This project will elucidate the nature and basis of social interactions in a model organism, the amoeba Dictyostelium discoideum. These normally single-celled amoebas come together to form a multicellular fruiting body. Most individuals in the fruiting body reproduce through formation of spores, but 20% die to form a stalk that helps disperse the spores. There is both cooperation and conflict over which cells get to become spores. This project will advance understanding of what keeps this conflict from destroying cooperation. D. discoideum is uniquely suitable as a model system to study cooperation and conflict at the molecular level. Genome sequences of many individuals will be analyzed to test the hypothesis that conflicts cause strong selection that molds the molecular variation that underlies the diversity of living species.
Many molecular and genetic resources are available for D. discoideum, whose genome includes many genes similar to human genes and is small enough for whole-genome sequencing. The insights gained from this model system will help us understand cooperation and conflict in obvious social animals such as bees, wolves, and humans, and also in single-celled organisms, including many that cause disease. D. discoideum thrives in the lab and has a short generation time, creating diverse excellent opportunities for students at all levels to participate in research-based learning. This project will provide educational opportunities to high school students through direct collaboration with one high school and development of materials for use by others, to grade school teachers, and to university students through provision of web-accessible novel educational materials. Dissemination of results will include articles for the lay public and general scientific readership, as well as publications for teachers.
The point of this project was to use an organism with a simple form of group living so we could investigate the way that natural selection operates on social systems. We could just as well call this an investigation into multicellularity because when the simple amoebae we study come together they form a multicellular body. The big questions involve how conflict is controlled so cooperation can emerge from grouping. This kind of question is important for understanding many different situations in the living world. Why do birds flock? Why are termites and ants so successful? Why do bacteria form biofilms that are much better at overwhelming our defenses than single bacteria? Why do cancer cells stream to a new location in groups? Why do groups of grasshoppers behave so differently when they reach a critical density? In short what are the molecular and genetic consequences of grouping? In the study system we use, social amoebae, or Dictyostelium discoideum, individuals aggregate when they starve. Since they are not necessarily of the same genotype, there may be genetic conflict over the next step after aggregation. That step involves some cells in the aggregate dying to make a strong stalk. The other amoebae swarm up that stalk and make spores at the top. The stalk makes it easier for them to be transported to a new place where the spores can hatch and the life cycle can continue. We have a sequenced genome of several clones and also social mutants that cheat their ancestor but differ at only one gene each. Furthermore, we have another kind of cooperation, that with the bacteria these amoebae feed on. These features overall make the system ideal to study the genetic roots to cooperation, conflict, and control of conflict. The funding provided by this award allowed us to show that the genes closely involved in cooperation show balancing selection. This means that when any one form gets too common, the other form has advantages that allow it to increase in frequency. This kind of evolution is common when conflicts of interest evolve, but it is not always easy to show. Our project on this topic involved sequencing the entire genomes of 20 different individuals and looking carefully at how the relevant genes evolved and comparing that to other genes. We have also found that there are a lot of specific genes that play important roles in the evolution of conflict control. Some of them have multiple functions. One kind of multiple function is when a gene controlling conflict also has an essential function, or similarly when an actor cannot tell exactly who the action will benefit. This latter kind of cooperation can involve what is called a veil of ignorance, so we wrote a review on that topic. A different kind of two-party evolution involves two or more species. We discovered that the amoebae we study not only eat bacteria, but also carry them through the social spore stage that was previously thought to be sterile. Some of these carried or farmed bacteria are good food and hatch out later and are eaten. But others are not food. They seem to harm other kinds of amoebae in interesting ways, which include producing small molecules that contain antibiotics of various kinds. The evolutionary interaction implications of this system are interesting. This work has involved a lot of researchers and students at all levels, including undergraduates, graduate students, postdoctoral fellows and research scientists. We have worked hard to give everyone ownership of their own work, and a clear understanding of its implications. We have had many writing workshops and exercises. We have run separate courses for the undergraduate researchers, which help them understand the research endeavor. We have also made opportunities for the more advanced researchers to learn effective teaching, communicating, and mentorship. We are strong supporters of under-represented students and have had many thrive in our research group. We want to be welcoming and worthwhile for all. As part of this we have a blog, which has just achieved 200,000 hits. The point of the blog is to teach people how to become effective, caring biology professors and researchers. It is directed at students and professors of all levels, though any given entry may target one specific level. We are firm believers in the community of science and put our resources in public access places, whether they be clones in stock centers, sequence on servers, or contributions to Wikipedia. We feel that the importance of our research into social interactions and molecular evolution will be best felt if we communicate in publications, poster sessions, talks, and public education. To some degree our efforts have succeeded.
