The ants are a diverse and abundant group of insects, numbering in excess of 12,000 species and comprising a greater biomass than most land-dwelling animals. When combined with their roles as predators, scavengers, and defenders of plants and insects, it is clear that these organisms are among the more influential animals found within terrestrial ecosystems. Although they are often thought of as carnivores, the ants have evolved to utilize a wide range of diets. Notably, several groups have come to feed at the base of the food chain, relying on low-quality, herbivorous diets that provide insufficient nutritional sustenance. While it is not known just how these insects can persist on such diets, symbiotic bacteria are hypothesized to play a central role. This research project will test this hypothesis, and investigate the roles of nutritional gut bacteria in the evolution of ant diets. Using molecular phylogenetics, the evolutionary relationships among ants from several geographic locations will be determined. Stable isotopes will be used to characterize the feeding positions of these species on the food chain, and DNA sequencing will be used to determine composition of their bacterial gut communities. These results will be combined to test whether ants feeding on low-quality diets have co-evolved with specialized, beneficial gut bacteria, and whether habitat disturbance or introductions of invasive ants have changed these dynamics. This synthetic approach will not only reveal features shaping bacterial communities from animal hosts, it will also determine whether symbiotic bacteria have played important roles in the evolution of ant diets, enabling the origins of novel lifestyles and proliferation within otherwise inhospitable habitats.
By examining native and invasive species from pristine and disturbed habitats, this work will reveal how environmental changes can alter interactions between animals and their gut microbes. Such work is broadly important to understanding the adaptability of animal species, for example, in dealing with climate change. This project will support the training of a postdoctoral scholar, graduate and undergraduates in interdisciplinary research, and provide opportunities for public involvement in the research through the Chicago Field Museum of Natural History.
Microbial symbionts are widespread across animals, playing integral roles in their evolution. Yet many ecologically important groups are under-explored from a perspective of microbial symbiosis. Our research was motivated by the lack of knowledge on the importance of bacterial symbionts across ants and by a striking pattern that identified common associations between ant-specific bacteria and several unrelated herbivorous ants. Our discoveries have revealed that related ants with similar diets harbor similar symbiotic communities of bacteria. In fact, ants at the extreme ends of the food chain appear to have separately acquired similar symbionts, which may play important roles in their use of these diets. These bacteria are from fairly ancient, ant-specific lineages, and can be highly prevalent if not ubiquitous within these ant herbivore or ant predator groups, suggesting very long-standing relationships. Importantly, a majority of surveyed ant species do not appear to engage in such symbioses. Instead these ants often harbor small quantities of bacteria, which typically do not come from ancient, ant-specialized lineages. Thus, "hot spots" for pervasive and specialized symbioses with gut associated bacteria may be an exception to a more general norm for ants—to harbor few symbiotic gut bacteria. The reasons for these patterns are completely unknown, and suggest the importance of future research on the interplay between ant immunity, antimicrobial compound production, diet, and the anatomical and physiological investment in defense and symbiosis. Anoter major finding from our research is the discovery that herbivorous ants benefit from nitrogen recycling bacteria in their guts, which can convert waste products (like those from bird droppings or urine) into amino acids that are then provided to the ants. Our analyses of symbiont genomes indicate that multiple species take part in various stages of this process. The distributionts of related bacteria across relatives of our focal ant species (the turtle ant Cephalotes varians), and the presence of related bacteria across several unrelated herbivorous ants suggests a broad importance for nutritional symbioses in ant herbivory. In addition, our discovery that a recently derived ant herbivore has little investment in symbioses with nitrogen-provisioning microbes argues that symbionts may not be essential for the origins of herbivorous diets. Instead, symbiotic bacteria may be a subsequent key to the evolutionary success and diversification of herbivorous ants.
