Setoseptoria guangxiensis Y.X. Shu & W. Dong

Shu, Yongxin, Senanayake, Indunil C., Boonmee, Saranyaphat, Doilom, Mingkwan, Dong, Zhangyong & Dong, Wei, 2024, Introduction of a novel freshwater species Setoseptoria guangxiensis sp. nov., with Setoseptoria baiyunensis nom. nov., in Lentitheciaceae, Pleosporales, Phytotaxa 645 (2), pp. 120-130 : 123-126

Setoseptoria guangxiensis Y.X. Shu & W. Dong , sp. nov. ( FIGURES 2 , 3 )

Index Fungorum number: IF 901555; Facesoffungi number: FoF 15507.

Etymology: referring to Guangxi Province, from where the holotype was collected.

Holotype: MHZU 23-0281

Saprobic on decaying culms of an unidentified grass submerged in freshwater habitats. Sexual morph: Ascomata 160– 200 μm high, 140–165 μm diam., perithecial, scattered or gregarious, semi-immersed to superficial, ovoid, subglobose, uni-loculate, black, coriaceous, central ostiolate, with a protruding, prominent, short papilla, lacking a clypeus. Peridium 15–40 μm thick, two-layered, outer layer consisting of few layers of brown, thick-walled, compressed cells of textura angularis, or undistinguished cells merging with host tissue; inner layer occupied the largest portion, consisting of several layers of hyaline, thin-walled, large cells of textura angularis. Pseudoparaphyses 2–4 µm wide, numerous, cellular, cylindrical, hyaline, septate, slightly constricted at septa, unbranched. Asci 95–140 × 15–18 µm (x̄ = 108 × 16 µm, n = 10), 8-spored, bitunicate, fissitunicate, cylindric-clavate, with narrower apex and short pedicellate, with an ocular chamber. Ascospores 21–34 × 4–6 µm (x̄ = 30 × 5 µm, n = 40), overlapping uni- to tri-seriate, narrowly fusiform, hyaline, 1-septate, deeply constricted at the septum, asymmetric, slightly constricted in the upper portion of the apical cell, occasionally 2–3-septate, straight or slightly curved, smooth-walled, surrounded by a gelatinous, defined, extended sheath at the base, showing an irregular-shaped sheath in Indian Ink which is similar to ascospore shape. Asexual morph: undetermined.

Culture characteristics: Ascospores germinating on PDA within 36 h and germ tubes produced from both ends at room temperature (~ 27 °C). Colonies on PDA reach 40 mm in a month, circular, with dense, velvety, white mycelia, margin sparse and wavy; in reverse, dark brown in the middle and pale brown at the margin, producing yellowishbrown pigmentation. Ascomata were produced in PDA after two months, with the asci measuring 110–120 × 15–20 µm (x̄ = 117 × 17 µm, n = 5). The ascospores have slightly greater length compared to those found in their natural substratum, measuring 28–40 × 6–7.5 µm (x̄ = 34× 6 µm, n = 20).

Material examined: China, Guangxi Province, Guilin City, on decaying culms of an unidentified grass submerged in Lijiang River , 3 June 2023, Y.X. Shu, LJ14 ( MHZU 23-0281 , holotype) , ex-type culture ZHKUCC 23-1034.

Notes: The BLASTn searches conducted on our collection ZHKUCC 23-1034 using the LSU sequence have revealed that it shares the closest relationships with species within the genus Setoseptoria , such as S. arundinacea (100%), S. arundelensis (100%), S. phragmitis (100%), S. scirpi (99.65%), and S. bambusae (99.65%). The BLASTn searches using SSU, ITS, and tef1-α sequences yield similar results, further indicating that ZHKUCC 23-1034 exhibits higher sequence similarities with S. bambusae . In the multi-locus phylogenetic analysis incorporating combined LSU, SSU, ITS and tef1-α sequence data, S. guangxiensis is clustered as a sister branch to S. bambusae ( FIGURE 1 ). Both species have similar morphology in their ascospores, but they can be distinguished by the characteristics of ascomata ( Yang et al. 2023). The ascomata of S. bambusae are immersed beneath raised host tissue, forming a clypeus-like structure, and have a small pore-like opening at the center. In contrast, those of S. guangxiensis are semi-immersed to superficial without a clypeus-like structure and lack a small pore-like opening; instead, they have a protruding, prominent, short papilla at the center. The peridium of S. bambusae consists of multi-layered, dark brown, thick-walled cells of textura angularis, becoming paler, smaller or elongated towards the inner layers. In contrast, the largest portion of the peridium of S. guangxiensis is composed of hyaline, thin-walled, large cells of textura angularis. In addition, the ascospores of S. bambusae are clearly 3-septate at maturity, whereas those of S. guangxiensis are 1-septate.

On PDA, the cultures of S. bambusae exhibit white mycelia in the center, dark brown at the edges, and produce dark brown pigmentation. In contrast, the cultures of S. guangxiensis on PDA solely exhibit white mycelia, and produce yellowish brown pigmentation. The comparison of DNA sequence data reveals a discrepancy of 0.59% (5 out of 850), 1.93% (11 out of 570) and 2.31% (21 out of 908) in nucleotide variations within the LSU, ITS and tef1-α, respectively, between S. guangxiensis ZHKUCC 23-1034 and S. bambusae GZCC 17-0044. Setoseptoria guangxiensis can also be distinguished from other species within the genus (see key below). Therefore, S. guangxiensis is introduced as a novel species herein.

Figures

FIGURE 1. The RAxML tree is generated from combined LSU, SSU, ITS and tef1-α sequence data analysis. Bootstrap support values for ML equal to or greater than 60% and Bayesian posterior probabilities (PP) equal to or greater than 0.95 are given above the nodes shown as “ML/PP”. The tree is rooted to Massarina cisti (CBS 266.62) and Massarina eburnea (CBS 473.64). Type strains are indicated in bold, new species is marked in red, and new name is indicated in blue.

FIGURE 2. Setoseptoria guangxiensis (MHZU 23-0281, holotype). a, b Appearance of ascomata on the host. c, d Section of ascomata. e Structure of peridium. f Pseudoparaphyses. g, h Asci. i–n Ascospores (n ascospore in Indian Ink). o Germinated ascospore. p, q Colony on PDA (p from above, q from below). Scale bars: c, d = 50 µm, e–o = 20 µm.

FIGURE 3. Setoseptoria guangxiensis (ZHKUCC 23-1034). a A two-month-old colony on PDA (from above). b Ascoma produced in PDA after two months. c Colony on PDA (from below). d, e Asci. f–i Ascospores (i ascospore in Indian Ink). Scale bars: d–i = 20 µm.