Many open questions surround the processes that led to the evolutionary origin of mutualism between hosts and endosymbionts. In particular, large effect size mutations and coevolution have both been hypothesized to be important. Here we conduct in silico experiments using the Symbulation platform to explore these questions. We find that increasing mutation size and mutation rate both promote the evolution of mutualism, as does faster generational turnover within symbionts. Our results support the idea that large effect size mutations are important for the de novo evolution of mutualism. Indeed, follow-up mathematical modeling suggests that large regions of the parameter space where mutualism evolves can be explained purely by mutation size and rate. However, we observe that mutualism evolves in a wider region of parameter space than we would expect under this simple probabilistic model. We hypothesize that coevolutionary forces are responsible for this discrepancy, a hypothesis that is further corroborated by phylogenetic data showing that partners in the first mutualism are often themselves descended from mutualists. Ultimateley, we conclude that both mutation size and coevolution play a role in the evolution of mutualism. We anticipate that our findings will generalize to other systems featuring evolution along the parasitism-mutualism spectrum. Our work furthers efforts to predict host-endosymbiont coevolution.

Endosymbiosis, symbiosis in which one symbiont lives inside another, is woven throughout the history of life and the story of its evolution. From the mitochondrion residing in almost every eukaryotic cell to the gut microbiome found in every human, endosymbiosis is a cornerstone of the biological processes that sustain life on Earth. While endosym-biosis is ubiquitous, many questions about its origins remain shrouded in mystery; one question in particular regards the general conditions and possible trajectories for its evolution. Modern science has hypothesized two possible pathways for the evolution of mutualistic endosymbiosis: one where an obligate antagonism is co-opted into an obligate mutualism (Co-Opted Antagonism Hypothesis), and one where a facultative mutualism evolves into an obligate mutualism (Black Queen Hypothesis). We investigated the viability of these pathways under different environmental conditions by expanding on the evolutionary agent-based system Symbulation. Specifically, we considered the impact of ectosymbiosis on de novo evolution of obligate mutualistic endosymbiosis. We found that introducing a facultative ectosymbiotic state allows endosym-biosis to evolve in a more diverse set of environmental conditions, while also decreasing the evolution of endosymbiosis in conditions where it can evolve independently.