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B. Fungal/ Insect Symbiosis & Commensalism

1. Tricomycetes

2. Ambrosia fungi


There is limited information about fungal/animal symbiosis in most mycology textbooks. One of the compelling reasons that animals need a symbiotic relationship with fungi is that animals cannot digest cellulose. Hey, horses and cows eat plants and seem to get by! But, they have bacteria or other organisms in their digestive system that can do the cellulose digestion.

1. Trichomycetes:

A commensal is a companion at meals. In biology, the term commensalism means a close association or union between two organisms in which one is benefited by the relationship and the other is neither benefited nor harmed. Some biologists view this as a form of symbiosis. Trichomycetes are the most striking example of commensalism among the fungi. They are associated with living arthropods such as insects, millipedes, and crustaceans, growing extensively in the hind gut (Fig.  14-81). Most all species grow internally within the hindgut of their host where they attach to the chitinous lining by means of a holdfast (Fig. 14-82)

Fig.  14-81. Highly branched filaments of Asellaria ligiae.

Fig. 14-82. The foot cell (or holdfast) of Asellaria ligiae.

The holdfast may be looked upon as somewhat a “sucker disc” that attaches the filamentous fungi to the hindgut. The mycelium apparently absorbs its nutrients from the contents of the hindgut. Dr. Robert W. Lictwardt of the University of Kansas is the world authority of this group of fungi. He and his colleagues have described a number or orders of Trichomycetes based upon the septation and branching of the filaments and on the types of spores they produce. Asexual reproduction may be by amoeboid cells, sporangiospores, or spores produced on a threadlike trichome (Fig. 14-83)



Fig. 14-83. Filaments of Smittium culicis showing attached trichospores.


That is why the spores are called trichospores and why this group of fungi is called the Trichomycetes. It has been suggested that the trichospores are instrumental in dissemination of the fungus, especially since insects molt and shed their exoskeleton and hind gut. Sexual reproduction has been found only in a few species of Trichomycetes such as species of Harpella where biconic spores, believed to by zygospores, are formed on the filaments.

Some of the Trichomycetes, such as species of Smittium that occur on mosquitoes, have received considerable attention. High populations of Smittium on mosquitoes have been shown to have a negative impact on mosquito colonies and thus might serve as a type of biological control. For those who want to learn more about this fascinating group of fungi they can refer to Lichwardt’s book (The Trichomycetes, Fungal Associates of Arthropods, Springer).

2. Ambrosia Fungi

Many fungi are known to be associated with ambrosia beetles. Most of them are conidial (asexual) states of Ascomycetes. Ambrosia means “food of the gods” and is commonly a name given to a fruit salad with shredded coconut and fruits. Ambrosia beetles are those beetles that produce piles of shredded wood and debris outside their galleries. There are several fungi that grow within this debris, the ambrosia fungi, and have been shown to have a mutualistic relationship with ambrosia beetles. The bark beetle that is currently devastating populations of southern yellow pines is one of the ambrosia beetles.  Batra (1979. Insect-Fungus Symbiosis. John Wiley & Sons, N.Y.276 pp.) list the following examples of ambrosia fungi: Absidia, Ambrosiella, Aspergillus, Cephalosporium, Ceratocystis, Chaetomium, Endomyces, Endomycopsis, Fusarium, Monilia, Penicillium, and Tuberculariella.

It has been shown that certain ambrosia fungi are required for reproduction of the beetle (French & Roper, 1972. Can. Entomol. 104:1635-1641, 1972). They influence the hatching of eggs, provide food for the developing larvae, and prevent the encroachment of pathogenic fungi.  The insect in turn provides plant debris and insect excrement as a food source for the fungi, and eventually is instrumental in disseminating the fungus. Many of these beetles have specialized compartments within their digestive system called mycangia. which hold fungal spores for later excretion whenever the insect arrives at a new site.  For illustrations of ambrosia fungi see File III, B,1.