Living together: evolutionary and ecological dimensions of protist endosymbiosis

Abstract Protists, which comprise the majority of eukaryotic diversity, frequently engage in endosymbiotic relationships with other unicellular eukaryotes or prokaryotes. These interactions have profoundly shaped eukaryotic evolution, not only through the origin of endosymbiotic organelles and the subsequent diversification of eukaryotes, but also via less studied endosymbioses that have influenced the evolution of diverse eukaryotic lineages. Endosymbioses often alter host metabolic capabilities, enabling the colonisation of new ecological niches and significantly contributing to ecosystem functioning. In recent years, interest in these interactions has increased, driven by methodological innovations and new discoveries that reveal the diversity, mechanisms, and ecological roles of protist endosymbioses. Despite these advances, key questions remain: How widespread and ecologically impactful are protist endosymbioses? What functions do symbionts provide, and how do associations form, persist, or break down? Addressing these questions requires systematic studies of protists in their natural environments, combining microscopy and sequencing using both high-throughput and single-cell approaches, along with experimental manipulations of host-symbiont interactions. Here, we review current knowledge, highlight recent breakthroughs, and discuss ongoing challenges in the study of protist endosymbioses.