The ecology of fungus farming: Role of worker behaviour and microbes in maintaining weed-free fungal gardens in the colonies of Odontotermes obesus

Abstract

Fungus-growing termites are canonical examples of the evolution of agriculture in animal societies that have domesticated a fungus, Termitomyces sp. as their food crop. However, just like human agricultural fields, these crops are also prone to invasion by other weedy fungi (primarily Pseudoxylaria sp.). Secondary bacterial symbionts, with generic fungicidal capabilities, are assumed to help prevent weed invasions, but as these can also act against Termitomyces, the precise mechanisms of how these microbes are topically utilized remains unknown. Using natural fungal combs, in dual choice assays, with Odontotermes obesus workers, I show Pseudoxylaria hyphae are physically removed and encased with soil boluses accompanied by ‘cleaning’ of the comb. With the rise in the magnitude of Pseudoxylaria infection, the behaviour shifts to encasement of the entire infected combs with soil boluses. Both these behaviors keep the fresh comb unaffected. These soil boluses were found to have generic fungistatic properties and could inhibit both Pseudoxylaria and Termitomyces. The major source of this fungistatic property was found to be the microbes associated with termites. Using Nanopore platform, the microbiome and mycobiome of these boluses were identified and several microbial strains were isolated from them to reconstitute a partial bolus microbiome. This reconstituted microbiome also showed inhibitory effects against Pseudoxylaria. Thus, O. obesus can employ several behavioural strategies against natural Pseudoxylaria infections, and by only encasing infected combs with soil boluses, which contain fungistatic microbes, can also protect the crop fungus from the effects of these fungicides. This is the first demonstration of how fungus-farming insects use such fungistatic remedies, selectively, to keep their crop free of weeds. In addition to the threat posed by the weedy fungus Pseudoxylaria, fungus-growing termites also have to address the challenges posed by disease causing pathogens, primarily entomopathogenic fungi which are commonly found to be associated with the nests of termites in soil. In my thesis, I investigated the role of secondary bacterial mutualists in O. obesus against the two entomopathogenic fungi Metarhizium and Purpureocillium. In-vitro interaction assays using bacterial strains isolated from the nests of O. obesus, suggest that some of these have the potential to inhibit such entomopathogenic fungi and possibly may play a role in the prevention and control of such pathogens. In conclusion, my thesis provides an insight to the role of microbes in O. obesus in disease and weed management