Every known group of coelomate animal hosts a massive diversity of bacteria in their gut. These two extremely dissimilar and distantly related groups of organisms have coevolved to develop what is now known to be an immensely complex symbiotic and evolutionarily adaptive relationship that is changing our understanding of animal physiology. These complex intestinal microbial communities are now collectively referred to as the gut microbiota, and the interactions between these communities and their hosts have become a particularly active area of study over the last decade. My thesis work investigated two projects:
- The bacterial gut microbiota of many animals is known to be important for many physiological functions including detoxification. The selective pressures imposed on insects by exposure to toxins may also be selective pressures on their symbiotic bacteria, who thus may contribute to the mechanism of toxin tolerance for the insect. Amatoxins are a class of cyclopeptide mushroom toxins that primarily act by binding to RNA polymerase II and inhibiting transcription. Several species of mycophagous Drosophila are tolerant to amatoxins found in mushrooms of the genus Amanita, despite these toxins being lethal to most other known eukaryotes. These species can tolerate amatoxins in natural concentrations to utilize toxic mushrooms as larval hosts, but the mechanism by which these species are tolerant remains unknown. Previous data has shown that a local population of D. tripunctata exhibits significant genetic variation in toxin tolerance. This study assesses the potential role of the microbiome in α-amanitin tolerance in six wild-derived strains of Drosophila tripunctata. Normal and antibiotic-treated samples of six strains were reared on diets with and without α-amanitin, and then scored for survival from the larval stage to adulthood and for development time to pupation. Our results show that a substantial reduction in bacterial load does not influence toxin tolerance in this system, while confirming genotype and toxin-specific effects on survival are independent of the microbiome composition. Thus, we conclude that this adaptation to exploit toxic mushrooms as a host is likely intrinsic to the fly’s genome and not a property of their microbiome.
- There is now a growing body of evidence suggesting that the gut microbiota have influence over complex physiological processes in their hosts. The Drosophila microbiome is known to play a significant role in metabolism, physiology, immune function, and mating behavior. However, little is known regarding the molecular and cellular mechanisms involved in host-microbiota interactions. Olfactory receptors and their complimentary odorant binding proteins (OBPs) have been implicated in several microbiome-influenced pathways. OBPs are most commonly associated with their role in olfaction, but they are also known to be ectopically expressed in the gut. Their specific purpose in olfaction is to transport odorant molecules to olfactory receptors in the sensilla, while their function in the gut is unknown. This preliminary study investigates the effect of the gut microbiota on the expression of several OBPs in the gut by rearing both normal and axenic flies and comparing their relative expression of OBPs by quantitative PCR analysis. Downregulation of OBPs by axenic flies could suggest an alternative function for OBPs as a signaling mechanism between Drosophila melanogaster and its microbiome.