Arthrobotrys oligospora

Arthrobotrys oligospora
Scientific classification
Domain:	Eukaryota
Kingdom:	Fungi
Division:	Ascomycota
Class:	Orbiliomycetes
Order:	Orbiliales
Family:	Orbiliaceae
Genus:	Arthrobotrys
Species:	A. oligospora
Binomial name
Arthrobotrys oligospora Fresen. (1850)
Synonyms
Orbilia auricolor (A. Bloxam) Sacc. (1889) Arthrobotrys superba var. oligospora (Fresen.) Coemans (1863) Didymozoophaga oligospora (Fresen.) Soprunov & Galiulina (1951)

Arthrobotrys oligospora was discovered in Europe in 1850 by Georg Fresenius.^[1][2] A. oligospora is the model organism for interactions between fungi and nematodes.^[2] It is the most common nematode-capturing fungus,^[3][4][5] and most widespread nematode-trapping fungus in nature.^[2][6] It was the first species of fungi documented to actively capture nematodes.^[2][6]

This fungus reproduces by means of two-celled, pear-shaped conidia, in which the cells are of unequal size with the smaller cell nearer to the attachment point on the conidiophore.^[4][7] During germination, the germ tube typically erupts from the smaller cell.^[7] In environments rich with nematodes, the spores range from 22-32 by 12-20 μm,^[4][7] though the spores are smaller in environments devoid of nematodes.^[4][7] Conidium germination has a success rate of 100% but the formation of trapping organs are not always observed.^[6] Conidia have been found to disintegrate both in the air and on impact with an agar plate.^[8] Conidiophores and conidia grow from hyphae sprouted outside of a trapped dead nematode,^[2] and conidiophores have been found to change and grow into part of the adhesive net.^[2] Under ideal conditions, a colony can reach 65 mm in diameter after seven days incubation,^[6] with colourless, pale pink or yellow mycelium.^[6] The optimal growth temperature for the fungus in nematode-free and nematode-infested environments is 20 °C (68 °F) and 25 °C (77 °F), respectively.^[6] The growth rate of colonies is greater in the presence of light than in darkness.^[6]

Arthrobotrys oligospora is considered a saprobe and is more saprotrophic than other nematode capturing fungi.^[2][6] At first the fungus was considered largely saprophytic in nature but this interpretation was later questioned.^[4] Saprophytic growth uses D-xylose, D-mannose, and cellobiose.^[6] The fungus uses nitrite, nitrate, and ammonium for its nitrogen sources and uses pectin, cellulose, and chitin for its carbon sources.^[6] When predating on nematodes, the fungus uses cellobiose, L-asparagine, L-arginine, DL glutamic acid for its carbon and nitrogen sources.^[6]

Predation of nematodes occurs in low nitrogen environments,^[9] as the nematode becomes the main nitrogen source for the fungi.^[2] It has been found that the presence of ammonium causes a higher decrease of predation when compared to presence of nitrate or nitrite.^[3] Adding green manure or carbohydrates has been found to increase nematode trapping behaviors.^[6] A complex 3-dimensional net of hyphae is formed to trap the nematodes under conditions of pH 4.9-8.1 and a temperature less than 37 °C (99 °F).^[5][6][8][9] Nematodes, and specifically "nemin" (an extract derived from nematodes) were found to stimulate net formation.^[2][6] Nematodes are not as attracted to A. oligospora colonies that have not manifested traps, suggesting that these structures serve an additional attractant role possibly through the expression of pheromones.^[9][10] It is possible some of these pheromones, such as methyl 3-methyl-2-butenoate, evolved as olfactory mimicry. They may mimic and interfere with sex pheromones of some nematode species (in the above case, the pheromone interferes with Caenorhabditis elegans).^[11]

A full net is not needed to catch nematodes as smaller nematodes can be caught with a single loop.^[2] Lectins are used in attaching nematode to fungi^[9] The entire surface of net is covered in adhesive material.^[2][8] Strong adhesion keeps the nematode trapped and when the nematode struggles, it often results in multiple points of adhesion of the nematode to the net.^[8][10] It was even found that the adhesion of the nematode to the fungus remained under washing of agar plate with water.^[8] The net is flexible which results in 'hyphal drag' tiring the nematode.^[8] Multiple points of adhesion and 'hyphal drag' allow the net to be capable of catching both large and small nematodes easily.^[8] In vitro, bait nematodes are consumed often leaving Bunonema nematodes.^[8]

A substance found in paralyzed nematodes was found to be capable of paralyzing healthy nematodes,^[6][8] and it was later determined that a paralyzing substance, Subtilisin (A serine protease),^[12] is excreted into nematodes.^[2][8] An unstable toxin was thought to be made by the fungus,^[6][8] and it was later found that toxic levels of linoleic acid for nematodes (lethal dose of linoleic acid for C. elegans is 5–10 μg/ml)^[13] were found in the fungus.^[5][13] Enzymatic hyphal invasion, likely using collagenases which are found in 'Arthrobotrys',^[2] of a trapped nematode is followed by the digestion of contents of the nematode.^[8][9] Shortly after hyphal invasion, a hyphal bulb appears where hyphae grow outwards from the bulb along the entire body of the nematode.^[8]

Not all nematodes are caught by the net as the nematode needs to be in contact with the net for a short period of time in order for adhesion to occur.^[8] Nematodes were found to quickly move away from any net followed by curling if instantaneous contact occurs.^[8][14] The nematode then proceeds to move forward until out of the area of the net and unless prolonged contact is made the nematode is safe.^[8] This means one or several instantaneous contacts are not enough for adhesion between the nematode and net to occur.^[8]

No competing fungi or bacteria are found in nematodes which are being consumed by the fungus which means it is possible an antibiotic is released inside the nematode.^[8] In 1993, secondary metabolites (oligosporon, oligosporol A, and oligosporal B) which can act as antibiotics were found in the fungus.^[2][13] Oligosporon, oligosporol A, oligosporal B have hemolytic effects and are cytotoxic to nematodes, however they are not toxic to the C. elegans.^[13] Other oligosporon-type secondary metabolites also found in A. oligospora include (4S,5R,6R)-4′,5′- dihydrooligosporon, (4S,5R,6R)-hydroxyoligosporon, and (4S,5R,6R)-10′11 ′-epoxyoligosporon.^[13]

A branch of hyphae grows out of a vegetative hyphae eventually arching back to the parent hyphae and fuses with it to make a loop.^[3][7][8] This process repeats from any hyphae along any existing branches or a new parent hyphae.^[3][8] The nets are immediately adhesive,^[8] and hyphae in the loop have different organelles to trap nematodes which are not found in vegetative cells.^[2]

Arthrobotrys oligospora has been found in many different geographical regions which include Asia, Africa, North America and South America and Australasia.^[2] Some countries it has been found in include Turkmenistan, Azerbaijan, Poland, Canada, New Zealand, and India.^[6] The presence of insects infected by nematodes increased presence of A. oligospora but not other nematode capturing fungi.^[2]

The fungus can be found in soil in grassland, shrubland, plantations, sheep and cattle yards,^[6] and domesticated and non-domesticated animal feces.^[2] It colonizes forest steppe soil, mixed forest soil, and Mediterranean brown soil (pH 6.9-8.0) where the pH can be as low as 4.5, but is typically above 5.5.^[6] The fungus has also been found in aquatic environments,^[2] and heavily polluted areas, specifically heavy metal poisoned mines, fungicide, or nematicide infested soil,^[2][5] decayed plant material, leaves, roots, moss,^[6] and in the rhizosphere of various bean plants, barley,^[2][6] and the tomato plant.^[2] Larger populations of the fungus can be found in late spring and summer.^[5]

The fungus is a biological indicator of nematodes.^[2] The annual global cost of plant-parasitic nematodes is approximately 100 billion USD.^[13] Nematode capturing fungi such as the A. oligospora can be used to control growth of nematodes.^[5][6] This means that they can be potentially used as a bio-control agent to protect crops against nematode infestations.^[2] This may not be feasible since the nematodes occasionally eat the fungi.^[6]