Microbes affect many aspects of daily human life, from health to agriculture. Despite such large influence, understanding of the ecology of microbes and their interactions is still in the early stages. This project will advance understanding of microbial interactions through study of one microbe, the eukaryotic social amoeba Dictyostelium discoideum, and its primitive agricultural association with bacteria. About one third of wild-collected Dictyostelium clones have the beneficial symbiotic behaviour of farming to facilitate future growth, and study of the variable symbiosis between Dictyostelium and bacteria can advance understanding of the ecology and evolution of microbial interactions. This project will apply the tools of experimental evolution and a suite of cellular and molecular analyses to this newly discovered novel symbiosis between a social amoeba and bacteria to elucidate the origins and maintenance of cooperative lifestyles.
The project also will foster integration of research and education to train undergraduates. Two female undergrads will be mentored and trained in field collection and cellular and molecular techniques and will develop substantive research projects. Undergraduate research assistants will participate in a bi-weekly seminar about the research questions addressed in the lab and their relevance to science and society. Additionally, the PIs will offer a freshman seminar to engage students' interest in and understanding of biological research.
Evolutionary costs and benefits of a newly discovered symbiosis between the social amoeba Dictyostelium and bacteria Microbes are everywhere and impact many aspects of daily human life, from health to agriculture. Interactions among microbes can impact whether they hurt or help humans, so this is a particularly important topic. Despite its importance, understanding how microbes interact with each other is still in its early stages. Our work focuses on a particular microbe that has a solitary and a social stage. This microbe is called Dictyostelium discoideum. It is a eukaryote, meaning it has a nucleus and mitochondria, and is in the same kingdom as humans. It shares at least 2000 genes with humans. Dictyostelium discoideum is also called a social amoeba, for the social stage. Most of the time these amoebas live alone and eat bacteria. Every few hours one amoeba divides by binary fission into two amoebas. During this time they sense the bacteria in their environment, expertly moving towards areas where bacteria are dense, or away from areas where they are scarce. Ultimately, however, they run out of their bacterial food and begin to starve. The starving amoebas have a special way of surviving starvation. They aggregate into a mound of tens of thousands of amoebas and turn themselves into a cooperative, motile, multicellular slug that crawls much more quickly than any one amoeba could. This slug moves towards heat and light. Ultimately, it forms into a fruiting body of 20% dead cells and 80% hardy spores that can last a long time without eating. Our research focused on an exciting new finding for this social stage. We found that some clones of social amoeba carry bacteria with them through the social stage. Before our work, it was thought that the social stage was entirely sterile, containing only pure amoeba cells, no bacteria. We discovered that about 33% of wild-caught social amoebas are farmers. They carry a small amount of bacteria with them as a kind of lunch kit through the social stage. This means that when the spores ultimately hatch again, the bacteria are released, and begin to eat nutrients and multiply. Then the social amoebas eat the bacteria, but are prudent predators that stop eating before wiping out all the bacteria, so some remain for their lunch kits. The symbiosis between Dictyostelium and bacteria make it an ideal system to study how microbes have evolved to both use and help each other. Using this newly discovered novel symbiosis between a eukaryote social amoeba and bacteria, we can discover much about an important element of the microbial world, cooperative life. We discovered that the amoebas that carry bacteria with them have a better food source than those that have to rely on random bacteria found in the soil. To document this, we showed that farmers produced more spores than non-farmers in two different soil locations. Clearly, relying on wild bacteria is not as good as carrying bacteria for later use. This is an advantage for the amoebas. For a mutualism to evolve, an advantage for both parties is necessary. Preliminary results indicate that the bacteria grow better in the presence of the amoebas. Another advantage for the bacteria is that they are transported to new locations along with the spores. This work promoted the integration of undergraduates in the lab, blending research and education. We mentored three female undergraduates, training them in field collection as well as an array of cellular and molecular techniques. These students presented their work at the Rice Undergraduate Research Symposium in the spring of 2010 and the spring of 2011. We also conducted an undergraduate bi-weekly seminar for a group of eight undergraduates working in our lab to educate them about the questions we are addressing and their relevance to science and society. We presented on this work at two meetings: The International Dictyostelium Conference and the Human Microbiome Meeting. Attending these meetings allowed Debbie Brock to develop contacts with the goal of establishing future collaborations. Debbie Brock also presented this work for invited lectures at St. Edward’s University, Austin, Texas and Washington University, St. Louis, Missouri. She attended the Quantitative Fluorescence Microscopy Course at Mount Desert Island, Maine and received training in advanced fluorescent microscopy. This training will be of great use for future work on this fascinating system.
