Biodiversity of “non-model” Rickettsiales and their association with aquatic organisms

Representatives of the order Rickettsiales are obligate intracellular bacteria, traditionally including well-studied pathogens of humans and other vertebrates, such as Rickettsia, Orientia, Anaplasma, and Ehrlichia. In the last two decades, studies based on molecular characterization techniques have reshaped our view on the biodiversity of Rickettsiales, and the eukaryotic hosts they can exploit. Several new genera have been described in “traditional” Rickettsiales families Rickettsiaceae and Anaplasmataceae. Moreover, a new family, “Candidatus Midichloriaceae,” displaying diversity at least comparable with the “traditional” ones, has been described. Recent data show that the majority of extant genera of Rickettsiales (16 out of 24) are hosted exclusively (or at least partly) by aquatic organisms, such as protists (e.g., ciliates, amoebas, flagellates), and animals (e.g., cnidarians, mollusks, tunicates, leeches), while only ten genera include some terrestrial host, such as arthropods. Given the highly interwoven phylogenetic relationships among Rickettsiales hosted by aquatic and terrestrial hosts, it is likely that the ancestral host of Rickettsiales was an aquatic protist organism, and the adaptation to terrestrial environments occurred independently in several distinct sublineages at least six times. Newly discovered lineages of “non-model” Rickettsiales present unforeseen features for the order such as the presence of flagella. Future investigations on “non-model” Rickettsiales are crucial to gain insight on the evolutionary history of the whole group, including the origin of molecular mechanisms involved in pathogenesis for humans and vertebrates.