Nematode trapping fungi, or “nematophagous fungi,” are carnivorous fungi that have developed methods and structures that enable them to successfully trap and consume nematodes. These fungi prey on nematodes and are in turn consumed by organisms on the next trophic level.
Nematophagous fungi use several methods to hunt their prey. Different varieties of nematophagous fungi have evolved alongside the nematodes that they consume and have developed methods to catch the nematodes.These methods include living within the nematode and slowly consuming them as well as spreading diseases through nematode populations. Predaceous fungi develop their mycelium predominantly outside their host, which means that they are also able to live as saprophytes in the soil and set traps for the nematodes. Thus only mobile nematode stages are caught.
The predators are divided into several groups concerning the trapping mechanisms. Some species are able to develop adhesive substance on any part of their mycelium, predaceous fungi develop special traps: adhesive three-dimensional networks, adhesive columnar branches, adhesive knobs, constricting rings and non-constricting rings.
At the place of adhesion, the cuticle of the nematode is perforated, a globular structure
called infection bulb formed, which give rise to hypha growing throughout the
body of the prey. Enzymes are secreted by the fungus to digest the prey.
Varieties of Nematode
Trapping Fungi
1)
Fungal Ring (Hyphal Rings) Producers
One method used by fungi to trap nematodes is the fungal ring. Under suitable conditions, the fungal conidia germinate directly
to produce constricting ring trapping devices, often referred to as 'conidial
traps'. The fungus produces hyphae that form an open constricting loop. When a
nematode swims through this loop, the loop suddenly fills with water. This
sudden change in the physiology of the loop causes the diameter of the inside
of the loop to narrow and in turn constricts around the nematode. The ring cells germinate, penetrate through the cuticle of the
nematode and germ tubes invade the body of the victim. These initiate the
assimilative hyphal system that digests and absorbs the contents of the
host. Arthrobotrys dactyloides/ Drechslerella dactyloides, Drechslerella anchonia, and Drechslerella brochopaga are examples of fungi producing
rings. Previously classified as an Arthrobotrys species (Arthrobotrys
dactyloides), now renamed as Drechslerella which includes all
the predatory fungi that capture nematodes using the constricting ring
device.
Constricting ring of Arthrobotrys dactyloides
2)
Adhesive Hyphae producers
Another method
commonly used by nematophagous fungi is the use of adhesive hyphae. The fungi
who utilize this method create adhesive knobs or nets out of adhesive hyphae to
trap the nematode within the structure, thus providing the fungus with a
nitrogen rich meal. Fungi that utilize adhesive hyphae include Dactylellina
candida, Drechmeria coniospora, and Arthrobotrys oligospora.
3)
Nematotoxin producers
Nematode trapping
fungi of the Pleurotaceae family of mushrooms utilize adhesive knobs to catch
nematodes, but once the nematode is caught the fungus secretes a nematacide
that kills the nematode and prevents its escape. Other notable nematotoxin
users include Coprinus comatus, Stropharia rugosoannulata, Arthrobotrys dactyloides etc.
4)
Endoparasitic Nematophagous Fungi
Fungi that live within the nematode can infect the nematode at any stage depending on the species of fungus. The infection can be done percutaneously which means through the skin. This is of significance for biological control of phytopathogenous nematodes. From within the nematode, the fungus can consume nutrients meant for the nematode's growth and spread to other nematodes when able. These endoparasitic fungi can spread throughout a community in a measurably short time and weaken a whole population of nematodes throughout the process.
Unlike nematode-trapping fungi, the endoparasitic fungi do not extensively produce mycelium external to nematode body. But they attack nematodes through many modifications brought about in conidia. The endoparasitic fungi are species of Cephalosporium, Meria, Verticillium, Catenaria, Meristacrum, etc.
Fungi that survive as
spores in the soil until the time is right include Drechmeria coniospora,
Pythium caudatum, and Harposporium leptospira.
Catenaria
anguillulae produces zoospores which track down nematodes by
swarming, eventually encyst near nematode body orifice (e.g. anus, buccal
cavity), penetrate and colonize the prey. The encysted zoospores produce germ
tube which penetrate nematode through orifice or by dissolving cuticle. The
infectious hyphae grow well inside nematode body, digest content and lyse the
prey. Zoosporangia are produced inside body from which numerous uniflagellated
zoospores are liberated.
Meristacrum asterospermum forms adhesive conidia which attach to the cuticle of nematode. It germinates to form the hypha and acts as infectious thallus. Similarly, adhesive spores are also produced by species of Meria, Cephalosporium and Verticillium.
The conidia attach to
nematode body, germinate and penetrate through cuticle. After penetration, an
infectious hypha in body cavity of nematode is formed, the amount of which
increases eventually with the result of nematode death.
5) Egg
Parasites:
The fungi that parasitize the nonmotile stages of nematodes, i.e. eggs, use a different strategy. Hyphae of Phythophthora chlamydospora and other fungi grow towards the eggs and penetrate the eggshell. The fungi then digest the contents of the egg, both immature and mature (containing juveniles) eggs.
Biological interaction
Nematophagous fungi and nematodes share a special predator-prey relationship. The nematodes secrete chemicals as they travel through the soil. Fungi that prey upon these nematodes detect and respond to the existence of these chemicals. Nematodes produce a highly conserved family of small molecules called ascarosides/nemins. These small molecules are unique to the nematodes and the fungi are able to detect them when nematodes come close. Different species of nematode produce different varieties of ascarosides. The fungus reacts to the ascarosides by constructing traps for the nematode.
Also, when the fungus is nutrient
deprived, and is in need of nutrients not readily available, it will create
traps in the area in which it detected the chemicals, thus effectively hunting
its prey.
Benefits
Having nematophagous fungi in the soil confers many benefits both environmentally and economically. Nematodes can affect both plants and animals. They cause several diseases through the use of their feeding stylet as well as some species of the worm living within the host.
Plant parasitic nematodes feed through a needle-like mouthpiece called a stylet. This mouthpiece is injected into the host and the nematode ingests the nutrients of its host. This feeding action deprives the host of nutrients and also creates wounds that can be entrypoints for invading microbes to colonize the plant and spread disease.
The feeding action of nematodes can
also spread disease by itself. By feeding from an infected host then moving to
an uninfected host, the nematode can directly inject the disease into the new
host.
By consuming nematodes, the fungi
creates biomass that can be consumed by other organisms that are higher up the
food chain. Having this microbial interaction promotes the cycling of nutrients
and diversifies the food web.
Nematophagous fungi are found in almost every natural soil and
play an important role as antagonists of nematodes. This interaction helps to
control the nematode population in check. Therefore there is great interest in
research of possibilities to use them in biological plant protection.
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