Phylogenetically distant intracellular symbionts in termites.

Cockroaches are known to harbour intracellular bacteria in specialised cells (mycetocytes, or bacteriocytes) of the fat body. In termites, mycetocyte bacteria have been observed only in Mastotermes darwiniensis. These symbionts are thought to have originated from a bacterium that infected an ancestor common to cockroaches and termites. Thus, loss of the infection should have occurred during evolution in all termite lineages, with the exception of that leading to M. darwiniensis. One might suspect that traces of the ancient infection may be present in some termites, in the form of non-mycetocyte intracellular bacteria (e.g. a small number of bacteria within normal cells). Indeed, circumstantial evidence for the presence of intracellular bacteria in two termite species has been reported. However, no data are available on the actual distribution of these bacteria in termites, or on their relationships with the mycetocyte bacteria of cockroaches and M. darwiniensis. In this paper we report results indicating that non-mycetocyte intracellular bacteria are widespread in termites. These results were obtained by electron microscopy on representatives of nine termite species. In addition, sequence analysis of the 16S rRNA genes indicated that the non-mycetocyte bacteria of termites belong to the wolbachia group of the alpha-2 subclass of the proteobacteria. These latter bacteria are not related to the mycetocyte bacteria of cockroaches and M. darwiniensis, which belong to the blattabacterium group of the flavobacteria-bacteroides. PCR analyses with primers specific for wolbachia or blattabacterium provided further support for the identification of the observed non-mycetocyte bacteria as members of the wolbachia group.