Microbial symbionts play key roles in shaping the diversity of life, from disease causing pathogens to beneficial microbes. The evolutionary dynamics of microbial interactions are understood best within single species or between pairs of interacting species; however, further multi-species studies are necessary to understand how these dynamics change within a broader community of symbionts. The fungus-growing ant (Apterostigma dentigerum) is ideal for addressing such questions, as it maintains multiple symbionts, including fungal cultivars, cultivar-attacking pathogens, and mutualistic bacteria whose antibiotic secretions combat infection. Preliminary work has shown that cultivars are not specialized to combat infection by their most common pathogen species; however, the bacteria are likely filling this role. This study will generate population genetic structure data for A. dentigerum and, coupled with bioassay experiments, seeks to understand how host/symbiont transmission and gene flow impact the evolution of specialization within a complex multi-species symbiosis.
This study applies long-standing concepts in population genetics and coevolutionary theory within a more complex framework. Because the proposed work speaks directly to the evolutionary dynamics of pathogen resistance and the maintenance of antibiotic potency, the results will have broad health implications. This study will rely on the assistance of undergraduates (including underrepresented minorities), who will become trained in a variety of molecular genetic and microbiological techniques. Finally, the results of this study will be incorporated into the researchers' microbial ecology and evolution course, in addition to annual K-12 public outreach events attended by the researchers.
With this award, we were able to study the complex relationships that exist between fungus-growing ants (Attini), Basidiomycetous fungi, which the ants cultivate for food, Ascomycetous fungal pathogens (Escovopsis), which parasitize cultivars, and Actinobacteria, which produce antibiotic compounds that suppress pathogen growth. For this research, we traveled across Central America collecting colonies of the fungus-growing ant Apterostigma dentigerum in order to isolate and study their bacterial symbionts, the complicated interactions between them, and how these interactions may shape the evolution of each member in the system. Among others, one of the main questions we wanted to answer was: Are populations of A. dentigerum-associated Pseudonocardia geographically structured within Central America? In order to answer this question, colonies of A. dentigerum ants were collected from one site in Costa Rica and seven sites in Panama. Samples of the symbiotic Actinobacteria were also taken by swabbing individual ants where the bacteria are known to be localized. These bacteria were then grown in pure culture and their DNA extracted and analyzed. Help with DNA sequencing was provided by an undergraduate researcher, who gained valuable hands-on laboratory experience through this project. In order to discern differences between populations of bacteria taken from geographically distinct ant colonies, DNA sequence information from six different genes was obtained from each of the collected bacterial isolates and run through a series of comparative analyses. Previous studies have suggested that the relationship between fungus-growing ants and their associated Pseudonocardia bacteria is much less specialized than previously thought, and may not be a mutualistic relationship at all. After analyzing our genetic data from many colonies across Central America, we saw the opposite. Overall, the population dynamics of the symbiotic Pseudonocardia we sampled were more consistent with a specialized mutualistic association than with the proposed models of low specificity and frequent horizontal acquisition. Discerning how individuals within a species are structured over geographic space is essential to understanding how populations evolve. Assessing the impact of population genetic structure is equally critical to understanding how interacting species coevolve. With this research, we were able to show a distinct mutualistic relationship between the fungus-growing ants Apterostigma dentigerum and their symbiotic bacteria, Pseudonocardia through genetic analyses. Our findings have helped identify local populations and define the geographic scale at which attine-microbe dynamics occur, which will benefit future studies seeking to find similar relationships.
