Bacterial genomes vary in size over two orders of magnitude. The Mycoplasma genitalium genome has historically defined the extreme small end of this spectrum, and has therefore heavily informed theoretical and experimental work aimed at determining the minimal gene content necessary to support cellular life. Recent genomic data from insect symbionts have revealed bacterial genomes that are incredibly small-two to four times smaller than M. genitalium-and these tiny genomes have raised questions about the limits of genome reduction and have blurred the once-clear distinction between autonomous cellular life and highly integrated organelle. New data from various systems with symbiotic bacterial or archaeal partners have begun to shed light on how these bacteria may function with such small gene sets, but major mechanistic questions remain.
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