The roles of dispersal limitation and pre-adaptation in shaping <i>Paraburkholderia</i> endosymbiont frequencies in social amoeba communities

Endosymbiotic interactions are ubiquitous in complex eukaryotes, as organelles such as mitochondria and chloroplasts represent the remnants of what were once free-living prokaryotes. However, how ecological processes facilitate the transition from free-living to host-associated is less understood. Selection is the most commonly invoked process to explain this transition: symbionts that are better at infecting hosts and potentially confer some benefit rise in frequency because they are selected for (and otherwise selected against). However, this only describes one fundamental process that can shape the ecology of symbiotic interactions. Here, we present evidence that the importance of dispersal (and its limitations) likely exceeds that of selection in shaping the distribution and frequency of <i>Paraburkholderia</i> endosymbionts in their dictyostelid social amoeba host communities. These findings highlight the need to consider regional ecological processes that operate at a scale beyond the individual when studying ecology and evolution of endosymbiotic interactions.