The ecology and evolution of heterorhabditid nematodes and their symbiotic bacteria: Phylogeny, species concepts, coevolution, and ecological observations

The entomopathogenic nematode Heterorhabditis and its bacterial endosymbiont, Photorhabdus, are promising agents for the biological control of insect pests. Together they provide an environmentally friendly alternative to conventional biocides. However, progress in many areas of biological control has been hampered by the lack of an evolutionary framework requisite for all research programs that utilize the comparative method. The primary goal of this research was to infer the historical relationships among species of Heterorhabditis using a ribosomal gene genealogy. This resulted in a re- examination of the concept of species in nematology. Recovered phylogenetic relationships also facilitated an analysis of the shared evolutionary history between Heterorhabditis and Photorhabdus. Preliminary to a more comprehensive analysis of heterorhabditid population structure and ecology, a Nebraska sandhills prairie was surveyed and described. The internally transcribed spacer region-1 of the ribosomal tandem repeating unit is shown to be an appropriate marker for inferring historical relationships among closely related heterorhabditid species. This genetic marker provides evidence that some sister species of Heterorhabditis are likely conspecific. It also suggests that other heterorhabditid lineages may consist of more than one species. Drawing on examples of Heterorhabditis and other nematodes, it is shown that the delimitation of species requires evidence that genealogical lineages have been evolving independently in the past and will continue to do so in the future. It also is demonstrated that the set of discovery operations required to identify species must include the anagenetic evolution of a uniquely derived character. A comparison of phylogenetic trees for Heterorhabditis and Photorhabdus reveals significant evidence of high host-symbiont coevolution. Ecologically, distributions of heterorhabditid nematodes in a Nebraska sandhills ecosystem appear to be distributed according to available soil moisture, and may influence insect and plant communities through trophic interactions.