ABSTRACT
Introduction ............................................................................................................ 192 Discovery and Phylogeny ...................................................................................... 193 Diversity of Rickettsia: Host Distribution and Evolutionary Patterns ................... 193 Prevalence of Rickettsia between and within Species ............................................ 195 Effects of Rickettsia on Host Fitness ..................................................................... 196 Distribution of Rickettsia within Host Tissues and Its Relationship with Symbiont Transmission .......................................................................................... 198 Rickettsia Movement .............................................................................................. 199 Horizontal Transmission ........................................................................................ 201 Acknowledgments ..................................................................................................202 References ..............................................................................................................202
The best-known members of the bacterial genus Rickettsia are associates of bloodfeeding arthropods and are pathogenic when transmitted to vertebrates. These species include the agents of acute human diseases such as typhus and spotted fever. However, recent surveys of bacteria associated with arthropods and other invertebrates have uncovered many other Rickettsia in hosts that have no relationship with vertebrates (Braig et al. 2008). It may therefore be more appropriate to consider Rickettsia as symbionts that are transmitted primarily vertically within arthropod hosts and occasionally horizontally between and within vertebrate hosts. Indeed, one of the most striking aspects of Rickettsia biology is the diversity of transmission modes. Perhaps even more than most other secondary symbionts, Rickettsia are characterized by multiple transitions between strains that are transmitted strictly vertically and those that exhibit mixed (horizontal and vertical) transmission. Moreover, the array of organisms that may harbor Rickettsia spans multiple kingdoms, including amoebae, annelids, arthropods, vertebrates, and plants. This unusually broad host range, combined with the ability of Rickettsia to actively move between and within living cells, makes this bacterium an excellent model system for studying both the evolution of transmission pathways and transitions between mutualism and pathogenicity (Merhej and Raoult 2010). Indeed, the major insights that have come from studying pathogenic Rickettsia will serve to illuminate our understanding of the evolution of infection strategies across this fascinating group of symbionts.
