Symbiosis With Fungi

Abstract

Abstract:Associations between termites and fungi are considered in two categories: firstly, interactions that affect the discovery and consumption of food or its nutrient value, but which fall short of mutualism; secondly, the cultivation of fungus-combs (Termitomyces spp.) within the nesting system by Macrotermitinae, where there is an obligate reliance on the fungus and which in most cases appears to contribute to the degradation of plant structural compounds, especially xylans. In the first category most information is available for pest termites and it is possible to show, in different species or circumstances, roles for fungal colonizers of decaying wood (or for their metabolic products) as attractants, repellents, conditioners, toxins, and nutritional supplements. Some entomophagous fungi are pathogenic to termites and may be potential agents of biological control, despite low host specificities. In the second category of association, the structures and dynamics of Macrotermitine fungus-combs are classified and data presented to illustrate the variety of net enzymatic synergisms (i.e. [mal hydrolytic activities and specificities in the termite gut) achieved by the mutualism in different host species and feeding groups. To reconcile the conflicting evidence on possible enzyme acquisition from the fungus by the termite, it is proposed that symbiosis in the Macrotermitinae shows evolutionary diversification, with some species of fungus producing a range of enzymes according to substrate and enviromnental conditions (generalists), others with a fixed range of enzymes not responsive to substrate (constitutive specialists) and yet others which produce little, if any, enzyme for termite use. The assay of enzyme productions in pure cultures of Termitomyces, and the host specificities of different species of the fungal partner are also reviewed.

The evolution of fungus-growing termites and their mutualistic fungal symbionts

“…The essentially ''symmetrical'' symbiosis with Termitomyces fungi allowed termites to occupy new food niches, and these diverging niches selected for fungal adaptations to combs built from different plant-derived materials. The roles of the fungal symbionts for termite nutrition therefore may differ across genera, as present data indicate (15). In contrast, the symbiosis between attine ants and fungi remained ''asymmetrical'' in most of the basal lineages, i.e., ants were obligatory dependent on fungal symbionts that were not necessarily obligatory dependent on them.…”

“…The fungi cultivated by the fungusgrowing termites also form a monophyletic group together with seven described morphospecies of the genus Termitomyces. The fruiting bodies of Termitomyces are always associated with termite nests (8,15,32,33), which implies that within the basidiomycetes there is a single evolutionary origin of mutualistic symbiosis with termites and that there are no known reversals to a nonsymbiotic lifestyle. These results are consistent with earlier findings (18,19,33) based on much less-complete data sets.…”

“…In most of the macrotermitine termites, on the other hand, associations between fungi and termites arise de novo in every generation such that active or passive selection of mutualistic partners must be the primary force inducing specificity. Once irreversibly committed to symbiosis with termites, Termitomyces clades apparently have evolved specific adaptations to the comb substrates built by the specific clades of termites (15), and the termites may have evolved active selection of specific symbiont clades. However, both niche specialization by the fungi and symbiont selection by the termites will be constrained by availability of comb substrates and fungal strains in the environment, and thus less-specific associations are likely to arise frequently.…”

“…The fungus helps the termites to degrade the plant-derived material (e.g., wood, dry grass, and leaf litter) on which they live (8). It grows on a special structure in the nest, the fungus comb, maintained by the termites through continuous addition of predigested plant substrate while the older comb material is consumed (15).…”

Insect‐mediated nitrogen dynamics in decomposing wood

“…The degree of N limitation faced by wood-feeding termites varies widely depending on the feeding habits of the particular groups. Subterranean termites (Rhinotermitidae) feeding in wood on the forest floor benefit from the high N contents of fungal tissues (Rouland-Lefèvre, 2000), for example, whereas dry-wood termites (Kalotermitidae) feeding above ground on dead limbs face more severe limitations. Regardless of feeding habit, termites tightly conserve nutrients within their colonies, owing to their habits of proctodeal trophallaxis and cannibalism (Wood, 1976;Lee, 1983).…”