Rafael Rodrigues da Costa:
The Physiology of Microbial Symbionts in Fungus- Farming Termites

Date: 15-09-2017    Supervisor: Michael Thomas-Poulsen




Approximately 30 MYA, a major evolutionary event took place when the subfamily Macrotermitinae engaged in a mutualistic relationship with the basidiomycete fungus Termitomyces. The association with Termitomyces is considered to have had major impacts on the way of living of these termites. The termites provide the fungus with optimal growth conditions (e.g., stable temperature and humidity), as well as with constant inoculation of growth substrate and protection against alien fungi. In reward, the fungus provides the termites with a protein-rich fungal biomass based diet. In addition to the Termitomyces, fungus-growing termites also maintain a symbiosis with gut bacteria, which seems to differ from to those of termites species. It is believed that this multipartite symbiosis is the key to the ecological success these termites achieved in subtropical areas in the Old World. These insects along with their symbionts are main decomposer of organic matter in Africa, and this is reflect of a metabolic complementarity to decompose plant biomass in the genome of the three organisms involved in this symbiosis. Many of the physiological aspects of this symbiosis remain obscure, and here I focus on physiology of microbial symbionts associated with fungus-growing termites. Firstly, by using a set of enzyme assays, plant biomass compositional analyses, and RNA sequencing we gained deeper understanding on what enzymes are produced and active at different times of the decomposition process. Our results show that enzyme activities and diversity differ at throughout the downwards plant biomass breakdown, as well as we indicate variance in enzyme expression across termite species. This could indicate specific metabolic adaptations in the microbial symbionts, giving that termite species collect different plant substrates. In addition, we demonstrated that this symbiosis is able to decompose and utilize a wide-range of complex plant components, thus signalizing the efficacy of this association. Our findings corroborate the “ruminant hypothesis” which suggests that the plant biomass-degradingenzymes could be complemented by the transport of those through the gut of termite workers. This is suggested a boost in the decomposition of plant substrate when it is incorporated to the fungus comb. The adoption of a fungiculture serving as a food source by fungus-growing termites led to shift in the gut bacteria community composition. With the prediction that a protein-rich fungal diet could directly reflect the functional roles of these gut symbionts, we analyzed CAZymes in the gut metagenomes of two species of fungus-growing termites and compared them to seven other gut metagenomes from termites with different diets. We indicate that the acquisition of a fungiculture did not only shift the composition of the gut microbiota community but also affected the roles of these gut symbionts. We demonstrated enrichment of GH families encoding fungal-cell-wall-degrading enzymes in the gut bacteria associated with fungusgrowing, in contrast to higher termites where this is either absent or minimal, and this indicates evolutionary adaptations to dietary intakes displayed by different termite species. In addition to these two physiological aspects of this tripartite symbiosis, we questioned the physiological mechanisms displayed by Termitomyces to explain the interaction specificity between the termite host and the fungal partner. Based on the fact that fungus-growing termites and Termitomyces have congruent phylogenies, where certain fungi will always be associated with specific termite genera or species, we used growth assays to better understand if Termitomyces’ growth capacities could explain interaction specificity of this relationship. Our results demonstrated that the growth patterns of Termitomyces symbionts fit the phylogenetic placement of these fungi, indicating that metabolic capacities displayed by the fungus symbiont could govern the degree of interaction specificity across different termite genera and species. We therefore suggest that this capacity could be complemented by the provision of specific plant substrate to Termitomyces by the termite host.