Rapid evolutionary tuning of endospore quantity versus quality trade-off via a phase-variable contingency locus

The UV resistance of bacterial endospores is an important quality supporting their survival in inhospitable environments and therefore constitutes an essential driver of the ecological success of spore-forming bacteria. Nevertheless, the variability and evolvability of this trait are poorly understood. In this study, directed evolution and genetics approaches revealed that the Bacillus cereus pdaA gene (encoding the endospore-specific peptidoglycan-N-acetylmuramic acid deacetylase) serves as a contingency locus in which the expansion and contraction of short tandem repeats can readily compromise (PdaA^OFF) or restore (PdaA^ON) the pdaA open reading frame. Compared with B. cereus populations in the PdaA^ON state, populations in the PdaA^OFF state produced a lower yield of viable endospores but endowed them with vastly increased UV resistance. Moreover, selection pressures based on either quantity (i.e., yield of viable endospores) or quality (i.e., UV resistance of viable endospores) aspects could readily shift populations between PdaA^ON and PdaA^OFF states, respectively. Bioinformatic analysis also revealed that pdaA homologs within the Bacillus and Clostridium genera are often equipped with several short tandem repeat regions, suggesting a wider implementation of the pdaA-mediated phase variability in other sporeformers as well. These results for the first time reveal (1) pdaA as a phase-variable contingency locus in the adaptive evolution of endospore properties and (2) bet-hedging between what appears to be a quantity versus quality trade-off in endospore crops.