Conserved rod-to-spherical shape transitions in Escherichia coli through primordial experimental adaptations

Cell shape plays a fundamental role in bacterial physiology, yet the evolutionary stability of morphological adaptations remains poorly understood. Our previous work suggested that Escherichia coli can evolve a spherical form under primordial-like conditions. Here, we extended experimental evolution for approximately 1000 generations in oleic acid vesicle (OAV)-supplemented media mimicking primordial environments, followed by 500 generations in glucose. We observed a robust and stable transition from rod-shaped to spherical morphology that persisted despite environmental reversal. This morphological shift was accompanied by mutations in cell-shape-related genes, and no reversion to the ancestral rod shape was observed during evolution in glucose, where mutations primarily affected metabolic and transcriptional pathways. These findings show that environmental pressures can drive heritable and evolutionarily stable morphological changes in bacteria. Our work provides insight into the genetic and environmental factors shaping bacterial cell geometry and is consistent with, though not conclusive for, the possibility that primordial cells may have been spherical.