2. Ecological Evolution and Applications
A. muscaria is a fungus belonging to the division of the Basidiomycota, the order of the Agaricales, and the family of the
Amanitaceae. The diameter of cap of the mushroom usually ranges from 4 to 21 cm, but can reach 50 cm. It is characterized by convex to flat shapes, and has bright red color (
Figure 1). However, there are different varieties with an important phenotypic diversity, especially regarding the cap color, e.g.,
Amanita muscaria var.
guessowii is yellow, and
A. muscaria var.
flavivolvata and
Amanita muscaria var.
inzengae are orange with yellowish warts.
A. muscaria var.
alba is entirely white, although it is still unclear if it is a variety of its own or a white variant of the variety
flavivolvata [3]. As a remnant of the universal veil tissues, the cap is covered with white warts and the stipe has a volva limited to concentric rings
[4][5].
Figure 1. Basidiocarps of A. muscaria observed in the French Prealps (Alpes-de-Haute-Provence, France) (Denis Charret ©).
The basidiospores are released from the basidia during the fruiting season, usually after a rain episode
[6][7]. It is an ectomycorrhizal fungus and it is usually associated with
Angiospermae trees, such as
Betula,
Castanea,
Cistus, and
Quercus species, and also coniferous ones, such as
Abies,
Picea, and
Pinus [5][8]. For this reason, on this day, it is impossible to grow
A. muscaria basidiocarps in a laboratory, although it is possible to grow mycelium on synthetic media, despite a very slow growth rate
[9]. A comparative genome analysis of the mitochondrial DNA suggested a common origin of the ectomycorrhizal
Amanita, forming a distinct clade from the saprophyte and, therefore, non-symbiotic
Amanita [10].
A. muscaria is adapted to the temperate and boreal regions of the northern hemisphere. Grown on synthetic media, temperature ranging from 11 to 23 °C had no effect on the mushroom’s respiratory activity
[11].
A. muscaria usually grows in podzolic soils from deciduous and coniferous forests
[12]. In these acidic soil conditions, rates of nitrogen mineralization of the leaf litter by bacteria are low, and nitrogen is mostly present under its poorly mobile ammonium and organic forms.
A. muscaria possesses a high-affinity ammonium importer gene and, through the mycorrhizal symbiosis, is therefore essential in the plant nitrogen uptake in such ecosystems
[13]. Phylogenetic studies indicate the existence of three distinct geographical clades (the “Eurasian”, “Eurasian-Alpine”, and “North American” groups) that evolved from an ancestral
A. muscaria population located in the unglaciated soils of the Siberian–Beringian region—connecting Eurasia and America—during the Neogene period. This population later underwent fragmentation during its expansion on the lands
[14]. This classification may evolve as new comparative genomic studies are carried out
[15]. The mushroom has also been reported in the southern hemisphere, usually as an accidental consequence of the introduction of non-native trees in Chile, South Africa, New Zealand, and Australia
[16][17][18][19]. In Colombia, where it was introduced through planting of non-native trees in an effort to mitigate deforestation, it has spread to form new symbiotic interactions with native trees
[20].
Fungal nutrition is carried out by absorbotrophy, meaning the mycelium absorbs and internalizes in its cells the nutrients present in the surrounding aqueous film
[21][22]. Mushroom composition regarding amino acids and sugars is, therefore, influenced by the type of topsoil the mycelium is growing in
[23]. This mode of nutrition also naturally allows the mushrooms to accumulate recalcitrant compounds—especially heavy metals—in their internal tissues and, notably, in the basidiocarp. This makes them interesting bioindicators for monitoring soil contaminants
[24]. To be a good quality bioindicator, an organism has to be able to accumulate a compound.
A. muscaria effectively accumulates Hg and labile forms of Cd, K, Rb, and V—up to 0.1% of the dry mass in the lower part of the stipe, the latter being stabilized with ligands in the mushroom as amavadin complex
[12][25][26]. A good bioindicator also has to be robust to the effect of the maturation stage on the composition of the monitored compound. This way,
A. muscaria revealed very little changes in the Cu, Zn, Mg, Mn, Se, V, Cd, Ag, and As composition of its tissue over time, unlike Rb, Cs, Pb, Sb, Tl, and Ba, which were higher in younger than in mature fruiting bodies, suggesting discontinuous absorption, translocation, and accumulation processes for these metals
[27]. This constant absorption of a given stable compound may vary with its radioactive isotope. In the case of radioactive cesium, an evolution of the concentration of the
137Cs isotope depending on the developmental stages of the mushroom has been observed
[28].
3. Chemical Composition, Toxicity, and Potential Pharmaceutical Applications
The fly agaric contains compounds that individually exhibit various biological activities, mainly the muscarine, the ibotenic acid, and the muscimol (
Figure 2,
Table 1). The combination of all these compounds can trigger a wide range of symptoms depending on the ingested quantities, the relative amounts of compounds in the mushroom, and the overall physical condition of the patient. These symptoms involve a state of inebriation, hallucinations, restlessness, increased psychomotor drive, central nervous system depression, and gastrointestinal disturbance. The usual clinical course after ingestion of a fly agaric usually starts after 30 min with nausea, vomiting, diarrhea, cramps, tremor, ataxia, and incoordination. After 60 min, there is an apparition of an altered mental status characterized by declining senses, alternating between agitation and obtundation, and bizarre behaviors, involving disorientation and depersonalization. Hallucinations under the form of visual and auditory distortions are also common. Finally, later effects imply lethargy, followed by deep sleep
[29][30]. There is no existing antidote, as symptoms are both cholinergic and anticholinergic. Treatments after ingestion usually involve gastric lavage, use of activated charcoal, and symptomatic supportive care
[31]. However, the ingestion of a cap is very unlikely to lead to death, and fatal cases are very scarce. Still, a lethal dose for a human adult was calculated to be equivalent to ingesting 15 caps
[32]. In addition to acute toxicity, and as previously mentioned, the mushroom is also consumed after detoxification as edible and the caps can accumulate contaminants. Therefore, the repeated consumption of mushroom harvested in contaminated areas may result in chronic toxicity related to long-term exposure to heavy metals.
Figure 2. Chemical structures of the main compounds found in A. muscaria.
Table 1. Compounds and their relative quantities in the basidiocarp.
Compounds |
Reported Quantities in a Mushroom (% Dry Weight) 1 |
Muscarine |
0.02 |
Ibotenic acid |
1 |
Muscimol |
0.09 |
Tropane alkaloids |
Traces |