Pythium xuzhouense Jia J. Chen & X.B. Zheng, 2020

Chen, Jia-Jia, Feng, Hui, Song, Wei & Zheng, Xiao-Bo, 2020, Two new Pythium species from southern China based on morphological and molecular characters, Phytotaxa 432 (3), pp. 263-273 : 269-270

Pythium xuzhouense Jia J. Chen & X.B. Zheng , sp. nov. ( Fig. 3 )

MycoBank no.: MB 822949

Differs from other Pythium species by filamentous or lobulated sporangia, smooth oogonia, mostly diclinous, sometimes monoclinous antheridia, subclavate, falcate or semicircle to subcircular antheridial cells, and plerotic or nearly plerotic and thin-walled oospores (0.5–1.5 µm).

Type.— CHINA. Jiangsu Prov.: Xuzhou, 117.05°E, 33.38°N, on roots of Glycine max , August 2016, Chen 136 (holotype, NJAU).

Etymology.— Xuzhouense (Lat.): referring to the location in China where the holotype was isolated.

Colonies submerged, with chrysanthemum pattern on CMA, rosette pattern on PCA, and rosette pattern on 10% V8A. Average growth rates 5 mm day−1 at 5°C, 9 mm day−1 at 10°C, 9 mm day−1 at 10°C, 12 mm day−1 at 15°C, 14 mm day−1 at 20°C, 17 mm day−1 at 25°C, 20 mm day−1 at 30°C, 15 mm day−1 at 35°C. Cardinal temperatures: minimum 2°C, optimum 30°C, maximum 40°C. Main hyphae hyaline, aseptate, up to 5.0 µm wide. No hyphal swellings. Sporangia lobulated, filamentous inflated or non-inflated ( Fig. 3A–D ). Homothallic; oogonia globose, smooth, terminal, 10–23 µm (mean 16.5 µm) in diameter.Antheridia mostly diclinous, sometimes monoclinous, one to three per oogonium ( Fig. 3E–I ); antheridial stalks unbranched; antheridial cells slightly elongated along the oogonial stalk, subclavate, falcate or semicircle to subcircular. Oospores plerotic or nearly plerotic, globose, 8–21.5 μm (mean 15.5 µm) in diameter, hyaline. Oospore wall 0.5–1.5 µm (mean 0.9 µm) thick.

Additional specimens examined. — CHINA. Jiangsu Prov.: Xuzhou, 117.05°E, 33.38°N, on roots of Glycine max , August 2016, Chen 153&215 (paratypes, NJAU).

Remarks.— Pythium xuzhouense is characterized by lobulated or filamentous sporangia, smooth oogonia, mostly diclinous, sometimes monoclinous antheridia, subclavate, falcate or semicircle to subcircular antheridial cells, and plerotic or nearly plerotic and thin-walled oospores (0.5–1.5 µm). The species in Pythium subclade B1a are characterized by filamentous inflated sporangia (except for P. angustatum ), and moderate growth rate (9–15 mm d –1; Lévesque & de Cock 2004). Phylogenetic analysis based on ITS and COI sequences indicated P. xuzhouense belongs to Pythium subclade B1a with full statistical supports ( Fig. 1 ). P. xuzhouense shares several morphological characteristics with other Pythium subclade B1a species, such as filamentous sporangia, and smooth oogonia. However, P. xuzhouense can be readily distinguished in having raletively fast growth rate (17 mm d –1) and low minimum growth temperature (2°C). P. xuzhouense may be confused with Py. biforme Uzuhashi & G. Okada in having filamentous inflated structures, plerotic oospores and similar growth rate (18 mm d –1). However, P. biforme differs from P. xuzhouense in its distinct globose hyphal swellings, and a lower maximum growth temperature (30°C, Table 4; Uzuhashi et al. 2015). P. angustatum Sparrow also has filamentous sporangia; however, it differs from P. xuzhouense in having a lower maximum growth temperature (35°C, Table 4), aplerotic oospores and slow growth rate (9 mm d –1; van der Plaats-Niterink 1981). Moreover, these two species and P. xuzhouense are phylogenetically different in the analysis based on the ITS+COI sequences ( Fig. 1 ).

Previous studies suggested that Pythium s.l. could be split into 11 clades (A–K), of which the large clade B consists of subclades B1 and B2 ( Lévesque & de Cock 2004). Most species in clade B formed filamentous sporangia of various shapes or sizes, consisting of a lobate or toruloid inflated element, or catenulate globose elements ( Uzuhashi et al. 2010). At present, 45 species were included in clade B.

BLAST analyses of ITS or COI sequences of Pythium subinflatum and P. xuzhouense , confirmed the generic placement of the two new species in Pythium subclade B1 and the two new species formed two monophyletic lineages with strong support. In the present study, P. subinflatum and P. xuzhouense are described based on morphological characters and molecular phylogenetic results. Moreover, comparisons of these two new species and their morphological and/or phylogenetically related species are also provided in Tables 3–4.

Pythium species are cosmopolitan with a variety of diverse habitats ( van der Plaats-Niterink 1981). To date, including 72 species of Pythium have been reported in China ( Ho 2013, Long et al. 2012, 2014, Chen et al. 2017).

In conclusion, as part of an ongoing study of Pythium species diversity in China, the five isolates represent two novel species within the genus Pythium , for which the names P. subinflatum and P. xuzhouense are proposed based on the phenotypic and phylogenetic characteristics.

Figures

FIGURE 1. Phylogeny of species in Pythium clade B species generated by maximum parsimony based on ITS+COI sequences. Branches are labeled with parsimony bootstrap proportions (before slanting line) high than 50% and Bayesian posterior probabilities (after slanting line) more than 0.95.

FIGURE 3. Asexual and sexual reproductive bodies of Pythium xuzhouense (Chen 136). A. Filamentous inflated sporangium. B. Filamentous non-inflated sporangia. C–D. Lobulate sporangia. E–F. Globose and smooth oogonia. G. Oogonium with a projection. H. Two diclinous antheridia. I. Plerotic oospore and three antheridia. Scale bars A–J=10 μm.