Panellus stipticus, commonly known as the bitter oyster, the astringent panus, the luminescent panellus, or the stiptic fungus, is a species of fungus in the family Mycenaceae. It is the type species of the genus Panellus. The fungus was given its current scientific name in 1879, but has been known by many names since French mycologist Jean Bulliard first described it as Agaricus stypticus in 1783. Molecular phylogenetic analysis revealed P. stipticus to have a close genetic relationship with members of the genus Mycena.
Starting out as tiny white knobs, over a period of one to three months, the mushrooms develop into fan- or kidney-shaped caps that measure up to broad. The caps are orange-yellow to brownish, and attached to the decaying wood by short stubby stalks that are connected off-center or on the side of the caps. Strains from eastern North America are typically bioluminescent, but those from the Pacific coast regions of North America and from other continents are not. The luminescence is localized to the edges of the gills and the junction of the gills with the stem and cap. Bioluminescence is also observable with mycelia grown in laboratory culture, and the growth conditions for optimal light production have been studied in detail. Several chemicals have been isolated and characterized that are believed to be responsible for light production. Genetic analysis has shown that luminescence is controlled by a single dominant allele.
A common and widely distributed species, it is found in Eurasia, Australia, and North America, where it grows in groups or dense overlapping clusters on the logs, stumps, and trunks of deciduous trees, especially beech, oak, and birch. Modern research has probed the potential of P. stipticus as a tool in bioremediation, because of its ability to detoxify various environmental pollutants.
Taxonomy
The species was first named Agaricus stypticus by the French botanist Jean Bulliard in 1783, and later sanctioned by Elias Magnus Fries under this name in 1821. Fries later changed the genus as well as the spelling of the epithet and called it Panus stipticus. The species has had an extensive taxonomic history and been shuffled to a number of genera by various authors, resulting in several synonyms: Agaricus flabelliformis (Johann Friedrich Gmelin, 1792), Pocillaria stiptica (Otto Kuntze, 1898), Rhipidium stipticum (Karl Friedrich Wilhelm Wallroth, 1833), Crepidopus stipticus (Samuel Frederick Gray, 1821), Pleurotus stipticus (Paul Kummer), Lentinus stipticus (Joseph Schröter, 1885), and Merulius stipticus (Jean-Baptiste Lamarck). It was Finnish mycologist Petter Karsten who in 1879 assigned its current name. Panellus stypticus is still used in the literature as a variant spelling.
Panellus stipticus is the type species of the genus Panellus, and, in Rolf Singer's authoritative 1986 classification of the Agaricales, it is also the type species of subgenus Panellus, an infrageneric (below the taxonomic level of genus) grouping of Panellus species characterized by the absence of cystidia on the sides of the gills. More recently, phylogenetic analyses of the sequences of their ribosomal large subunit genes have concluded that Panellus stipticus is closely related to the poroid mushroom Dictyopanus pusillus. The molecular analysis supports a previous assessment by mycologists Harold Bursdall and Orson K. Miller, who in 1975 suggested merging Dictyopanus into Panellus based on similarities in spore shape, stem structure, and the ability of dried fruit bodies to revive when moistened. Formerly grouped in the family Tricholomataceae, a wastebasket taxon of gilled mushrooms with white spores, P. stipticus is now classified in the Mycenaceae, after a large-scale phylogenetic analysis revealed "a previously unsuspected relationship between Mycena and Panellus (including Dictyopanus)".
The fungus is commonly known as the bitter oyster, the luminescent panellus, the astringent panus, or the styptic fungus. The specific epithet stipticus refers to its purported value in stopping bleeding. Etymologically, it is a Greek equivalent to the Latin word astringens, deriving from στυπτικός (styptikós), itself from the verb στύφειν (styphein), "to contract".
Description
The fungus normally exists unseen, in the form of a mass of threadlike vegetative cells called a mycelium, inhabiting rotting wood; only when suitable environmental conditions of temperature, moisture, and nutrient availability are achieved does the fungus produce the reproductive structures known as fruit bodies, or mushrooms. The cap of the fruit body is kidney- or clamshell-shaped, convex to roughly flat, with dimensions of by . The edges of the caps are scalloped with small rounded teeth, and curved slightly inward. The cap surface is dry, with a pattern of block-like areas similar to cracked dried mud; the surface is also covered with small fine hairs that give it a somewhat woolly consistency. It may have several concentric ridges or zones. Fresh fruit bodies range in color from yellowish-orange to buff to cinnamon; when dried they may be various shades of tan, brown or clay. The faded colors of dried fruit bodies tend to revive when moistened. On the underside of the cap, the gills are narrow and spaced closely together, often forked, brownish to buff, and with numerous interconnecting cross-veins.
Supporting the cap is a stem that is long by thick and has an off-center attachment to the cap, either at or near the cap side. The dull-white stem is covered with minute silk-like fibers and is narrower at the base where it attaches to the substrate. Fruit bodies do not have any distinctive odor. The flesh is thin, tough, and dark yellow-brown to cream-colored.
Fruit body development Fruit bodies first appear as tiny white knobs less than a cubic millimeter in size. In a day or two the knobs grow into a horizontal pyramidal mass, increasing in height as the hyphae lengthen. This is soon followed by the formation of a minute cap, and lengthening of the stem. The stem is about 1 mm long when the cap first begins to form. The hyphae that comprise the stem gradually cease to grow at their ends, and then start to branch, with many of the branches growing in a horizontal direction. This growth, indicated by the flattening and broadening out of the top of the stem, gives rise to the cap. The horizontally aligned hyphae grow vertical branches which remain more or less parallel, ultimately forming the dorsal tissue of the cap. Other similar downward-growing branches form the fertile hymenium, which can be seen when the cap is about 2 mm in diameter.
The young cap is spherical and its growth is at first epinastic, its margin being curved inwards and pressed against the stem. In this way, the hymenium begins its development within a special enclosed chamber. As the hymenial surface increases and keeps pace with the growth of the dorsal tissue of the cap, the latter expands and exposes the gills. The gills are formed by the continual downward growth of some of the hyphae. The gills are exposed before the cap is completely developed, and before the spores are mature. Spores can be produced by fruit bodies as small as broad, and liberation of the spores continues until the fruit body is fully grown—a period of one month to three months, depending on the conditions of temperature and moisture. The mature spores are disseminated by the wind. When the fruit body is nearing maturity, some of the terminal portions of the hyphae of the dorsal surface of the cap separate, and as a consequence, the upper surface of the fruit body becomes granular in appearance.
The fruit body projects out horizontally from the growing surface. If the position of a log is altered after young fruit bodies with the beginnings of gills have appeared, the stems of these attempt to readjust themselves in order to place the cap in a horizontal position. The cap are sometimes zonate (marked with concentric lines that form alternating pale and darker zones); this depends on changes in the humidity of the environment, as variations in the amount of moisture will cause alternating periods of acceleration or slowing of growth.
A yellowish-brown pigment is diffused through the cytosol of the hyphae and is much deeper in color just below the cuticle of the cap. In very young fruit bodies, stems and caps are very pale buff, but soon the color of the cap deepens and becomes cinnamon. The intensity of the color appears to be dependent on light, for when fruit bodies are grown in diffuse light (temperature and humidity being constant) they are a uniform pale buff color, but in bright light they are cinnamon or tan.
Reproduction Panellus stipticus uses a heterothallic, tetrapolar mating system: each basidiospore develops into a self-sterile mycelium which, when grown alone, remains homokaryotic (i.e., with all cells genetically identical) indefinitely. Researchers have paired collections of P. stipticus from Japan and Eastern North America, and later, collections from New Zealand and Russia. Although the separated allopatric populations differ in bioluminescence and taste, the results revealed a universal intercompatibility group over these geographical regions. In a 2001 study, Jin and colleagues also paired geographically representative collections of the fungus, but observed a reduced ability to cross between Northern Hemisphere and Oceanian collections, as well as between and within Oceanian collections.
Microscopic features Various microscopic characteristics may be used to help identify the fungus from other morphologically similar species. A spore print of P. stipticus, made by depositing a large number of spores in a small area, reveals their color to be white. Viewed with a microscope, the spores are smooth-walled, elliptical to nearly allantoid (sausage-shaped), with dimensions of 3–6 by 2–3 μm. Spores are amyloid, meaning that they will absorb iodine and become bluish-black when stained with Melzer's reagent, but this staining reaction has been described as "relatively weak".
The basidia (the spore-bearing cells) are 15–20 by 2.5–3.5 μm, club-shaped, and clamped at the base. The spores are attached to the end of the basidia by four projections called sterigmata that are 1–3.5 μm long. Cystidia are hyphal cells in the hymenium that do not produce basidiospores; they also make up the surfaces of the fruit body (the pelli or "cuticles"). They are usually structurally distinct from the basidia, and their features may be used as microscopic characters to help distinguish and differentiate similar fungi. The cheilocystidia are found on the gill edge; in P. stipticus they are narrowly club-shaped, cylindrical, spindle-shaped to bifurcate at the apex. They are also thin-walled, hyaline (translucent), abundant and crowded, and measure 17–45 by 3.5–6 μm. The pleurocystidia, located on the gill face, are 17–40 by 3–4.5 μm, spindle- or club-shaped, sometimes bifurcate at the apex, thin-walled, and hyaline. They are scattered or in dense clusters, mostly embedded in hymenium, occasionally protruding up to half the width of the hymenium.