Termitomyces fungus combs - formation, structure, and functional aspects

Many insects that rely on food sources with low nutritional content establish obligate symbioses with prokaryotic and/or eukaryotic microorganisms. The gut symbionts of termites including both bacteria and protists which allow the insects to feed on dead wood and plant litter. Fungus-growing termites have evolved an additional symbiosis with fungi of the genus Termitomyces. Their fungal symbionts are housed in specialized fungus combs located within nest chambers of termite mounds. In the fungus combs, the symbiotic fungi degrade cell wall polysaccharides and lignin of foraged plant matter and enrich nitrogen for the benefit of the host colonies. As a reciprocal service, the insects provide the fungi a continuous supply of food and an optimized growth environment. In this chapter, we review what is presently known about the physical and biochemical features and functional aspects of Termitomyces fungus combs. While the focus is on the termite genus Macrotermes, enhancement of food quality within fungus combs is shared across the whole subfamily Macrotermitinae. We conclude that fungus combs represent the central innovation that has allowed fungus-growing termites to radiate into a wide variety of semi-arid and arid ecosystems in sub-Saharan Africa. Fungal symbionts degrade lignocellulose of harvested plant material and the gained energy and nutrients are utilized by the whole symbiotic consortium. In addition, fungus combs allocate nitrogen to fungal nodules which are then specifically supplied to support egg production and larval growth. While recent studies on termite symbioses have already increased our understanding tremendously, many fascinating aspects in these complex biological systems remain obscure and require more research.