Dictyostelium, the Social Amoeba Joan E. Strassmann1, Sandra L. Baldauf2 1Washington University in St. Louis MO USA 2Uppsala University, Uppsala Sweden [email protected] [email protected] Glossary entries: Altruism: A behavior that is costly to the performer’s fitness, but beneficial to others. Greenbeard gene: A gene that affects copies of itself via three effects: production of trait, recognition of the trait in others, and differential treatment based on that trait. Sometimes not considered as part of kin selection because benefits go not to relatives but to actual bearers of the gene. Mutualism: An interaction that benefits both parties. Can be used for interactions within and between species. Social amoeba: A eukaryote in the Dictyostelia, a kingdom in the Amoebozoa. Social evolution: Evolution of traits of organisms that have fitness consequences for others of the same species, in particular those traits that may benefit others at a cost to oneself. Evolution of social interactions. Sociogenomics: Study of the genetic and genomic foundations of social behaviors. Symbiosis: Living together in close association in ways that may be beneficial or harmful for either party. Keywords Altruism Dictyostelium Greenbeard gene Mutualism Protist Social amoeba Social evolution Sociogenomics Symbiosis Abstract: The Dictyostelia present a splendid opportunity for the study of mutualism, sociality and genetic conflicts of interest. These amoebae aggregate upon starvation to form cooperative multicellular structures in which some formerly independent cells die to form a stalk. This serves to lift the other cells above the substrate where their chances of dispersal are greatly enhanced, for example by sticking to passing invertebrates. Dictyostelia vary in their social organization and the cells can be cultured from soil samples from nearly all parts of the world. Furthermore, they have complex symbiotic interactions with bacteria. Together these make many kinds of studies possible. Genome sequences are also available for increasing numbers of species; many molecular pathways are known; and experimental evolution is feasible. Who lives, who dies, and how sociality and mutualism are structured are great questions that are easily addressed in this group. INTRODUCTION The social amoeba Dictyostelium discoideum is an odd model system for behavior: it lacks a nervous system, is not an animal, and is only briefly multi-celled (Kessin, 2001, Baldauf and Strassmann, 2017). On the other hand, it is hard to imagine an organism more ideally suited to advancing our understanding of social behavior. Its social life is fascinating and tools developed by hundreds of cell and molecular biologists over the last few decades allow a gene-based approach to understanding its sociality (Strassmann and Queller, 2011, Madgwick et al., 2018). Studies of Dictyostelium provide a crucial independent test of social evolution theories, since these theories were developed with social insects and vertebrates in mind, not social amoebae. D. discoideum is a eukaryote that lives most of its life dispersed as independent amoebae primarily in the forest soil. They eat bacteria, and divide around every 4 h when food is abundant. But when they run out of food, a much more intense social stage begins (Figure 1). The amoebae aggregate by the hundreds of thousands and form a multicellular motile organism (slug). Ultimately, the multicellular slug organizes itself further into a fruiting body in which about 25% of the cells die to form a rigid cellulose-walled stalk while the remaining cells ascend this stalk and form a bolus of hardy spores at the top of the stalk (sorus). In this way, these tiny amoebae greatly increase their chances to be dispersed (Smith et al., 2014). This is essentially one-stop sociality, with a single, magnificent altruistic act whereby a subset of formerly independent cells benefits the rest. It can be compared both to a major transition to multicellularity and to the altruism of social insect workers. In some ways, the social-insect comparison is apt because, unlike most multicellular organisms, which consist of clones of cells, Dictyostelium sp. arrive at multicellularity by aggregation. Therefore, as in social insects, we might expect both altruism favored by kin selection and conflicts between the different genotypes in an aggregate. Given the genetic tools available for dicty, there is great potential for understanding the mechanisms of altruism and the control of conflict in this organism, making it a rich field for graduate students. This piece introduces the group, points out some of the most important molecular and genomic tools, summarizes what is known of its social behavior, and suggests promising future directions. BACKGROUND Where Is Dictyostelium on the Tree of Life? D. discoideum is the best-studied member of the Dictyostelia which is in the Amoebozoa, a superkingdom closely related to animals and fungi (Sheikh et al., 2017, Schaap et al., 2006). We will henceforth call D. discoideum by its vernacular name, dicty. The rest of the approximately 150 described species of Dictyostelia are much less well studied and will be referred to here by their scientific names. Dicty occupies a fascinating place on the Tree of Life, with important cellular traits shared uniquely with fungi and animals, including humans. Given the ancient age of Dictyostelia of about ~600 mya (Fiz-Palacios et al., 2013) and the small number of described species, it is highly likely that much of its diversity is undiscovered. This should change greatly with sequencing of more wild-collected clones as well as direct sequencing (culture independent sampling) of soils from around the world (Baldauf et al., 2018). The taxonomy of Dictyostelia has recently been revised to match the new molecular trees (Sheikh et al., 2017). As a result the rather simple traditional morphology-based taxonomy, which recognized three genera, has now been replaced with two orders, four families and 12 genera (Fig. 2, (Sheikh et al., 2017). Dicty is found in the Dictyosteliaceae, home to most of the hardiest and easily cultured species (Dictyostelium sp. and Polysphondylium sp.). Polysphondylium sp. are some of the most striking Dictyostelids. Their fruiting bodies are decorated with delicate whorls of side branches that are evenly spaced along the stalk and each topped by a deep purple sorus. (Fig 2). Like dicty, the polysphondylids tend to have large, robust, easily-cultured fruiting bodies built from hundreds of thousands of formerly independent amoebae. Another notable group of Dictyostelids are the heterostelids (Heterostelium sp.). Many of these also form ornate polysphondylid-type sorocarps, but these are pale in color and tiny, thus requiring the cooperation of far fewer amoebae to build the sorocarps. Heterostelium is quite a distant relative of Polysphondylium, meaning that this very distinctive morphology has evolved at least twice . At the other extreme are the Acytostelium sp.. These also have a social stage, but one that does not require the sacrifice of any cells to build the sorocarp. Thus, no cells die in the formation of an acytostelid sorocarp. Instead, it forms tiny stalks made entirely from cellular secretions, and all the aggregating amoebae survive to form spores. Although this behavior was once considered primitive (for dictyostelids), the molecular trees show instead that acytostelid simplicity is derived, that is, it evolved from an ancestor with altruistic development. This also means that dictyostelid altruism was probably lost and regained at least once. All this social variation in the Dictyostelia greatly enhances their value as a model social group. Where Dicty Lives Dictyostelia live in the upper layers of soil where they are predatory on bacteria, eating them by engulfment (Raper, 1984). Dicty is particularly common in autumn when leaf litter is abundant. Some species are more widespread than others, with D. mucoroides and Polysphondylium violaceum among the most ubiquitous. Dicty was first described by Kenneth Raper from a site just off the Blue Ridge Parkway near Mount Mitchell, NC, USA. It is abundant in forest soils of the Appalachians above about 1000 m elevation, but it also occurs generally in the eastern United States, with collections made from Houston, TX, to northern Minnesota and Massachusetts. Other samples assigned to this species have been collected as far South as Costa Rica. It has also been found along the eastern coast of Asia, including China, Japan and India, but not in Europe or Africa. However, other Dictyostelia can be found throughout the world, including clones detected in Antarctic lake sediments and New Zealand forest canopy soils (Baldauf et al. 2018). Life Cycle There are three important cycles in the life of dicty, asexual division, sexual aggregation and meiosis, and the social cycle (Figure 1). During the feeding stage of their life, dicty exists as independent amoebae that move through the soil by advancing pseudopods and engulfing any bacteria they encounter (Bonner, 1967). The amoebae divide about every 4 h when bacteria are plentiful (Figure 1, steps 2 and 3). At this stage in their life, their existence is essentially solitary since they do not depend on others to eat, move, or divide. However, it is clear that communication among amoebae is maintained through small signaling proteins like CMF and PSF, which function as quorum sensing molecules and more (Kessin, 2001). This communication is important because starvation