Physarum fragile A. Vlasenko, 2025

Vlasenko, Anastasia V., Zibzeev, Evgeny G., Baturin, Sergei O. & Vlasenko, Vyacheslav A., 2025, A new species of Physarum (Physaraceae) with a long stalk, Phytotaxa 702 (2), pp. 213-220 : 215-217

Physarum fragile A. Vlasenko , sp. nov. Fig. 1 .

MycoBank no: 857414

Etymology:— Refers to the fragile stalk.

Sporocarps scattered, sporotheca white or light grey, spherical, 0.2–0.3 mm diam., up to 1–1.5 mm high, on a very long stalk. Stalk is not subulate, its thickness is approximately equal at the bottom and at the top, bicoloured, lower part dark grey-greenish, middle and apical parts yellow, very fragile, breaks off in the apical part when dry. Peridium thin, filmy, iridescent, covered with a thin layer of white lime. Capillitium well developed, threads of capillitium light, thin, long, dichotomously branching and anastomosing. Nodes of the capillium small, more often rounded, sometimes less than 9 µm in diam. and elongated, contain yellow lime. When observed with SEM, threads of capillitium are covered with warts. Columella absent. Spores brown in mass, dark brown, spherical in transmitted light, 9–10 µm diam., with a double ornamentation; shell of spore ornamented with large coral-shaped warts and small rounded warts are located between them on the surface of the spore.

Holotype:— RUSSIA. Trans-Baikal Territory : Gazimuro-Zavodsky municipal district, left bank of the Gazimur river, larch-birch mixed grass and horsetail forest, on the bark of Betula platyphylla , 51.7370° N, 118.3977° E, 674 m a.s.l. Substrate samples collected 15 August 2022, E. G. Zibzeev, moist chamber culture 6 February 2023 cult. and ident. A. V. Vlasenko, NSK 1016826 ; GenBank: PP237263 (18 S).

Additional specimen examined:— RUSSIA. Novosibirsk Region: Iskitimsky district, near village of Ust-Chem, floodplain of the Chem River, aspen-willow forest, on bark of Populus tremula , 54.6955° N, 83.7680° E. Substrate samples collected 1 August 2021, S. O. Baturin, moist chamber culture 20 May 2022, cult. and ident. A. V. Vlasenko, NSK 1013218.

Habitat:— Bark of deciduous living trees.

Ecology:— Epiphyte.

Distribution:— Currently known only from North Asia, Russia, Trans-Baikal Territory and Novosibirsk Region.

Comments:— The new species, Physarum fragile resembles a group of species with a very long stalk and nodes of threads of capillitium with yellow lime— P. bethelii T. Macbr. ex G. Lister (1911: 57) , P. flavicomum Berk. (1845: 66) , P. galbeum Wingate (1899: 53) , P. karamanicum Eroglu (2023: 100) , P. oblatum T. Macbr. (1893: 384) , P. viride (Bull.) Pers. (1796: 6) ( Table 2).

capillitium.

The new species resembles the recently described ( Li et al. 2024) P. guangxiense X.F. Li, B. Zhang & Y. Li (2024: 21) in shape and size, but P. fragile is distinguished by darker and yellow nodes of capillitium and a brighter bicoloured dark grey-greenish and yellow stalk. Pysarum fragile , as well as P. subviride X.F. Li, B. Zhang & Y. Li (2024: 23) have yellow nodes of capillitium, but differ in shape, colour, size of the sporotheca, and shape of the stalk. Pysarum subviride has sporotheca that are yellowish-green, spherical or hemi-spherical, often slightly umbilicate, 0.3–0.4(−0.5) mm in diam., while P. fragile has sporotheca that are white, light grey, spherical, 0.2–0.3 mm in diam. Physarum subviride has a subulate stalk, whereas P. fragile has a stalk that is not subulate, the thickness of the stalk is approximately equal at the bottom and at top.

The distinctive features of Physarum fragile from the group of the above-mentioned species are presented in Table 2. The Ml analysis based on the 18S nrDNA region ( Fig. 2 ) showed that P. fragile is an independent species, closest to P. citrinum Schumach. (1803: 201) . The genetic distance of the P. fragile branch on the SSU tree is 0.029, with 100% bootstrap support. Phylogenetically similar species to P. fragile are also P. cinereum (Batsch) Pers. (1794: 89) , and P. vernum Sommerf. (1829: 146) , but morphologically these species have absolutely no similarities.

Physarum fragile has significant morphological differences from P. citrinum . Physarum fragile has a very long fragile stalk and small sporotheca. The ratio of stalk to sporotheca is on average five diameters of the sporotheca, whereas P. citrinum has a solid stalk not exceeding two diameters of the sporotheca. The hypothallus of P. citrinum is yellow, while that of P. fragile is black. Ornamentation of the surface of the spores in these two species is also different. However, the capillitium of P. citrinum and P. fragile are identical – the capillitium is dense, thin, and consists of many small yellow rounded nodes connected by many colourless tubes. Physarum cinereum and P. vernum cannot be confused with P. fragile , since they have fruit bodies in the form of plasmodiocarps and sessile sporangia without a stalk. In addition, the nodes of capillitium of P. cinereum and P. vernum are white, large, angular, and in P. fragile the nodes of capillitium are small and yellow.

Figures

FIGURE 1. Physarum fragile (holotype): A, C, D. Sporocarps (RL). B, E. Broken stalks (RL). F, G. Spores and threads of capillitium (TL). H. Threads of capillitium (SEM). I. Spore (SEM). J. Ornamentation on surface of the spore (SEM). Arrows point to bicoloured stalks. Scale bars: A = 0.2 mm, C, D, E = 0.2 mm, F, G = 20 µm, H = 5 µm, I = 2 µm, J = 800 nm.

FIGURE 2. UPGMA trees based on SSU sequences showing phylogenetic relationships between the new species and other related species of Physarum.