RESEARCH ARTICLE Two Different Rickettsial Bacteria Invading

Background Bacteria of the family Rickettsiaceae are principally associated with arthropods. Recently, endosymbionts of the Rickettsiaceae have been found in non-phagotrophic cells of the vol-vocalean green algae Carteria cerasiformis, Pleodorina japonica, and Volvox carteri. Such endosymbionts were present in only C. cerasiformis strain NIES-425 and V. carteri strain UTEX 2180, of various strains of Carteria and V. carteri examined, suggesting that rickettsial endosymbionts may have been transmitted to only a few algal strains very recently. Howev-er, in preliminary work, we detected a sequence similar to that of a rickettsial gene in the nu-clear genome of V. carteri strain EVE. Methodology/Principal Findings Here we explored the origin of the rickettsial gene-like sequences in the endosymbiont-lack-ing V. carteri strain EVE, by performing comparative analyses on 13 strains of V. carteri. By reference to our ongoing genomic sequence of rickettsial endosymbionts in C. cerasiformis strain NIES-425 cells, we confirmed that an approximately 9-kbp DNA sequence encom-passing a region similar to that of four rickettsial genes was present in the nuclear genome of V. carteri strain EVE. Phylogenetic analyses, and comparisons of the synteny of rickettsi-al gene-like sequences from various strains of V. carteri, indicated that the rickettsial gene-like sequences in the nuclear genome of V. carteri strain EVE were closely related to rickett-sial gene sequences of P. japonica, rather than those of V. carteri strain UTEX 2180