Cotoneaster densiflorus M.W. Li, Q. Fan & W. B. Liao, 2023

Li, Mingwan, Li, Dan, Lu, Mengfei, Mo, Shuangfeng, Ding, Shen, Chen, Yuanyuan, Lai, Yong, Zhang, Dangquan, Liao, Wenbo & Fan, Qiang, 2023, A new species of Cotoneaster (Rosaceae) from western Sichuan, China, PhytoKeys 236, pp. 39-52 : 39

Cotoneaster densiflorus M.W. Li, Q. Fan & W. B. Liao sp. nov.

Type.

China. Sichuan Province, Baoxing County, Qiaoqi Town, Zegen Village , on the cliff of steep slopes, 30°43′N, 102°45′E, 2180 m a.s.l., 7 Dec 2016, Q. Fan & M.W. Li 14925 (holotype: SYS; isotype: SYS) (Figs 1 , 2 )

Diagnosis.

Morphologically, Cotoneaster densiflorus is similar to C. salicifolius , but differs in its leaf blade of ovate-lanceolate to obovate shape (vs. elliptic-oblong to ovate-lanceolate), smaller length-width ratio of 2.37 ± 0.31 (vs. 3.17 ± 0.32), slightly conduplicate (vs. not conduplicate), fewer lateral veins of 6-8 pairs (vs. 12-16 pairs), upper surface slightly rugose (vs. rugose), leaf margin plane (vs. revolute), lower surface densely grey tomentose (vs. grey tomentose, with bloom), greater corolla diameter of 7-9 mm (vs. 5-6 mm), styles 2 (vs. 2-3) and pyrenes 2 (rarely 3). Although there is a closer phylogenetic relationship between the new species and C. rhytidophyllus , it is easy to distinguish them by the indumentum color of branchlets, leaves and inflorescences, rugose leaf upper surface, fruit shape and pyrenes number. See Table 2 , Figs 1 , 2 for a detailed comparison.

Description.

Evergreen shrubs, rarely semi-evergreen, up to 5 m tall, with spreading to erect branches; stems 5 cm in diameter; branchlets terete, stout, reddish-brown, initially sparsely tomentose, glabrous when old. Petiole red, robust, 4-7 mm long, tomentose; stipules linear, 4-7 mm, tomentulose, caducous; leaf blades ovate-lanceolate to obovate, 25-72 × 12-33 mm, leathery, lightly conduplicate along the mid-vein, mid-vein conspicuously raised abaxially and deeply impressed adaxially, lateral veins 6-8 pairs, rarely 5 or 9, impressed, lower surface densely grey tomentose, apex acute or obtuse, rarely abruptly mucronate, base cuneate, margin entire, plane, upper surface initially sparsely pilose, subglabrous when old, slightly rugose. Compound corymbs 25-40 mm long, 17-43 mm diam., with (5-)10- to 50-(61)-flowered per inflorescence; rachis and pedicels densely white pilose; peduncles 2-3 cm; bracts linear, tomentulose, caducous, 2-4 mm long; pedicel 2-4 mm. Flowers 7-9 mm diam.; hypanthium campanulate, abaxially densely white tomentose; sepals triangular, apex acute, pilose; petals spreading, white, glabrous, suborbicular, ca. 3-4 mm and nearly as broad, apex obtuse, base shortly clawed; stamens 20, slightly longer than or subequal to petals, anthers purple, filaments white; styles 2, free, slightly shorter than stamens; carpels 2, ovary apically pilose. The ripe pome obovoid or subglobose, 5-7 mm diam., red, sparsely pilose; 2 pyrenes per fruit.

Pollen morphology.

The pollen grains of C. densiflorus are tricolpate. Polar axis (P) = 46.15 ± 3.09 µm, equatorial axis (E) = 22.64 ± 1.28 µm, the P/E value (proportion of polar axis to equatorial axis length) = 2.05 ± 0.12. The P/E values of the new species is obviously larger than C. salicifolius (1.19 ± 0.05). The surface is mainly striate-foveolate ornamentation (Fig. 3 , Table 3 ).

Leaf epidermis morphology.

According to previous studies on leaf epidermis type of Cotoneaster Cotoneaster species ( Ding et al. 2008), the leaf epidermis of this species could be classified as type W, with irregular stomata surrounded by 4-5 corneous papillae, while C. salicifolius and C. rhytidophyllus are presented as type I, with irregular stomata surrounded by various ridges protruding from the stratum corneum (Fig. 3 , Table 3 ).

Phenology.

Flowering from June to July, fruiting from November to December.

Etymology.

The specific epithet refers to the compact compound corymbs with (5-)10- to 50-(61)-flowered per inflorescence.

Distribution and habitat.

C. densiflorus is currently known only from the type locality, Zegen Village, Baoxing County, Sichuan Province, China. This population includes nearly 60 individuals, with about 40 densely distributed individuals and 20 scattered shrubs on a steep slope of sunny sparse forest along the National Highway at altitudes of about 2180 m a.s.l. The associated tree species include C. dielsianus , Coriaria nepalensis , Indigofera szechuensis , Desmodium elegans , and Elaeagnus bockii .

Phylogenetic analysis.

The complete chloroplast genome of C. densiflorus exhibited characteristic quadripartite structure with 159,759 bp in total length, including a pair of inverted repeat (IRA and IRB) region of 26,371 bp, separated by a larger single-copy (LSC) region of 87,807 bp and a small single-copy (SSC) region of 19,210 bp with an overall GC content of 36.60%. A total of 111 unique genes were encoded, including 78 protein-coding genes (PCGs), 29 transfer RNA (tRNA) genes and four ribosomal RNA (rRNA) genes, while 17 genes duplicated in the IR regions.

Phylogenetic analyses constructed from 72 Cotoneaster Cotoneaster chloroplast genomes resulted in the ML tree topology as shown in Table 1 . Two main clades ( Cotoneaster Cotoneaster and Cotoneaster Chaenopetalum ) with well-supported values are presented in Fig. 4 , which was consistent with the previous phylogenetic studies. The new species was placed in Clade Cotoneaster Chaenopetalum and clustered with C. rhytidophyllus of Ser. Cotoneaster Salicifolii , but with weak support values (BS = 44, BI = --).

Chromosome ploidy analysis.

The results of flow cytometry analysis displayed a mean genome size (2C-value) of 2.33 ± 0.19 pg for C. densiflorus (Table 3 ). Comparing with ploidy levels and genome sizes of Cotoneaster Cotoneaster species reported in the previous studies ( Rothleutner et al. 2016; Ksinan et al. 2021), this species was inferred as tetraploid (2n = 4x =68), while its closely-related species, C. salicifolius (2C-value = 1.13 ± 0.16; unpublished data) and C. rhytidophyllus (2C-value = 1.55 ± 0.10; unpublished data) were diploid (2n = 2x = 34) ( Fryer and Hylmö 2009).

Conservation status.

Only one large population was found with nearly 60 mature and juvenile individuals on steep slopes about 2 km along the highway. Its habitat is affected and threatened by the violent geological, climate and artificial activities with frequent construction of highways, mud-rock flows, landslides, and even earthquakes in the last few decades. Therefore, the species could be considered as CR (Critically Endangered) status according to IUCN Red List Criteria (IUCN 2022).

Additional specimens examined

(paratypes). China. Sichuan: Baoxing County, Qiaoqi Town, Zegen Village , 30°43′N, 102°45′E, 2180 m a.s.l., 17 June 2023, M.W. Li 20230617007 (SYS) .

Figures

Figure 1. Cotoneaster densiflorus A habit B leaf, adaxial surface C leaf, abaxial surface D flowers E vertical section of flower F petals G stamens H styles I fruiting branch J pome K transverse section of pome L pyrenes. Illustration by Zhengmeng Yang based on living field-collected materials (Q. Fan & M. W. Li 14925, M. W. Li 20230617007).

Figure 2. Cotoneaster densiflorus sp. nov. A habitat B habit C branchlets D petal, stamen, style, and vertical section of flower E ovate-lanceolate to obovate shape of leaves F leaf, abaxial surface G 2 pyrenes per fruit H inflorescence I fruiting branch.

Figure 3. SEM micrographs of pollen grains of Cotoneaster densiflorus (A-C), light microscope and SEM micrographs of leaf epidermis of C. densiflorus (D-F) A equatorial view B polar view C striate-foveolate ornamentation D upper epidermis E under epidermis F stomata and corneous papillae of under epidermis.

Figure 4. The phylogenetic tree of 65 Cotoneaster Cotoneaster taxa based on 72 whole chloroplast genomes showing the position of Cotoneaster densiflorus (bold and highlighted with green) in the genus. Numbers near the branches are ML and BI bootstrap values. GenBank accessions were provided after underlines.