我们对很多不同细菌形态的形成机制还很不了解。在这项研究中，Yves Brun及同事发现Caulobacter 和 Asticcacaulis sp.两种细菌的spmX基因（它编码溶菌酶家族的定域化因子SpmX）是对细胞形态有重要影响的一个发育调控因子。SpmX决定向细胞被膜(cell envelope)的柄状延伸(stalk-like extension)的位置是在两极还是在两侧，而这两个位置分别是Caulobacter 和 Asticcacaulis的特点。系统发生分析显示，spmX 一个特定区域的逐步演变导致一个新功能的获得和这一蛋白的定位，从而驱动“柄位置”(stalk position)的依次转变。
What mechanisms underlie the transitions responsible for the diverse shapes observed in the living world? Although bacteria exhibit 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 subcellular positions of stalks within a rod-shaped cell body: polar in the genus Caulobacter and subpolar or bilateral in the genus Asticcacaulis. Here we show that a developmental regulator of Caulobacter crescentus, SpmX, is co-opted in the genus Asticcacaulis to specify stalk synthesis either at the subpolar or bilateral positions. We also 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 changes in 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.