Robert Murphy:
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Symbiosis with microorganisms is a globally ubiquitous and taxonomically widespread evolutionary innovation. Such symbiosis frequently takes the form of mutualisms where both partners gain a net benefit from the interactions. Nutritional mutualisms are a common form of symbiosis, often allowing host and symbiont to explore new niches by expanding the range of available nutrients the host can rely on. Microbial mutualists can also be major contributors of anti-pathogen, parasite and predator defence. Such functions are often mediated through the production of secondary metabolites, for which the microbe can be coopted for.
The fungusfarming termites subfamily (Macrotermitinae, Termitidae: Blattodea) engage in a tripartite obligate mutualism with a single genus of basidiomycete fungus, Termitomyces (Agaricales: Lyophyllaceae) and complex bacterial communities within the guts and externally maintained fungus gardens (combs). The termites provide constant plant biomass for the fungus that reside inside the dense nests with strictly controlled microenvironments. Lignocellulosic rich plant biomass is exceptionally recalcitrant presenting a significant barrier to digestion, yet the fungus-farming termite symbiosis efficiently utilise it.
Termitomyces, along with bacterial comb communities, fully degrades the plant biomass and Termitomyces acts as a nutritious food source for the termites. In return, the fungus benefits from a constant supply of growth substrate and optimal growth conditions. Despite the fungal crop being maintained in monoculture and the termites foraging in pathogen rich environments, this symbiosis demonstrates a robustness towards disease and infection. Many factors are thought to contribute to this effective defence of which secondary metabolite production by bacterial mutualists and Termitomyces themselves are thought to be one.
Collectively, this thesis demonstrates the vast potential of microbial symbionts to contribute to both nutrient acquisition and colony defence of the fungus-farming termite symbiosis and that the harsh internal microclimate likely contributes to the latter. Furthermore, we show that both Termitomyces and the bacterial symbionts harbour vast potential for novel compound discovery that should be further explored to understand its ecological relevance.