. In addtion, phylogenetic analyses of cpDNAs variations suggested these two species did not comprise a monophyletic clade. Furthermore, the Himalayan E. nepalense is disjunctly separated by a distance of at least 1200 air kilometres from the Chola Shan (Sichuan, SW China), where E. nanum is endemic (Fig. 3). Eutrema nanum is also closely related to E. sinense in the phylogenetic analyses of the cpDNA sequence variations, but both are easily distinguished from each other. Eutrema nanum is a small and weak herb with entire leaves (0.5–1.5 cm long) and small flowers (petals 2–3 mm long), whereas E. sinense is obviously stout with entire or toothed leaves (1.5–8 cm long) and distinctly larger flowers (petals 8–15 mm long) and fruit (10–20 mm long).

Pylogenetic relationships amongst E. nanum, E. nepalense and E. sinense are incongruent between ITS and cpDNA trees. This incongruence may suggest possible hybridisations or incomplete lineage sorting during the rapid and recent species diversifications (Soltis and Soltis 2000, 2009). However, it is not possible at present to determine which of these two factors had caused the incongruent phylogenies observed here. More analyses and molecular data, especially based on more individuals and genomic evidence, are needed to solve these phylogenetic inconsistences.

Acknowledgements

This research was supported by grants from the National Natural Science Foundation of China (31700164 and 31590821).

References

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