Nemania serpens is a species of wood-inhabiting ascomycete fungus in the family Xylariaceae. It is the type species of the genus Nemania. The fungus produces spreading, dark brown to black, carbonaceous stromata on wood. It is particularly well documented as a saprobe on strongly decayed hardwood, but it has also been isolated as an endophyte from living plant tissues.
The species has played an important role in defining the modern concept of Nemania. During much of the twentieth century it was treated in Hypoxylon, while its asexual state was separately named Geniculosporium serpens. Morphological, cultural and later molecular work supported recognition of Nemania as distinct from Hypoxylon.
Taxonomy
Christiaan Hendrik Persoon described the species as Sphaeria serpens in 1796. Samuel Frederick Gray transferred it to his genus Nemania in 1821, producing the combination Nemania serpens.
Gray's original concept of Nemania contained a heterogeneous group of fungi. In 1964, M. A. Donk selected Sphaeria serpens as the type of the genus, thereby making N. serpens the species to which the generic name Nemania is nomenclaturally attached.
For many years, fungi now classified in Nemania were instead included in a broad concept of Hypoxylon, and N. serpens was commonly known as Hypoxylon serpens. Zdeněk Pouzar reassessed the Hypoxylon serpens complex in 1985 and reinstated Nemania as a separate genus on morphological grounds. Cultural studies by Liliane Petrini and Jack D. Rogers subsequently supported the distinction, particularly through differences between the asexual forms characteristic of Nemania and Hypoxylon.
The asexual state of the species had itself been described in 1964 as Geniculosporium serpens, the type species of the genus Geniculosporium. Under the modern system in which a fungus has a single accepted name rather than separate names for sexual and asexual states, Nemania has priority over Geniculosporium and is the recommended generic name.
The original material associated with the name was inadequate for modern identification. Granmo, Læssøe and Schumacher therefore designated a Norwegian collection, TROM 174, as the neotype in their 1999 monograph. The specimen was sufficiently preserved for morphological study and was also analysed using DNA sequencing.
Description
The sexual fruiting structures form superficial, spreading to cushion-shaped stromata that are firmly attached to the underlying wood. A modern redescription based on Iranian material recorded stromata up to long and wide. Their exposed surface is dark brown to black, with the outlines of the underlying perithecia often producing conspicuous mounds. The tissue immediately beneath the surface is carbonaceous, while tissue between and below the perithecia is black to dark brown.
The individual perithecia are obovoid, measuring approximately 0.35–0.65 mm high and 0.25–0.4 mm wide. Their ostioles project through the stromatal surface as papillate to coarsely papillate openings.
The asci are cylindrical and long-stalked. In the Iranian material, the stalk reached about 130 μm long and the spore-bearing portion measured approximately 55–70 by 7–9 μm.
A useful microscopic character is the response of the apical apparatus of the ascus to iodine reagents. It does not turn blue in Melzer's reagent, but gives a dextrinoid red to reddish-brown reaction in Lugol's solution. The combination is unusual within Nemania and is useful in separating N. serpens from morphologically similar species.
The ascospores are smooth, one-celled and pale brown to brown. They are ellipsoid and somewhat unequal-sided, with narrowly to broadly rounded ends. Iranian specimens measured 10–14 by 4–5 μm, occasionally reaching 6 μm in width. Nordic material falls within a closely similar range.
Each ascospore possesses a straight germ slit that is considerably shorter than the length of the spore. Together with the broadly rounded spore ends, relatively large perithecia and distinctive iodine reaction of the ascus, this short and inconspicuous germ slit forms part of the diagnostic combination used to recognise the species.
Asexual form and growth in culture
The asexual form is described as geniculosporium-like. Its conidiophores vary considerably in length and range from colourless to light brown. The conidiogenous cells may reach about 60 μm long and 2.5–3.2 μm wide. The conidia are colourless, ellipsoid and have a truncate base, measuring approximately 3–4.8 by 2–3.5 μm.
In culture on oatmeal agar, an Iranian isolate filled a 9 cm Petri dish in about 18 days. The colony was initially white and later became vinaceous, with a felt-like, non-zonate surface. After about 50 days it developed amber to honey-coloured pigmentation.
Cultural characters were historically important in distinguishing Nemania from Hypoxylon. Species of Nemania typically develop geniculosporium-like asexual structures, whereas the more restricted modern concept of Hypoxylon is associated with nodulisporium-like asexual forms.
Ecology
Decaying wood
Nemania serpens is principally a wood-inhabiting fungus. In the Nordic countries it produces stromata on strongly decayed, usually barkless wood belonging to numerous woody plant species. Members of the beech family, Fagaceae, and willow family, Salicaceae, were especially frequent substrates in the material examined by Granmo and colleagues.
Recorded Nordic substrates included wood of oaks, willows, poplars, maples and rowans, among other deciduous trees. Fruiting bodies may occur on the same fallen trunk as other species of Nemania, including N. atropurpurea, N. aureolutea, N. colliculosa and N. prava.
Specimens confirmed from northern Iran using both morphology and DNA were collected on fallen branches of Persian ironwood, Parrotia persica. Collections were made in Mazandaran and Guilan provinces in the forests south of the Caspian Sea.
Endophytic occurrence
The species can also occur within living plant tissues without producing visible disease symptoms. A study of fungal endophytes in symptomless leaf petioles of European ash in southern Poland identified N. serpens as the most frequently recovered ascomycete in the survey. It occurred in 38% of colonised petioles examined.
The same study tested the isolated fungi against Hymenoscyphus fraxineus, the causal agent of ash dieback, in laboratory cultures. N. serpens showed strong antagonistic activity and was capable of overgrowing colonies of the pathogen. The experiment was conducted in vitro and did not establish that the fungus suppresses ash dieback under field conditions.
A separate study isolated N. serpens as an endophyte from a Riesling grapevine (Vitis vinifera) in the Niagara region of Canada. That isolate was subsequently investigated for its production of specialised metabolites.
Association with tea wood rot
Plant-pathology literature has reported a fungus identified under the older name Hypoxylon serpens as a cause of wood rot in tea. The disease has been studied particularly in tea-growing regions of southern India, where affected stems develop dark fungal fructifications and deterioration of the underlying wood.
Because historical usage of the name H. serpens encompassed a complex of morphologically similar fungi, older plant-disease records made without modern molecular comparison need to be interpreted with caution. Modern taxonomic sources use Nemania serpens for the species represented by the neotype and its closely matching DNA sequence.
Distribution
Nemania serpens is especially well documented in Europe and has been described as very common there. Granmo and colleagues confirmed it from Norway, Sweden, Finland and Denmark after reassessing Nordic material under a restricted species concept.
The fungus has also been confirmed from northern Iran. Collections from Mazandaran and Guilan represented the first records of the species from that country and were supported by both morphological examination and DNA sequence data.
Endophytic isolates identified as N. serpens have additionally been reported from Poland and Canada. Interpretation of older geographic records is complicated by the historically broad application of the name and by later discoveries of incorrectly identified reference cultures.
Molecular identification
The molecular identification of N. serpens has an unusual history. When Granmo and colleagues designated neotype specimen TROM 174 in 1999, they generated an ITS ribosomal DNA sequence from it. The sequence was published as part of a graphical alignment in the paper but was not submitted to a public sequence database.
As a result, later phylogenetic studies used other cultures labelled as N. serpens without having a readily accessible sequence from the neotype for comparison. Pourmoghaddam and colleagues revisited the problem in 2022. They manually transcribed the neotype ITS sequence from the original 1999 alignment and deposited it in GenBank under accession OP289675.
Their study also generated ITS, LSU, RPB2 and TUB2 sequences from Iranian culture MUCL 57702. The ITS sequence of this isolate was nearly identical to that of the Norwegian neotype, differing by three nucleotide substitutions and three gaps. The two sequences clustered together with strong support, independently supporting the morphological identification of the Iranian material.
The comparison exposed errors in previously available sequence records. Culture HAST 235, which had been used as N. serpens in earlier phylogenetic work, did not group with the neotype lineage and was considered misidentified. Culture CBS 679.86 had likewise been deposited in GenBank under the name N. serpens, but comparison with type-derived sequence data showed that it represented Nemania prava.
These findings are significant because N. serpens is the type species of Nemania. A sequence securely tied to its neotype provides a reference point for determining the limits and evolutionary relationships of the genus.
Secondary metabolites
The Canadian grapevine endophyte studied by Ibrahim and colleagues produced a diverse group of polyketide metabolites in culture. Ten compounds newly characterised in the study were named epoxynemanione A, nemanifuranones A–F and nemanilactones A–C. Four previously known metabolites were also recovered.
Nemanifuranone A has an unusual C2 hemiacetal structure. In laboratory assays, the compound was active against both Gram-positive and Gram-negative bacteria. The reported activity concerned an isolated fungal metabolite tested experimentally and does not establish a medical use.
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