sources. The proteins of the Caulobacter cell cycle control system are widely co-conserved across the alphaproteobacteria, but the ultimate function of this regulatory system varies widely in different species. These evolutionary changes reflect enormous differences between the individual species in fitness strategies and ecological niches. For example, Agrobacterium tumefaciens is a plant pathogen, Brucella abortus is an animal pathogen, and Sinorhizobium meliloti is a soil bacterium that invades, and becomes a symbiont in, plant root nodules that fix nitrogen yet most of the proteins of the Caulobacter cell cycle control are also found in these species. The specific coupling between the protein components of the cell cycle control network and the downstream readout of the circuit differ from species to species. The pattern is that the internal functionality of the network circuitry is conserved, but the coupling at the “edges” of the regulatory apparatus to the proteins controlling specific cellular functions differs widely among the different species. The evolution of stalk positioning in the Caulobacter clade Caulobacter crescentus is a member of a group of bacteria that possess the stalk structure, a tubular extension from the cell body. However, the positioning of the stalk is not necessarily conserved at the pole of the cell body in different closely related species. Specifically, research has shown that not only the position of the stalk can change, but the number can vary as well in the closely related genus Asticcacaulis. SpmX, a polarly localized protein in Caulobacter crescentus, has been shown to manipulate stalk positioning in these Asticcacaulis species. Presumably, It does so by a gain of function after protein expansion from around 400 amino acids in Caulobacter crescentus to more than 800 amino acids in Asticcacaulis species. Caulobacter aging Caulobacter was the first asymmetric bacterium shown to age. Reproductive senescence was measured as the decline in the number of progeny produced over time. On the basis of experimental evolution studies in C. crescentus, Ackermann et al. suggested that aging is probably a fundamental property of all cellular organisms. A similar phenomenon has since been described in the bacterium Escherichia coli, which gives rise to morphologically similar daughter cells. Cell polarity regulation In C. crescentus, cell polarity is readily apparent by the assembly of polar organelles and by the polarization of the division plane, which results in the generation of stalked progeny that are longer than swarmer progeny. The formation of new cell