Biodiversity and systematics of nematode-bacterium entomopathogens

Nematodes are one of the most abundant animals on earth, and bacteria comprise the most biologically and phylogenetically diverse domains of organisms. On at least two separate occasions a soil dwelling nematode and a bacterium have entered into a mutualistic, insecticidal association. From such origins arose two distinct lineages of nematode-bacterium entomopathogens, Steinernema-Xenorhabdus and Heterorhabditis-Photorhabdus. Herein, we present a summary and discussion of the known evolutionary diversity and systematics of these two groups relative to other nematodes and bacteria, and their shared evolutionary history.