Armillaria gallica

Source: Wikipedia, the free encyclopedia.

Armillaria gallica
A group of five yellow-brown mushrooms clustered together. The mushroom caps are roughly convex, and have their edges rolled inwards towards the stem. The cap surfaces are covered with small short yellow scales. The stems are thick, with a thickness of about a third to a half the width of the caps. The mushrooms are growing in the dirt.
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Fungi
Division: Basidiomycota
Class: Agaricomycetes
Order: Agaricales
Family: Physalacriaceae
Genus: Armillaria
Species:
A. gallica
Binomial name
Armillaria gallica
Marxm. & Romagn.
Synonyms

Armillaria bulbosa (Barla) Kile & Watling
Armillaria inflata

Velen.

Armillaria lutea Gillet
Armillaria mellea var. bulbosa Barla
Armillariella bulbosa (Barla) Romagn.

Armillaria gallica
View the Mycomorphbox template that generates the following list
Gills on hymenium
Cap is convex
Hymenium is adnate
Stipe has a ring
Spore print is white
Ecology is
saprotrophic
or
parasitic
Edibility is edible

Armillaria gallica (

temperate regions of Asia, North America, and Europe. The species forms fruit bodies singly or in groups in soil or rotting wood. The fungus has been inadvertently introduced to South Africa. Armillaria gallica has had a confusing taxonomy, due in part to historical difficulties encountered in distinguishing between similar Armillaria species. The fungus received international attention in the early 1990s when an individual colony living in a Michigan forest was reported to cover an area of 15 hectares (37 acres), weigh at least 9.5 tonnes (9,500 kg; 21,000 lb), and be 1,500 years old. This individual is popularly known as the "humongous fungus", and is a tourist attraction and inspiration for an annual mushroom-themed festival in Crystal Falls. Recent studies have revised the fungus's age to 2,500 years and its size to about 400 tonnes (400,000 kg; 880,000 lb), four times the original estimate.[1]

Armillaria gallica is a largely subterranean fungus, and it produces fruit bodies that are up to about 10 cm (3.9 in) in diameter, yellow-brown, and covered with small scales. On the underside of the

plant pathogen, its ability to bioluminesce, its unusual life cycle, and its ability to form large and long-lived colonies
.

Phylogeny, taxonomy and naming

A. solidipes

A. gemina

A. calvescens

A. gallica (ST22)

A. gallica (ST23)

A. gallica (M70)

NABS X

A. nabsnona

A. tabescens

A. mellea

Phylogeny and relationships of A. gallica and related North American species based on amplified fragment length polymorphism data. SY22, ST23, and M70 are A. gallica specimens collected from Michigan, Wisconsin, and British Columbia, respectively.[2]

Confusion has surrounded the nomenclature and taxonomy of the species now known as Armillaria gallica, paralleling that surrounding the genus Armillaria.[3] The type species, Armillaria mellea, was until the 1970s believed to be a pleiomorphic species with a wide distribution, variable pathogenicity, and one of the broadest host ranges known for the fungi.[4] In 1973, Veikko Hintikka reported a technique to distinguish between Armillaria species by growing them together as single spore isolates on petri dishes and observing changes in the morphology of the cultures.[5] Using a similar technique, Kari Korhonen showed in 1978 that the European Armillaria mellea species complex could be separated into five reproductively isolated species, which he named "European Biological Species" (EBS) A through E.[6] About the same time, the North American A. mellea was shown to be ten different species (North American Biological Species, or NABS I through X);[7] NABS VII was demonstrated shortly after to be the same species as EBS E.[8] Because several research groups had worked with this widely distributed species, it was assigned several different names.

The species that Korhonen called EBS B was named A. bulbosa by Helga Marxmüller in 1982,

type locality, and incomplete collection notes.[12] A. inflata (Velenovský, 1920) may represent another synonym, but the type specimens were not preserved, so it is considered a dubious name (nomen dubium).[17] As of 2010, both the Index Fungorum and MycoBank consider Armillaria gallica Marxm. & Romagn. to be the current name, with A. bulbosa and A. lutea as synonyms.[18][19]

Phylogenetic analysis of North American Armillaria species based on analysis of amplified fragment length polymorphism data suggests that A. gallica is most closely related to A. sinapina, A. cepistipes, and A. calvescens.[2] These results are similar to those reported in 1992 that compared sequences of nuclear ribosomal DNA.[20]

The

type locality.[22] The prior name bulbosa is Latin for "bulb-bearing, bulbous" (from bulbus and the suffix -osa).[21][22] Armillaria is derived from the Latin armilla, or "bracelet".[23]

Description

The underside of a mushroom cap showing whitish cottony tissue connecting the edge of the brown cap with the whitish stem.
Young fruit bodies have a cottony partial veil that protects the developing gills.
The underside of a mushroom cap showing numerous closely spaced gills. A small ring of whitish cottony tissue can be seen at the stem where it attaches the cap.
Mature gills

The fruit bodies of Armillaria gallica have caps that are 2.5–9.5 cm (1.0–3.7 in) broad, and depending on their age, may range in shape from conical to convex to flattened. The caps are brownish-yellow to brown when moist, often with a darker-colored center; the color tends to fade upon drying. The cap surface is covered with slender fibers (same color as the cap) that are erect, or sloping upwards.

When the fruit bodies are young, the underside of the caps have a cottony layer of tissue stretching from the edge of the cap to the stem—a

rhizomorphs, black root-like structures 1–3 mm in diameter. While the primary function of the below-ground mycelia is to absorb nutrients from the soil, the rhizomorphs serve a more exploratory function, to locate new food bases.[24][25]

Microscopic features

When the spores are seen in deposit, such as with a spore print, they appear whitish. They have an ellipsoid or oblong shape, usually contain an oil droplet, and have dimensions of 7–8.5 by 5–6 µm. The spore-bearing cells, the basidia, are club-shaped, four-spored (rarely two-spored), and measure 32–43 by 7–8.7 µm.[26] Other cells present in the fertile hymenium include the cheilocystidia (cystidia present on the edge of a gill), which are club-shaped, roughly cylindrical and 15–25 by 5.0–12 µm. Cystidia are also present on the stem (called caulocystidia), and are broadly club-shaped, measuring 20–55 by 11–23 µm.[27] The cap cuticle is made of hyphae that are irregularly interwoven and project upward to form the scales seen on the surface. The hyphae that make up the surface scales typically measure 26–88 µm long by 11–27 µm thick and can be covered with a crust of pigment. Clamp connections are present in the hyphae of most tissues.[26]

Edibility

Like all Armillaria species, A. gallica is considered

edible. Thorough cooking is usually recommended, as the raw mushroom tastes acrid when fresh or undercooked.[24] One author advises to consume only a small portion initially, as some people may experience an upset stomach.[28] The taste is described as "mild to bitter", and the odor "sweet",[29] or reminiscent of camembert cheese.[27]

Similar species

Metabolites

Armillaria gallica can produce

aryl ester.[32] Although the specific function of arnamiol is not definitively known, similar chemicals present in other Armillaria species are thought to play a role in inhibiting the growth of antagonistic bacteria or fungi, or in killing cells of the host plant prior to infection.[33]

Bioluminescence

The

oxidation of a luciferin (a pigment).[35] The biological purpose of bioluminescence in fungi is not definitively known, although several hypotheses have been suggested: it may help attract insects to help with spore dispersal,[36] it may be a by-product of other biochemical functions,[37] or it may help deter heterotrophs that might consume the fungus.[36]

Humongous fungus

Two clusters of mushrooms growing in a bed of green moss. The mushroom caps are densely covered with small scales and are a reddish-brown that gets deeper in the center. Some caps appear shiny as is covered with a translucent slime. The mushroom stems are club-shaped and a very light reddish-brown.
The fruit bodies—the visible manifestation of A. gallica—belie an extensive underground network of mycelia.

Researchers reported finding Armillaria gallica in the

giant redwood.[41]

The fungus has since become a popular tourist attraction in Michigan, and has inspired a "Humongous Fungus Fest" held annually in August in Crystal Falls.[42] The organism was the subject of a Late Show Top Ten List on Late Night with David Letterman,[43] and an advertising campaign by the rental company U-Haul.[39]

Life cycle and growth

The

genetic mosaic.[45] These regular and repeating haploidization events result in increased genetic diversity, which helps the fungus to adapt to unfavorable changes in environmental conditions, such as drought.[46][47][48]

The growth rate of A. gallica rhizomorphs is between 0.3 and 0.6 m (1.0 and 2.0 ft) per year.[49] Population genetic studies of the fungus conducted in the 1990s demonstrated that genetic individuals grow mitotically from a single point of origin to eventually occupy territories that may include many adjacent root systems over large areas (several hectares) of forest floor.[40][50][51] Based on the low mutation rates observed in large, long-lived individuals, A. gallica appears to have an especially stable genome.[52] It has also been hypothesized that genetic stability may result from self-renewing mycelial repositories of nuclei with stem cell-like properties.[53]

Specific mechanisms of somatic growth have been proposed to explain how species such as A. gallica keep

mutations. At the somatic growth front of A. gallica mutation rate was proposed to be kept low by cells dividing infrequently, but giving rise to cells behind the growth front that divide rapidly thus promoting tissue growth although at the expense of a higher mutation rate.[54]

Habitat and distribution

Several clusters of light brown mushrooms growing in moss on the base of a large tree.
Young fruit bodies growing in clusters at the base of a tree

Armillaria gallica can normally be found on the ground, but sometimes on stumps and logs.

Field studies suggest that A. gallica prefers sites that are low in organic matter and have high soil pHs.[62][63]

In North America, it is common east of the

White Fir.[65] It was found to be the most common Armillaria species in hardwood and mixed oak forests in western Massachusetts.[66]

A Chinese study published in 2001 used the

DNA sequence between 23 A. gallica specimens collected from the Northern Hemisphere. The results suggest that based on the restriction fragment length polymorphism patterns observed, there are four global A. gallica subpopulations: the Chinese, European, North American–Chinese, and North American–European geographical lineages.[67] A 2007 study on the northeastern and southwestern Chinese distribution of Armillaria, using fruit body and pure culture morphology, concluded that there are several unnamed species (Chinese biological species C, F, H, J and L) that are similar to the common A. gallica.[56]

Ecology

An aggregation of long, thin translucent cells that are multiply branched. Some of the terminal branches have a small circular cell at their tips.
The soil-dwelling fungal pathogen Trichoderma harzianum can parasitize A. gallica rhizomorphs.

Armillaria gallica is a weaker

Rías Baixas in northwestern Spain. The latter infestation "may be related to the fact that the vineyards from which they were isolated were located on cleared forestry sites".[76] When A. solidipes and A. gallica co-occur in the same forest, infection of root systems by A. gallica may reduce damage or prevent infection from A. solidipes.[77]

Six mushrooms of various shape and either brown or whitish in color, picked and laid in a row on a bed of moss. The two brown mushrooms have stems and caps. The smallest mushroom also has stem and cap, but is whitish-gray. Three other whitish-gray mushrooms are irregularly shaped and lumpy.
A. gallica may be parasitized by the fungus Entoloma abortivum, resulting in grayish-white, malformed fruit bodies.

Armillaria gallica can develop an extensive subterranean system of rhizomorphs, which helps it to compete with other fungi for resources or to attack trees weakened by other fungi. A field study in an ancient broadleaved woodland in England showed that of five Armillaria species present in the woods, A. gallica was consistently the first to colonize tree stumps that had been

Fractal geometry has been used to model the branching patterns of the hyphae of various Armillaria species. Compared to a strongly pathogenic species like A. solidipes, A. gallica has a relatively sparse branching pattern that is thought to be "consistent with a foraging strategy in which acceptable food bases may be encountered at any distance, and which favours broad and divisive distribution of potential inoculum."[25] Because the rhizomorphs form regular networks, mathematical concepts of graph theory have been employed to describe fungal growth and interpret ecological strategies, suggesting that the specific patterns of network attachments allow the fungus "to respond opportunistically to spatially and temporally changing environments".[78]

Armillaria gallica may itself be parasitized by other

soil flora. Several species of the fungus Trichoderma, including Trichoderma polysporum, T. harzianum and T. viride, are able to attack and penetrate the outer tissue of A. gallica rhizomorphs and parasitize the internal hyphae. The infected rhizomorphs become devoid of living hyphae about one week after the initial infection.[79] Entoloma abortivum is another fungus that can live parasitically upon A. gallica. The whitish-gray malformed fruit bodies that may result are due to the E. abortivum hyphae penetrating the mushroom and disrupting its normal development.[80]

See also

References

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