Mortierella solitaria Telagathoti, M. Probst & Peintner sp. nov.

Index Fungorum number: IF 557446; MycoBank number: MB 557446; Facesoffungi number: FoF 09208; Figs. 1, 2, 3

Etymology – Latin solitaria means solitary, living / acting on its own. This refers to i) the unbranched sporangiophores, which usually appear solitary or in small groups on the mycelium. ii) The phylogenetic position without a clear sister–group. iii) The solitary status of people in quarantine during the COVID 19 pandemic.

Holotype – OAS3

Saprobic barren or poorly vegetated ground in alpine areas. Asexual morph Colonies rapidly forming characteristic concentrically zonate pattern consisting of broad lobes is produced on LCA media, whereas this pattern is inconspicuous under the thick cottony layer of mycelium, which readily appears on PDA from day 1. A pronounced smell of garlic at 10 °C changes to a faded slightly fruity odour when incubated at the higher temperatures. Colonies exhibiting faster growth rates at temperatures>10 °C, forming thick mycelial layers, but no or very few sporangia. Sporangia hyaline, many–spored, globose with a smooth-walled surface. They are 15–22 µm diam. with a deliquescent wall. A conspicuous collarette is left after spore liberation. Sporangiophores arise from both the aerial and substrate mycelium. They are simple and unbranched, 50–200 µm (–500 µm) tall, non–septate. Sporangiophores are tapering from 4–9 µm at the base to 3–4 µm near the tip. One of the isolates (OAS4) formed part of the sporangiophores with a swollen base similar to those formed by M. alpina. Sporangiophores typically awl–like, sometimes slightly widening to an inconspicuous 6–8 µm wide apophysis–like inflation. The columella is slightly bulged, but without spinose outgrowing typical for M. gamsii. Sporangiospores are subglobose to cylindrical–ovoid, smooth-walled, (6–) 7–8 (–10) µm×(5.5–)6–8 µm (x̅=7.8×6.9 μm, n=30), often with an oil drop. Gemme with 5.3–5.7 µm diam., appears macroscopically as white-spotted clusters. They were observed in the substrate mycelium on Hempseed agar at 25 °C. Chlamydospores and stylospores undetermined. Sexual morph Undetermined. The formation of zygospores is initiated, but aborted on PDA and SNA at 20 °C: suspensor cells and the progametangia are formed but do not further develop: empty hyphal compartment remained after plasmogamy.

Culture characteristics– Colonies rapidly growing, white, and filling the agar plate (8 cm) within 5–10 d of incubation (Fig. 2).

Material examined – AUSTRIA, Obergurgl, Rotmoos glacier forefield 2300 msl., barren ground with scattered Saxifraga aizoides and Persicaria vivipara, 10 September 2016, A. Telagathoti, J. Falbesoner P. Dresch (OAS3, holotype), ex–type living culture JMRC: SF013920; OAS4 (JMRC: SF013921) and OAS5 (JMRC: SF013922) are from the same origin, but different sampling sites.

GenBank numbers – ITS = MT279272, MT279273, MT279274, LSU=MT279275, MT279276.

Notes – The rDNA ITS sequences generated from our Mortierella solitaria isolates showed<95% sequence similarity to the sequences generated from the type materials of M. gamsii and of M. fluviae. The phylogenetic analysis (maximum likelihood, parsimony bootstrap analysis, and Bayesian inference analysis; Fig. 3) indicates that M. solitaria could belong to M. gamsii species complex. However, M. solitaria lineage is clearly distinct from M. gamsii, M. fluviae, M. exigua, M. elongata, M. alpina, M. schmuckeri, M. cogitans and M. sclerotiella (Fig. 2). An analysis of the concatenated ITS and LSU sequences confirmed that M. solitaria constitutes a distinct lineage of Mortierella (Fig. 4), but relationships have not yet been further resolved. Mortierella solitaria differs in several morphological features from similar species. Based on the classical concepts (Linnemann 1941), M. solitaria species could either be placed in M. simplex or M. alpina morpho-groups. However, species in M. simplex group usually have sporangiophores longer than 200 µm, and typical representative species of M. alpina morpho-group have sporangiophores with a typically widening base. All currently described species of M. alpina group differ due to e.g., presence of chlamydospores or different spore morphology. According to our studies, only one (OAS4) of the three isolates occasionally forms sporangiophores with a swollen base. However, the sporangiophore base is only slightly widened. In M. simplex group, M. rostafinskii could be similar, but it differs by larger sporangiophores and smaller spores 5–6.5×3–5 µm. Sequences of these species (including sequences from type strains) are available in public databases, further confirming that M. solitaria is a morphologically and phylogenetically distinct species.

Figure 1 – Mortierella solitaria (OAS3, holotype). a, b Sporangiophores formed on aerial or basal hyphae. c Sphaerical sporangial head with intact smooth peridium and small apophysis-like widening of the sporangiophore. d Tip of a sporangiophore with collarette and spores. e Sporangiospores released from the sporangium. f–h Initial stages of gametangiogamy with empty, aborted zygospore. Scale bars: a, b=20 µm, c–h=10 µm

Figure 2 – Sporangia of Mortierella solitaria. a Length of the sporangia (–500 µm tall). b Sporangiophore tips showing collarette and the spores. c Details of the peridial wall of the sporangia. Scale bars: a–c=10 µm

Figure 3 – Growth rates of Mortierella solitaria as boxplots. a Boxplot displaying the mean 25th and 75th percentiles as box and outside values as dots. b, c Seven days old culture of the isolates on LcA and PDA

Figure 4 – Phylogenetic relationship of Mortierella solitaria with related taxa based on the nucleotide sequences of the ITS. The maximum likelihood (ML) tree (-ln likelihood=1899.6675) was constructed in MEGA X (Kumar et al. 2018). Five representatives of Mortierella alpina complex are used as the outgroup taxa. The dataset includes 494 total characters. Phylogenetic trees obtained from ML and Bayesian inference posterior probabilities (BYPP) were similar in topology. Bootstrap support on the nodes represents MPBS/BYPP. The new species M. solitaria is highlighted in blue in the yellow box

Figure 5 – Phylogenetic relationship of Mortierella solitaria with related taxa based on the concatenated nucleotide sequences of the ITS and LSU. The maximum likelihood (ML) tree (-ln likelihood=5141.7516) was constructed in MEGA X (Kumar et al. 2018). Mortierella horticola and M. globalpina are used as the outgroup taxa. The dataset includes 1439 total characters. Phylogenetic trees obtained from ML, and Bayesian inference posterior probabilities (BYPP) were similar in topology. Bootstrap support on the nodes represents MPBS/BYPP. The new species Mortierella solitaria is highlighted in blue in the yellow box