The study of the bacteria of the genus Rickettsia provided an opportunity to investigate whether the population structure would be one in which all bacterial cells within a host seemed to behave (at the population level) as if they represented a single "individual", with a bottleneck occurring at each transfer between hosts. This in fact seems to be the case, since population diversity within a species of Rickettsia is among the lowest reported for any bacterial group, and is equivalent of that found for vertebrate species. Further, however, the study of Rickettsia provided an opportunity to make comparisons between a bacterial system and the diversity observed in a mitochondrial gene system. The latter was the focus for other studies done in collaboration with Dr. Maruyama and Dr. C.W. Birky. In fact Rickettsia has been suggested to have many characteristics expected in a close bacterial relative of the prokaryotic progenitor of the mitochondrion.

The studies which we initiated have focusing on several genes, including the anonymous gene segments visualized as RFLP variation, the small subunit rRNA gene, and the 17kD surface antigen protein gene. The studies have resulted in a series of papers written primarily in collaboration with three of my former doctoral students. These papers have laid the framework for a re-examination of our understanding of the natural history of populations of Rickettsia and of the host-parasite interactions of these obligate intracellular bacteria with their arthropod hosts.