What Is a Pseudomonas syringae Population?

Abstract

Although they are often best known as causative agents of agricultural disease, many phytopathogen lineages, like Pseudomonas syringae, have been sampled across a wide range of environmental contexts. These may be frequently isolated as epiphytes on disease-free plants as well as from sources associated with the water cycle like rivers, lakes, rain, snow, and clouds. The ability of these bacteria to persist across such diverse environments poses a great challenge for understanding population dynamics because adaptation likely occurs across numerous distinct niches and evolutionary parameters and will likely differ widely depending on specific contexts. Within the literature, there is an intrinsic tendency to treat all strains within these lineages the same, but such a treatment likely obscures interesting and important nuances between isolates. In this chapter, I will focus on P. syringae and explore what is known about the evolutionary dynamics of this group at the levels of genomes, phylogroups, and (broadly defined) species. I will highlight many ways in which populations could differ and will touch upon what is known and has been learned from numerous genome sequencing efforts, which hopefully shine a light toward a path forward to resolve numerous nomenclatural challenges. I will point toward the generality of what is known about P. syringae and how this may apply to other environmental systems. While there remains much to learn, the ever-increasing rate of accumulation of genomic data from diverse sources has certainly helped our ability to at least frame the evolutionarily important questions. Building from these, an impending wave of future data promises to be a powerful tool for resolving some of these discussions.