What mechanisms underlie the transitions responsible for the diverse shapes observed in the living world? While bacteria display a myriad of morphologies, the mechanisms responsible for the evolution of bacterial cell shape are not understood. We investigated morphological diversity in a group of bacteria that synthesize an appendage-like extension of the cell envelope called the stalk,. The location and number of stalks varies among species, as exemplified by three distinct sub-cellular positions of stalks within a rod-shaped cell body: polar in the Caulobacter genus, and sub-polar or bi-lateral in the Asticcacaulis genus. Here we show that a developmental regulator of Caulobacter crescentus, SpmX, was co-opted in the Asticcacaulis genus to specify stalk synthesis at either the sub-polar or bi-lateral positions. We show that stepwise evolution of a specific region of SpmX led to the gain of a new function and localization of this protein, which drove the sequential transition in stalk positioning. Our results indicate that evolution of protein function, co-option, and modularity are key elements in the evolution of bacterial morphology. Therefore, similar evolutionary principles of morphological transitions apply to both single-celled prokaryotes and multicellular eukaryotes.
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