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The growth rate of ''A.&nbsp;gallica'' rhizomorphs is between {{convert|0.3|and|0.6|m|ft|abbr=on|1}} per year.<ref name=Rishbeth1991/> [[Population genetics|Population genetic]] studies of the fungus conducted in the 1990s demonstrated that genetic individuals grow [[mitosis|mitotically]] from a single point of origin to eventually occupy territories that may include many adjacent root systems over large areas (several [[hectare]]s) of forest floor.<ref name=Smith1992/><ref name=Rizzo1995/><ref name=Saville1996/> Based on the low [[mutation rate]]s observed in large, long-lived individuals, ''A.&nbsp;gallica'' appears to have an especially stable [[genome]].<ref name=Hodnett2000/> It has also been hypothesized that genetic stability may result from self-renewing mycelial repositories of nuclei with [[stem cell]]-like properties.<ref name=Gladfelter2009/>
The growth rate of ''A.&nbsp;gallica'' rhizomorphs is between {{convert|0.3|and|0.6|m|ft|abbr=on|1}} per year.<ref name=Rishbeth1991/> [[Population genetics|Population genetic]] studies of the fungus conducted in the 1990s demonstrated that genetic individuals grow [[mitosis|mitotically]] from a single point of origin to eventually occupy territories that may include many adjacent root systems over large areas (several [[hectare]]s) of forest floor.<ref name=Smith1992/><ref name=Rizzo1995/><ref name=Saville1996/> Based on the low [[mutation rate]]s observed in large, long-lived individuals, ''A.&nbsp;gallica'' appears to have an especially stable [[genome]].<ref name=Hodnett2000/> It has also been hypothesized that genetic stability may result from self-renewing mycelial repositories of nuclei with [[stem cell]]-like properties.<ref name=Gladfelter2009/>


==Habitat and distribution==
==Ecology, habitat, and distribution==
[[image:Armillaria gallica 57632.jpg|thumb|left|Young fruit bodies growing in clusters at the base of a tree|alt=Several clusters of light brown mushrooms growing in moss on the base of a large tree.]]
[[image:Armillaria gallica 57632.jpg|thumb|left|Young fruit bodies growing in clusters at the base of a tree|alt=Several clusters of light brown mushrooms growing in moss on the base of a large tree.]]
''Armillaria gallica'' can normally be found on the ground, but sometimes on stumps and logs.<ref name=Roody2003/> Mushrooms that appear to be terrestrial are attached to plant roots underneath the surface.<ref name="urlArmillaria gallica (MushroomExpert.Com)"/> It is widely distributed and has been collected in North America, Europe,<ref name=Rogers/> and Asia (China,<ref name=Qin2001/> Iran,<ref name=Asef2003/> and Japan<ref name=Ota2009/>). The species has also been found in the [[Western Cape]] Province of South Africa, where it is thought to have been [[introduced species|introduced]] from potted plants imported from Europe during the early colonization of Cape Town.<ref name=Coetzee2003/> In Scandinavia, it is absent in areas with very cold climates, like Finland or Norway, but it is found in southern Sweden. It is thought to be the most prevalent low altitude species of ''Armillaria'' in Great Britain and France. The upper limits of its altitude vary by region. In the French [[Massif Central (geology)|Massif Central]], it is found up to {{convert|1100|m|ft|abbr=on}}, while in Bavaria, which has a more [[continental climate]], the upper limit of distribution reaches {{convert|600|m|ft|abbr=on}}.<ref name=Guillaumin1993/>
''Armillaria gallica'' can normally be found on the ground, but sometimes on stumps and logs.<ref name=Roody2003/> Mushrooms that appear to be terrestrial are attached to plant roots underneath the surface.<ref name="urlArmillaria gallica (MushroomExpert.Com)"/> It is widely distributed and has been collected in North America, Europe,<ref name=Rogers/> and Asia (China,<ref name=Qin2001/> Iran,<ref name=Asef2003/> and Japan<ref name=Ota2009/>). The species has also been found in the [[Western Cape]] Province of South Africa, where it is thought to have been [[introduced species|introduced]] from potted plants imported from Europe during the early colonization of Cape Town.<ref name=Coetzee2003/> In Scandinavia, it is absent in areas with very cold climates, like Finland or Norway, but it is found in southern Sweden. It is thought to be the most prevalent low altitude species of ''Armillaria'' in Great Britain and France. The upper limits of its altitude vary by region. In the French [[Massif Central (geology)|Massif Central]], it is found up to {{convert|1100|m|ft|abbr=on}}, while in Bavaria, which has a more [[continental climate]], the upper limit of distribution reaches {{convert|600|m|ft|abbr=on}}.<ref name=Guillaumin1993/>


In North America, it is common east of the [[Rocky Mountains]], but rare in the [[Pacific Northwest]].<ref name=Ammirati2009/> In California, where it is widely distributed, the fungus is found in a variety of [[plant community|plant communities]], including aspen, coastal oak woodland, [[Douglas Fir]], [[Klamath National Forest|Klamath mixed conifer]], montane hardwood, montane hardwood-conifer, montane [[riparian zone|riparian]], Redwood, Sierran mixed conifer, [[valley oak]] woodland, valley-foothill riparian, and [[White Fir]].<ref name=Baumgartner2001/> A study of ''Armillaria'' distribution in New York forests showed that ''A.&nbsp;gallica'' was found primarily at sites that were low in [[organic matter]] and had high [[soil pH]]s.<ref name=Blodgett1992/>
In North America, it is common east of the [[Rocky Mountains]], but rare in the [[Pacific Northwest]].<ref name=Ammirati2009/> In California, where it is widely distributed, the fungus is found in a variety of [[plant community|plant communities]], including aspen, coastal oak woodland, [[Douglas Fir]], [[Klamath National Forest|Klamath mixed conifer]], montane hardwood, montane hardwood-conifer, montane [[riparian zone|riparian]], Redwood, Sierran mixed conifer, [[valley oak]] woodland, valley-foothill riparian, and [[White Fir]].<ref name=Baumgartner2001/> [[Field study|Field studies]] suggest that ''A.&nbsp;gallica'' prefers sites that are low in [[organic matter]] and have high [[soil pH]]s.<ref name=Blodgett1992/><ref name=Oliva2009/>


A Chinese study published in 2001 used the [[molecular biology|molecular biological]] technique [[restriction fragment length polymorphism]] to analyze the differences in [[DNA sequence]] between 23 ''A.&nbsp;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.&nbsp;gallica'' subpopulations: the Chinese, European, North American–Chinese, and North American–European geographical [[lineage (evolution)|lineages]].<ref name=Qin2001/>
A Chinese study published in 2001 used the [[molecular biology|molecular biological]] technique [[restriction fragment length polymorphism]] to analyze the differences in [[DNA sequence]] between 23 ''A.&nbsp;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.&nbsp;gallica'' subpopulations: the Chinese, European, North American–Chinese, and North American–European geographical [[lineage (evolution)|lineages]].<ref name=Qin2001/>
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==Parasitism==
==Parasitism==
[[image:Trichoderma harzianum.jpg |thumb|right|The soil-dwelling fungal pathogen ''Trichoderma harzianum'' can parasitize ''A.&nbsp;gallica'' rhizomorphs''.|alt=An aggregation of long, thin translucent cells that are multiply branched. Some of the terminal branches have a small circular cell at their tips.]]
[[image:Trichoderma harzianum.jpg |thumb|right|The soil-dwelling fungal pathogen ''Trichoderma harzianum'' can parasitize ''A.&nbsp;gallica'' rhizomorphs''.|alt=An aggregation of long, thin translucent cells that are multiply branched. Some of the terminal branches have a small circular cell at their tips.]]
''Armillaria gallica'' is a weaker [[plant pathogen|pathogen]] than the related ''A.&nbsp;mella'' or ''[[Armillaria ostoyae|A.&nbsp;ostoyae]]'', and is considered a secondary parasite—typically initiating infection only after another fungus has weakened the host's defenses. Fungal infection can lead to [[root rot]] or [[butt rot]].<ref name=Healy2008/> As the diseased trees die, the wood dries, increasing the chance of catching fire after being struck by lightning. The resulting forest fire may in turn kill the species that killed the trees.<ref name=Stamets2005/> Plants that are under [[water stress]] caused by dry soils or [[waterlogging (agriculture)|waterlogging]] are more susceptible to infection by ''A.&nbsp;gallica''.<ref name=Packham2007/> It has been shown to be one of several ''Armillaria'' species responsible for widespread mortality of [[oak]] trees in the Arkansas [[Ozark]]s.<ref name=Kelly2009/> The fungus has also been shown to infect [[Daylily]] in [[South Carolina]],<ref name=Schnabel2005/> [[Northern highbush blueberry]] (''Vaccinium corymbosum'') in Italy<ref name=Prodorutti2006/> and vineyards (''[[Vitis]]'' species) of [[Rías Baixas (DO)|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".<ref name=AguinCasal2003/> When ''A.&nbsp;ostoyae'' and ''A.&nbsp;gallica'' co-occur in the same forest, infection of root systems by ''A.&nbsp;gallica'' may reduce damage or prevent infection from ''A.&nbsp;ostoyae''.<ref name="urlCFS:Armillaria gallica"/> ''A.&nbsp;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.&nbsp;gallica'' was consistently the first to colonize tree stumps that had been [[coppicing|coppiced]] the previous year.<ref name=Rishbeth1991/> [[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.&nbsp;ostoyae'', ''A.&nbsp;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."<ref name=Mihail1995/>
''Armillaria gallica'' is a weaker [[plant pathogen|pathogen]] than the related ''A.&nbsp;mella'' or ''[[Armillaria ostoyae|A.&nbsp;ostoyae]]'', and is considered a secondary parasite—typically initiating infection only after another fungus has weakened the host's defenses. Fungal infection can lead to [[root rot]] or [[butt rot]].<ref name=Healy2008/> As the diseased trees die, the wood dries, increasing the chance of catching fire after being struck by lightning. The resulting forest fire may in turn kill the species that killed the trees.<ref name=Stamets2005/> Plants that are under [[water stress]] caused by dry soils or [[waterlogging (agriculture)|waterlogging]] are more susceptible to infection by ''A.&nbsp;gallica''.<ref name=Packham2007/> It has been shown to be one of several ''Armillaria'' species responsible for widespread mortality of [[oak]] trees in the Arkansas [[Ozark]]s.<ref name=Kelly2009/> The fungus has also been shown to infect [[Daylily]] in [[South Carolina]],<ref name=Schnabel2005/> [[Northern highbush blueberry]] (''Vaccinium corymbosum'') in Italy<ref name=Prodorutti2006/> and vineyards (''[[Vitis]]'' species) of [[Rías Baixas (DO)|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".<ref name=AguinCasal2003/> When ''A.&nbsp;ostoyae'' and ''A.&nbsp;gallica'' co-occur in the same forest, infection of root systems by ''A.&nbsp;gallica'' may reduce damage or prevent infection from ''A.&nbsp;ostoyae''.<ref name="urlCFS:Armillaria gallica"/> ''A.&nbsp;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.&nbsp;gallica'' was consistently the first to colonize tree stumps that had been [[coppicing|coppiced]] the previous year.<ref name=Rishbeth1991/> [[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.&nbsp;ostoyae'', ''A.&nbsp;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."<ref name=Mihail1995/>


''Armillaria gallica'' may itself be parasitized by other [[soil life|soil flora]]. Several species of the fungus ''[[Trichoderma]]'', including ''[[Trichoderma polysporum]]'', ''[[Trichoderma harzianum|T.&nbsp;harzianum]]'' and ''[[Trichoderma viride|T.&nbsp;viride]]'', are able to attack and penetrate and the outer tissue of ''A.&nbsp;gallica'' rhizomorphs and parasitize the internal hyphae. The infected rhizomorphs become devoid of living hyphae about one week after the initial infection.<ref name=Dumas1992/>
''Armillaria gallica'' may itself be parasitized by other [[soil life|soil flora]]. Several species of the fungus ''[[Trichoderma]]'', including ''[[Trichoderma polysporum]]'', ''[[Trichoderma harzianum|T.&nbsp;harzianum]]'' and ''[[Trichoderma viride|T.&nbsp;viride]]'', are able to attack and penetrate and the outer tissue of ''A.&nbsp;gallica'' rhizomorphs and parasitize the internal hyphae. The infected rhizomorphs become devoid of living hyphae about one week after the initial infection.<ref name=Dumas1992/>
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<ref name=OEDlatin>{{cite dictionary | encyclopedia=Oxford Latin Dictionary |edition=combined |editor=P.G.W. Glare |year=1982 |origyear=1976 |publisher=Clarendon Press |location=Oxford |isbn=0-19-864224-5}} ss. vv. "Gallia", p. 753; "bulbosus", p 244.</ref>
<ref name=OEDlatin>{{cite dictionary | encyclopedia=Oxford Latin Dictionary |edition=combined |editor=P.G.W. Glare |year=1982 |origyear=1976 |publisher=Clarendon Press |location=Oxford |isbn=0-19-864224-5}} ss. vv. "Gallia", p. 753; "bulbosus", p 244.</ref>

<ref name=Oliva2009>{{cite journal |author=Oliva J, Suz LM, Colinas C. |year=2009 |title=Ecology of ''Armillaria'' species on silver fir (''Abies alba'') in the Spanish Pyrenees |journal=Annals of Forest Science |volume=66 |issue=6 |pages=603–13 |doi=10.1051/forest/2009046}}</ref>


<ref name=Packham2007>{{cite book |author=Packham JR, Thomas PDG. |title=Ecology of Woodlands and Forests: Description, Dynamics and Diversity |publisher=Cambridge University Press |location=Cambridge, UK |year=2007 |page=205 |isbn=0-521-83452-X |url=http://books.google.com/books?id=0Ntvos9aaC8C&lpg=PA205&dq=Armillaria%20gallica&lr=&pg=PA205#v=onepage&q=Armillaria%20gallica&f=false |accessdate=2010-02-17}}</ref>
<ref name=Packham2007>{{cite book |author=Packham JR, Thomas PDG. |title=Ecology of Woodlands and Forests: Description, Dynamics and Diversity |publisher=Cambridge University Press |location=Cambridge, UK |year=2007 |page=205 |isbn=0-521-83452-X |url=http://books.google.com/books?id=0Ntvos9aaC8C&lpg=PA205&dq=Armillaria%20gallica&lr=&pg=PA205#v=onepage&q=Armillaria%20gallica&f=false |accessdate=2010-02-17}}</ref>

Revision as of 16:47, 18 March 2010

Template:FixBunching

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
Kingdom:
Fungi
Division:
Basidiomycota
Class:
Agaricomycetes
Order:
Agaricales
Family:
Physalacriaceae
Genus:
Armillaria
Species:
A. gallica
Binomial name
Armillaria gallica
Synonyms

Armillaria bulbosa (Barla) Kile & Watling
Armillaria inflata Velen.
Armillaria lutea Gillet
Armillaria mellea var. bulbosa Barla
Armillariella bulbosa (Barla) Romagn.

Template:FixBunching

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

Template:FixBunching Armillaria gallica is a species of honey mushroom in the Physalacriaceae family of the Agaricales order. The species is a common and ecologically important wood-decay fungus that can live as a saprobe, or as an opportunistic parasite in weakened tree hosts to cause root or butt rot. It has a widespread distribution, being found in temperate regions of Asia, North America, and Europe, and forms fruit bodies singly or in groups in soil or rotting wood. It has been inadvertently introduced to South Africa. The fungus was formerly known as Armillaria bulbosa and has a confusing taxonomic history. An individual colony living in a Michigan forest caused a media stir in the early 1990s after it was reported to cover an area of 15 hectares (37 acres), weigh at least 9,500 kilograms (21,000 lb), and be 1,500 years old. This individual is popularly known as the "humungous fungus", and is a tourist attraction and inspiration for an annual mushroom-themed festival in Crystal Falls.

The largely subterranean fungus 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 caps are gills that are white to creamy or pale orange. The stem may be up to 10 cm (3.9 in) long, with a white cobwebby ring that separates the surface color of the stem into pale orange to brown above, and lighter-colored below. The fungus can develop an extensive system of underground root-like structures, called rhizomorphs, that help it to efficiently decompose dead wood in temperate broadleaf and mixed forests. Armillaria gallica has been the subject of considerable scientific research due to its importance as a plant pathogen, its ability to bioluminesce, its unusual life cycle, and its ability to form large and long-lived colonies.

Taxonomy and naming

Confusion has surrounded the nomenclature and taxonomy of the genus Armillaria.[1] Various biological studies conducted by the Finnish mycologist Kari Korhonen in 1978 showed that there were five different Armillaria species in Europe with rings on the stem; this contrasted with the more than 50 specific epithets that had been published in the scientific literature.[2] Korhonen's analysis reduced the European "Armillaria mellea" species complex to five species, which he named A through E. The taxon he called "species E" was later renamed and described as Armillaria gallica by Helga Marxmüller in 1987.[3] However, Marxmüller had earlier referred to this species in 1982—when it was thought to be "species B"—and called it A. bulbosa.[4] This name was given to reflect presumed identity with Armillaria mellea var. bulbosa, first described by Joseph Barla in 1887.[5]

In 1927, Josef Velenovský raised the variant bulbosa to the species level.[6] In 1973, the French mycologist Henri Romagnesi, unaware of Velenovský's findings, published a description of the species Armillariella bulbosa based on specimens he had found near Compiègne and Saint-Sauveur-le-Vicomte in France. These specimens were later demonstrated to be the same species as the "species E" of Korhonen. Further, Velenovský's description was not based on fresh material. Rather, it was based on Barla's 1887 description. Thus, both Romagnesi and Marxmüller proposed the name Armillaria gallica for "species E", but they mistakenly indicated Armillariella bulbosa (Barla) Romagnesi as the basionym. Marxmüller considers she and Romagnesi to have joint authorship of the species, explaining, "as we had clearly mentioned that we accepted neither the typus of Barla, nor Romagnesi's combination, the mistake does not justify changing the name gallica."[7] This naming, however, has not been universally accepted. For example, the third volume of Flora Agaricina Neerlandica, a comprehensive guide to Dutch mushrooms, uses the name Armillaria lutea Gillet.[8] As of 2010, however, both the Index Fungorum and MycoBank consider Armillaria gallica Marxm. & Romagn. to be the current name, with A. lutea as a synonym.[9][10]

The specific epithet gallica is botanical Latin for "French" (from Gallia, "Gaul")[11], and refers to the type locality.[12] The prior name bulbosa is Latin for "bulb-bearing, bulbous" (from bulbus and the suffix -osa).[12][11] Armillaria is from the Latin armilla, or "bracelet".[13]

Description

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 partial veil—which serves to protect the developing gills. As the cap grows in size the membrane is eventually pulled away from the cap to expose the gills. The gills have an adnate (squarely attached) to somewhat decurrent (extending down the length of the stem) attachment to the stem. They are initially white, but age to a creamy or pale orange covered with rust-colored spots. The stem is 4–10 cm (1.6–3.9 in) long and 0.6–1.8 cm (0.24–0.71 in) thick, and almost club-shaped with the base up to 1.3–2.7 cm (0.5–1.1 in) thick. Above the level of the ring, the stem is pale orange to brown, while below it is whitish or pale pink, becoming grayish-brown at the base. The ring is positioned about 0.4–0.9 cm (0.16–0.35 in) below the level of the cap, and may be covered with yellowish to pale-brownish woolly cottony mycelia. The base of the stem is attached to 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.[14][15]

Microscopic features

Observing the spores and tissues of A. gallica fruit bodies with a light microscope reveals microscopic characteristics that may be used to help distinguish the fungus from related species. When the spores are seen in deposit, such as with a spore print, they appear whitish.[8] 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.[8] 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.[16] 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.[8]

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.[14] One author advises to consume only a small portion initially, as some people may experience an upset stomach.[17] The taste is described as "mild to bitter", and the odor "sweet",[18] or reminiscent of camembert cheese.[16]

Similar species

Armillaria calvescens is rather similar in appearance, and can only be reliably distinguished from A. gallica by observing microscopic characteristics. A. calvescens has a more northern distribution, and in North America, is rarely found south of the Great Lakes.[18] A. mellea has a thinner stem than A. gallica, but can be more definitively distinguished by the absence of clamps at the base of the basidia.[19] Similarly, A. cepistipes and A. gallica are virtually identical in appearance (especially older fruit bodies), and are identified by differences in geographical distribution, and microscopic characteristics.[20] Molecular methods have been developed to discriminate between the two species by comparing DNA sequences in the gene coding translation elongation factor 1-alpha.[16]

Metabolites

Armillaria gallica can produce cyclobutane-containing metabolites such as arnamiol,[21] a natural product that is classified as a sesquiterpenoid aryl ester.[22] 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.[23][24]

Bioluminescence

The mycelia (but not the fruit bodies) of Armillaria gallica are known to be bioluminescent. Experiments have shown that the intensity of the luminescence is enhanced when the mycelia are disturbed during growth or when they are exposed to fluorescent light.[25] Bioluminescence is caused by the action of luciferases, enzymes that produce light by the oxidation of a luciferin (a pigment).[26] 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,[27] it may be a by-product of other biochemical functions,[28] or it may help deter heterotrophs that might consume the fungus.[27]

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 Upper Peninsula of Michigan in the early 1990s, during an unrelated research project to study the possible biological effects of extremely low frequency radio stations, which were being investigated as a means to communicate with submerged submarines. In one particular forest stand, Armillaria-infected oak trees had been harvested, and their stumps were left to rot in the field. Later, when red pines were planted in the same location, the seedlings were killed by the fungus, identified as A. gallica (then known as A. bulbosa). Using molecular genetics, they determined that the underground mycelia of one individual fungal colony covered 15 ha (37 acres), weighing over 9,500 kilograms (21,000 lb), with an estimated age of 1,500 years.[29][30] The analysis used restriction fragment length polymorphism (RFLP) and random amplification of polymorphic DNA (RAPD) to examine isolates collected from fruit bodies and rhizomorphs (underground aggregations of fungal cells that resemble plant roots) along 1-kilometer (0.6 mi) transects in the forest. The 15-hectare area yielded isolates that had identical mating type alleles and mitochondrial DNA restriction fragment patterns; this degree of genetic similarity indicated that the samples were all derived from a single genetic individual, or clone, that had reached its size through vegetative growth. In their conclusion the authors noted: "This is the first report estimating the minimum size, mass, and age of an unambiguously defined fungal individual. Although the number of observations for plants and animals is much greater, members of the fungal kingdom should now be recognized as among the oldest and largest organisms on earth."[31] After the Nature paper was published, major media outlets from around the world visited the site where the specimens were found; as a result of this publicity, the individual acquired the common name "humongous fungus".[30] There was afterward some scholarly debate as to whether the fungus qualified to be considered in the same category as other large organisms such as the blue whale or the giant redwood.[32]

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.[33] The organism was the subject of a Late Show Top Ten List on Late Night with David Letterman,[34] and an advertising campaign by the rental company U-Haul.[30]

Life cycle and growth

The life cycle of A. gallica includes two diploidization–haploidization events. The first of these is the usual process of cell fusion (forming a diploid) followed by meiosis during the formation of haploid basidiospores.[35] The second event is more cryptic, and occurs before fruit body formation. In most basidiomycetous fungi, the hyphae of compatible mating types will fuse to form a two-nucleate, or dikaryotic stage; this stage is not observed in Armillaria species, which have cells that are mostly monokaryotic and diploid. Genetic analyses suggest that the dikaryotic mycelia undergo an extra haploidization event prior to fruit body formation to create a genetic mosaic.[36] These regular and repeating haploidization events result in increased genetic diversity, which helps the fungus to adapt to unfavorable changes in environmental conditions, like drought.[37][38][39]

The growth rate of A. gallica rhizomorphs is between 0.3 and 0.6 m (1.0 and 2.0 ft) per year.[40] 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.[31][41][42] Based on the low mutation rates observed in large, long-lived individuals, A. gallica appears to have an especially stable genome.[43] It has also been hypothesized that genetic stability may result from self-renewing mycelial repositories of nuclei with stem cell-like properties.[44]

Ecology, 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.[45] Mushrooms that appear to be terrestrial are attached to plant roots underneath the surface.[18] It is widely distributed and has been collected in North America, Europe,[17] and Asia (China,[46] Iran,[47] and Japan[48]). The species has also been found in the Western Cape Province of South Africa, where it is thought to have been introduced from potted plants imported from Europe during the early colonization of Cape Town.[49] In Scandinavia, it is absent in areas with very cold climates, like Finland or Norway, but it is found in southern Sweden. It is thought to be the most prevalent low altitude species of Armillaria in Great Britain and France. The upper limits of its altitude vary by region. In the French Massif Central, it is found up to 1,100 m (3,600 ft), while in Bavaria, which has a more continental climate, the upper limit of distribution reaches 600 m (2,000 ft).[20]

In North America, it is common east of the Rocky Mountains, but rare in the Pacific Northwest.[50] In California, where it is widely distributed, the fungus is found in a variety of plant communities, including aspen, coastal oak woodland, Douglas Fir, Klamath mixed conifer, montane hardwood, montane hardwood-conifer, montane riparian, Redwood, Sierran mixed conifer, valley oak woodland, valley-foothill riparian, and White Fir.[51] Field studies suggest that A. gallica prefers sites that are low in organic matter and have high soil pHs.[52][53]

A Chinese study published in 2001 used the molecular biological technique restriction fragment length polymorphism to analyze the differences in 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.[46]

Parasitism

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 pathogen than the related A. mella or A. ostoyae, and is considered a secondary parasite—typically initiating infection only after another fungus has weakened the host's defenses. Fungal infection can lead to root rot or butt rot.[54] As the diseased trees die, the wood dries, increasing the chance of catching fire after being struck by lightning. The resulting forest fire may in turn kill the species that killed the trees.[55] Plants that are under water stress caused by dry soils or waterlogging are more susceptible to infection by A. gallica.[56] It has been shown to be one of several Armillaria species responsible for widespread mortality of oak trees in the Arkansas Ozarks.[57] The fungus has also been shown to infect Daylily in South Carolina,[58] Northern highbush blueberry (Vaccinium corymbosum) in Italy[59] and vineyards (Vitis species) of 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".[60] When A. ostoyae and A. gallica co-occur in the same forest, infection of root systems by A. gallica may reduce damage or prevent infection from A. ostoyae.[61] A. 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 coppiced the previous year.[40] 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. ostoyae, 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."[15]

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 and 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.[62]

See also

References

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