FUNGI & INSECT ASSOCIATIONS

Endosymbiosis

Many arthropods contain specific microorganisms, usually bacteria and yeasts, which are present in every individual and transmitted from one generation to the next by various elaborate mechanisms. These endosymbionts, which were first studied in detail by Buchner ( 1965) and his students, occur in at least some members of several insect orders, including Blattodea, Isoptera (Mastotermitidae), Orthoptera, Phthiraptera, Hemiptera (all Sternorrhyncha, most Auchenorrhyncha, Peloridiidae, Cimicidae, Reduviidae, Lygaeidae, Pentatomidae, etc.), Diptera (Glossinidae, Pupipara), Hymenoptera (Formicidae) and Colcupteru (Lucanidae, Passalidae, Scarabaeidae, Nosodendridae, Bostrichidae. Anobiidae, Silvaniduc, Tenebrionidae, Cerambycidae, Chrysomelidae and Curculionidae). Endosymbionts may occur intracellularly in gut epithelium, Malpighian tubules, or fat body, or they may be located in special fermentation chambers, gastric caecae or crypts. In some cases the microorganisms are confined to special cells or mycetocytes and these may be grouped to form organs called myce[omes.

The identity of the endosymbionts is often difficult to determine, since many cannot be cultured, but they may be bacteria (actinomycetes or rickettsias) rather than true fungi. Those of Anobiidae and Cerambycidae are anamorphic yeasts (Fungi Anamorphici: Saccharomycetales); those of the former are species of Candida (formerly as Torulopsis in some publications). Many Coccoidea (Hemiptera) also have yeast-like endosymbionts. The associations of insects with their endosymbionts is generally considered to be mutualistic, because of the constant occurrence of the micro-organisms and the various methods which have evolved to ensure their transmission to the next generation. One method of inoculating eggs is by the transference of symbiotic micro-organisms to special pouches at the base of the ovipositor and their subsequent transmission to the egg. Another method involves an invasion of the ovarioles, with the symbionts passing from the nurse cells to the oocytes. The exact nature of the contribution of endosymbionts to the well-being of the host insect is often not clearly understood. Cellulose digestion has been ruled out in most cases. Kukor et al. (1988) demonstrated that cellulose digestion in Cerambycidae is dependent on the ingestion of fungal enzymes and suggested that the same was probably true for Anobiidae and Buprestidue. Jurzitza (1979) showed that endosymbionts of Anobiidae can supply vitamins, a sterol and essential amino acids to their hosts, but that their main function is in the recycling of excreted nitrogen and the adaptation of plant proteins to the demands of the host

(Crowson, 1981; Dolling. 1991; Houk & Gritfiths, 1980; Koch. 1967; Trembiay, 1977; Wigglesworth. 1972).

DUNG BEETLES

Scarabaeiformia

The larvae of many Searabaeoidca (including Lucanidae, Passalidae and the scarabaeid subfamilies Dynastinae and Cetoniintte) feed on rotten wood, and others are commonly associated with other kinds of rotting vegetation and herbivore dung. These beetles certainly depend to some extant on fungal and bacterial breakdown and may have hind gut fermenation chambers and bacterial endosymbionts (Buchner, 1965). However, little information is available on possible mycosymbionts in the group.

. Two scarabaeines appear to be obligate fungivores, in that adults construct brood balls from mushrooms, and larvae are not only capable of developing on this food type but apparently do not survive on dung. These are the Australian Oruholrhagus dunningi Harold (Bornemissza, 1971) and the Mexican Phanaeus halffterorum Edmonds (1979).

Termites feed primarily on wood, but also on a variety of other plant materials. They can break down cellulose using mid gut cellulases supplemented, in all families except Termitidae, by enzymes produced by symbiotic flagellate protozoa in the hind gut. Moreover, many are attracted to wood which has been attacked by certain fungi and some exhibit an increased viability when fed on such wood. This may be due to the greater nutritional value of the partly decayed wood, the breakdown of a toxic substance in the wood by the fungi, or a combination of both. One of the commoner species of wood-rotting fungi eliciting positive responses from various termites is Gloeophyllum trabeum  (Pers. : Fr.) Murrill, which produces a cubical brown rot. However. Gilbertson (1984) has shown that both white rot and brown rot fungi may he involved, and some fungus species may be repellent to termites.

Fungus-growing termites

The associations between species of the busidiomycete genus Termiromyces (Agaricales: Amanitaceae) and various members of the termite subfamily Macrolermitinae (Isoptera: Termitidae) are mutualistic, involving the construction and maintenance of a fungus comb. Thus, termite faecal material serves as a food source for the fungal hyphae and a surface area for the production of white bodies or 'mycotetes' on which the termites feed. The Macrotermitinae are dominant termites in the savannas and forests of tropical Africa and the Indo-Malaysian region, where they are general detritivores, playing a major role in litter removal. Like other members of the Termitidae, they lack intestinal protozoa and rely on their own digestive enzymes, as well as other microorganisms in the gut. Termitumyces is a genus of Amanitaceae (Agaricales), which reproduces asexually by means of mycotetes, consisting of sphaerocysts bearing conidia of blastosporic ontogeny, and sexually by means of small, ailled basidiomata, which may either penetrate the soil or walls of the termite mounds or, sometimes, arise from comb fragments brought to the surface by termite workers. Ascomycete fungi of the genus Xylaria may also occur as hyphae in fungal combs and were forrnerly thought to represent the symbiotic fungus. However, they appear to be obligate saprotrophs which become active only when combs die or are removed from the nest. Termites can control the microclimate of the nest and suppress the growth of competing fungi, including Xylaria, by means of oral secretions

Termites evolved from cockroach-like ancestors that fed on decaying plant material. and throughout their evolution fungi have been involved in their diet. Dead plant material has a high carbon:nitrogen ratio and is difficult to digest. Termites accomplish this digestion through the combined enzyme system in their gut and gut symbionts. The ability to exploit dead plant material is limited by the number of symbionts that can be accommodated in the gut. Macrotermitinae have bypassed this limitation by cultivating Termitomyces outside the gut. The fungus is supplied with large quantities of comminuted food through which hyphue can rapidly spread, in a favourable microclimate in which competitors are suppressed. Termitomyces degrades lignin and cellulose, the termites re-ingest the comb along with the nitrogen rich mycotetes and this food is further degraded in the termite's gut by acquired enzymes produced by the fungus

 (Batra ~ Batra, 1979; Martin, 194 r; Martin & Martin, ¹9^-N, Sands. 1969 Wood & Thomas. 1989j.

Fungus-growing ants

The tribe Attini (Formicidae: Myrmicinac) includes 12 genera and almost 200 species restricted to the Nearctic and Neotropical regions.They are unique among ants in their obligate dependence on symbiotic fungi as a source of larval food. The more primative attines cultivate their fungal symbiont on insect faeces, dead insects and dead plant matter, whilst members of the most derived genera Acromyrmex and Atta (known as leaf cutter ants) use living plant material as fungal substrata. The latter produce extremely large, polymorphic colonies and are the dominant exploiters of living vegetation in tropical America. The fungus gardens of Atta consist of many small pieces of' substratum held together by dense mycelial growth. Worker ants provide the substratum by cutting leaves and other plant parts & chewing them to form a pulpy mass. The fungal symbiont produces peculiar hyphae with swollen tips called staphylae, which are cropped by worker ants, deposited in their infrabuccal pocket and either fed to the larvae or passed into the crop and mid gut. Workers rely on the staphylae for about 5%of their energy requirement, the balance coming from

plant sap ingested while cutting leaves. Martin (1987) has shown that ingested fungal enzymes also play a role in the symbiosis, in that workers reinoculate the fungus garden with a concentration of enzymes in their liquid faeces, and thus promote more rapid growth of the fungus. Attines are highly polyphagous, utilising a wide variety of plant species, and Cherrett et al. (1989) argued that the main advantage of this mutualism is to permit the two organisms, each with very different morphology and physiology, to cooperate in breaking down the physical and chemical plant defences which would form effective barriers to either organism acting alone.

All above extracts are from “Fungi of Australia vol  1 & 2”