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Symbiotic interactions are fundamental to life on Earth and were critical for the evolution of organelles that led to the success of eukaryotes on the planet. Such mutualistic interactions between unicellular microorganisms and multicellular plants and animals are pervasive in natural and agricultural ecosystems (1). In contrast, very little is known about symbiotic interactions between unicellular partners. Recent studies have revealed single-celled nitrogen-fixing symbioses that require different mechanisms to maintain symbiosis than seen in multicellular systems.