Didymodon ussuriensis Ignatova & Fedosov, 2025

Ignatova, E. A., Fedosov, V. E., Fedorova, A. V., Kuznetsova, O. I. & Ignatov, M. S., 2025, Two new species of Didymodon (Pottiaceae, Bryophyta) from Asian Russia, Arctoa 34 (1), pp. 24-30 : 28-29

Didymodon ussuriensis Ignatova & Fedosov , species nova. Fig. 3 .

TYPE: Russia, southern part of Primorsky Territory, Ussurijsky State Reserve , 43.638943°N, 132.556431°E ,

ca. 177 m a.s.l., vicinity of Peyshula Field station , Zmeinaya Mt. northern slope. On cliff. 19.VIII.2022, Fedosov 21-02-11 (Holotype MW9131243 !, isotype MHA9132130 !) .

Diagnosis: the new species is similar to Didymodon glaucus Ryan in plant habit and leaf shape but differs from it in having leaves with 2–3(–4)-layered lamina in 1–4 marginal rows, basal laminal cells weakly differentiated, and in absence of brood bodies.

Etymology: the species name refers to the collecting locality of the type specimen, Ussurijsky Reserve and the name of the river Ussuri, which means ‘black as soot” in Manchu.

Description. Plants small to medium-sized, in loose or dense tufts, green or dark-green above, occasionally slightly glaucous, light brownish below, not glossy. Stems 0.7–1.5 cm long, repeatedly branched, with large central strand. Leaves loosely appressed, slightly flexuose when dry in proximal parts of shoots, upper leaves somewhat crisped, widely spreading when wet, 1.7–2.3× 0.18–0.32 mm, linear lanceolate to linear, widely keeled distally, with narrow, 2–3 cells wide, 3–4-layered, round in transverse section apical portions 0.1–0.3 mm long, not decurrent at bases; margins plane or weakly recurved in mid-leaf, entire; costa strong, 65–80 µm wide at leaf base, weakly tapering upwards, percurrent, strongly prominent dorsally, flat ventrally, with quadrate cells on both surfaces, papillose on ventral side, without a pad of large, translucent cells below leaf apex, smooth or weakly papillose on dorsal side, semicircular in transverse section, guide cells in 2 layers, 2–5 cells in each layer, dorsal stereid band 1-2-layered, ventral stereid band absent, ventral and dorsal epidermis differentiated; lamina unistratose near costa and in midleaf, with 1–4 rows of 2–3(–4)-stratose cells at margins; cells in distal part of leaves round, short elliptic and oblate, 6–10×µm, with moderately thickened, straight walls, with 1–4 low, bifurcate papillae on both surfaces; basal laminal cells weakly differentiated, chlorophyllose, rectangular, 20–40×9–11 µm, smooth, with moderately thickened walls. KOH reaction yellowish-green or yellow. Brood bodies absent. Dioicous, sporophytes unknown.

Distribution and ecology. In addition to the type locality in Ussurijsky Reserve, Didymodon ussuriensis was repeatedly collected in Primorsky Territory on Lozovy Range and found once on Russkij Island. It was also revealed in collections from Amur Province (Tukuringra Range in Zeisky Reserve) and from Zabaikalsky Territory (Daursky Reserve). It grows on dry calcareous rocks and outcrops in oak and birch forests; it was also collect- ed on marble blocks on the bank of Zeya reservoir.

Differentiation. Didymodon ussuriensis is habitually similar to D. glaucus in having linear-lanceolate leaves crisped at shoot tips in dry condition, occasionally also with glaucous tint. These two species share a specific structure of leaf tips, 2–3 cells wide, 3–4-layered, round in transverse section. However, D. ussuriensis possesses strongly thickened leaf margins, 2–3(–4)-layered in 1–4 rows of cells, flat or weakly recurved at short distance at mid-leaf, whereas in D. glaucus leaf margins are always unistratose and recurved. Also D. ussuriensis lacks brood bodies (in D. glaucus they are frequently numerous), and its basal laminal cells are poorly differentiated, chlorophyllose. The latter character distinguishes it also from D. australaisiae , which shares with it having 2–3-stratose leaf margins; D. australaisiae has shorter, lanceolate leaves, costa with a pad of large, translucent cells on ventral surface below leaf apex, and rhizoidal tubers. The latter species is resolved in the molecular-phylogenetic tree ( Fig. 1 ) in the clade representing the genus Trichostomopsis sensu Jiménez et al. (2022) , whereas Didymodon ussuriensis is found in the terminal clade of Didymodon s.str. There is some resemblance between D. ussuriensis and D. sibiricus in leaf shape, as they have long and narrow, liner-lanceolate leaves with partially bistratose lamina, costa with guide cells in 2–3 layers, lacking ventral stereids; however, D. sibiricus possesses wider leaf bases, 0.3–0.4 mm wide (vs 0.20–0.35 mm wide), leaf laminae with bistratose patches not only at margins, and smooth laminal cells (densely papillose in D. ussuriensis ).

Additional specimens examined: RUSSIA: Zabaikalsky Territory: Borzya District, Daursky Reserve , 16.VII.2010 E. S . Prelovskaya ( LE B-0047621). Amur Province: Zeysky Reserve, Zeya Reservoir coast between Razvedochny and Sukhoy Gulfs, outcrops, S. V . Dudov & M. N . Kozhin 2016_ Br _1006 ( MW9111441 ). Primorsky Territory: Partizansk District, Lozovy Range, Ignatov , Ignatova & Cherdantseva 06-2814 ( MHA9109004 ); Russkij Island , vicinity of Melkovodnaya Bay , Fedosov 24-351-18 ( MW9133389 ) .

Figures

J L M N OK Fig. 3. Didymodon ussuriensis Ignatova & Fedosov (from holotype). A: habit, dry; B: habit, wet; C– F: leaf transverse sections; G: stem transverse section; H: median leaf cells; I: upper leaf cells; J–O: leaves; P: basal leaf cells. Scale bars: 5 mm for B; 2 mm for A; 1 mm for J–O; 100 µm for C–I, P.

Fig. 1. Bayesian phylogenetic tree of Didymodon s.str. inferred from the com- bined sequences of nr ITS region and plastid atpB–rbcL, trnG & trnL–trnF. Bayesian posterior probabilities (above 0.7) inferred from the dataset with indels coded using simple indel coding approach and Bootstrap values (above 60) obtained from 1000 pseudoreplicates of ultrafast bootstrapping as implemented in iQ-tree are shown above the branches. The group- ings considered in the Taxonomy section are boldfaced.