ABSTRACT
This chapter presents a view of the class of gammaproteobacteria that focuses on taxa that are not pathogenic to animals or humans. It examines a member of the latter order, the olive tree killer Xylella. The class of gammaproteobacteria has the largest number of genomes and protein sequences in National Center for Biotechnology Information databases. Human pathogens, which are so common among gammaproteobacteria, will not be described for multiple reasons. A brief premise on the tools of molecular phylogeny is necessary before discussing aspects of gammaproteobacterial phylogenesis. Gammaproteobacteria form a sister group to betaproteobacteria, both stemming from the ancestors of alphaproteobacteria after the separation of zetaproteobacteria. The class of gammaproteobacteria includes many species that live as endosymbionts of insects. Insects can harbor multiple species of endosymbionts and it is thus necessary to distinguish between primary and secondary symbionts among the increasing number of gammaproteobacteria intimately associated with insects.
