The nematophagous fungi are of three main types on the basis of ecological habit: 1. Nematode-Trapping Fungi 2. Endoparasitic Fungi and 3. Egg Parasites.
Type # 1. Nematode-Trapping Fungi:
Fungi capturing nematodes are called nematode-trapping fungi. Such fungi have evolved structural adaptations to trap or penetrate their prey. They may be predatory or endoparasites. There are varieties of ways by which fungi trap nematodes resulting in their death.
These methods are discussed as below:
(a) Adhesive Hyphae:
The fungal hyphae form adhesive which capture nematodes. These hyphae produce adhesive at any point in response to nematode contact or the hyphae are coated with adhesive along their entire surface. At the point of hyphae where contact is made for capture, a thick and yellowish chemical material is secreted for example, Stylopage hadra.
Thereafter, an outgrowth of hyphae similar to appressorium develops. When the nematode is trapped, it becomes inactive first and killed in the last after penetration of hyphae. After penetration, elongate, unbranched absorptive hyphae grow along the nematode body and completely exploit the contents.
(b) Adhesive Branches:
The nematode trapping fungi produce the most primitive and simple organ of capture, the adhesive branches, which are a few cells in height. From the main prostrate hyphae short laterals grows as erect branches on or below the substrate. Over the whole surface of branch a thin film of adhesive material is coated. Examples of adhesive branch producing fungi are Dactylella cionopaga and D. gephyropaga (Fig. 28.5 A).
(c) Adhesive Nets:
Nets are formed by fungal hyphae which are adhesive in nature. Nets may be in the form of a single hoop-like loop (e.g. Arthrobotrys musiformis) to a complex multi-branched networks (e.g. A. oligospora). Upon observation with electron microscope it appears that the hyphae are coated with adhesive material.
As the nematode comes in contact of hyphae it is attached at many points resulting in penetration by infectious hyphae. Initially penetration is accompanied by the formation of infectious bulb which leads to form hyphae which grow inside nematode. Hyphal growth exploit nutrients and results death of the prey.
(d) Adhesive Knobs:
Morphologically a distinct adhesive cell, globose to sub-globose in structure, is produced at the apex of a slender non-adhesive stalk containing 1-3 cells. A thin film of adhesive material is produced over the surface of knob.
If a nematode is caught by a knob, soon it is attacked by several knobs with subsequent penetration. The immobilized nematode is destroyed thereafter. Examples of adhesive knob-producing fungi are Dactyleria Candida, Dactylella and Nematoctonus.
(e) Non-Constricting Rings:
From the prostrate creeping septate hyphae there arise erect and lateral branches which form non-constricting rings, initially the branch is slender but widens subsequently and being curved to form a circular structure. At the point where tip of branch makes contact with supporting stalk, cell walls get fused. Thus it results in formation of three-celled ring with a stalk. A nematode enters the ring and moves forward.
This results in marked constriction in cuticle. Generally rings are impossible to dislodge. During struggle the rings break from the weak point. Therefore, nematodes containing rings can move. Initially rings do not have any harmful effect on nematode but eventually nematode is penetrated and its body content is consumed. Dactylaria Candida and D. lysipage produce non-constricting rings, inspite of producing the adhesive knobs.
(f) Constricting Rings:
The constricting rings are produced similar to non-constricting rings but the supporting stalk is shorter and stouter (Fig. 28.5 B). In this case also a three celled ring is formed. It is a most sophisticated ring formed by predaceous fungi such as. Arthrobotrys anchonia, Dactylaria brachopaga and Dactylella which are abundant in soil.
The nematode is captured by garroting action of the ring cell. By swallowing the ring cell grasps the nematode in a single hold when a nematode enters into the ring, friction created by its body induces to swell the rings soon. The cells swell inwardly by three times greater than the original one within 1/10 of a second resulting in body of nematode deeply constricted.
Struggle between nematode and fungi goes on for a few minutes. Thereafter nematode becomes still and hyphae from ring cell penetrate the body and exploit the nutrients of nematode with the consequences of death. The most potential predatory fungi are the species of Dactylaria.
(g) Mechanism of Interaction:
The body of nematode consists of a low molecular weight peptide (or possibly a single amino acid) which is called nemin. Nemin is water soluble and potential stimulant for trap-formation. It causes morphological changes in nematophagous fungi. The process of nematode-fungus interaction is accomplished through a series of molecular events resulting in nematode death.
Pramer and Kuyama (1963) identified on the trap of A. oligospora the initiator of prey-predator recognition as lectin. The lectin of fungus binds especially to the sugar, N-acetyl-D-galactosamine, present on nematode cuticle. The lectin has also been purified. It is a protein of molecular weight 22,000 daltons.
The trapped nematodes secrete mucilage which has been identified under electron microscope. The nematode cuticle is lysed at the point where lectin combines with N-acetyl-D-glucosamine. Within an hour fungal hyphae penetrate the prey.
The enzyme collagenase is secreted by the fungus which dissolves collagen protein of nematode cuticle. The hyphae which have penetrated the nematode digest the body content and translocated to rest of the parts of hyphae for fungal growth and reproduction.
Type # 2. Endoparasitic Fungi:
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.
Catenaria anguillulae produces zoospores which track down nematodes by swarming, eventually encyst near nematode body orifice (e.g. anus, vulva and 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.
In addition, Meristacrum asterospermum forms adhesive conidia which attach to the cuticle of nematode. It germinates to form the hypha which swells and acts as infectious thallus.
Similarly, adhesive spores are also produced by species of Meria, Cephalosporium and Verticillium. In M. coniospora an adhesive bud develops at the distal end of tear-drop shaped spores. 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.
Type # 3. Egg Parasites:
There are a few saprophytic fungi which attack on nematode eggs. When a fungal hypha comes in contact of an egg, a swollen structure at terminal portion develops at the point of contact. It gets attached to the egg where from a narrow infectious tube develops that penetrates the shell of the egg.
After penetration, the infectious hyphae swell up and form a post- penetration bulb which looks like appressorium. From this structure there develops numerous irregularly branched absorption hyphae that consumes egg nutrients. Examples of egg parasites are Dactyllela oviparasitica and Paecilomyces lilacinus that penetrate root-knot or cyst nematode.