THE ESTABLISHMENT OF INTRACELLULAR SYMBIOSIS IN AN ANCESTOR OF COCKROACHES AND TERMITES

All cockroaches examined so far have been found to harbour a bacterial endosymbiont in specialized cells of the fat body, whereas Mastoterme s darwiniensis is the only termite currently known to harbour an intra cellular symbiont. The localization and mode of transmission of these bacteria are surprisingly similar, but so far no data have been publis hed on their phylogenetic relationships. To address this issue, molecu lar sequence data were obtained from the genes encoding the small subu nit ribosomal RNA of the M. darwiniensis endosymbiont, and compared wi th those obtained from endosymbionts of seven species of cockroaches. Molecular phylogenetic analysis unambiguously placed all these bacteri a among the flavobacteria-bacteroides, indicating that the endosymbion t of M. darwiniensis is the sister group to the cockroach endosymbiont s examined. Additionally, nucleotide divergence between the endosymbio nts appears to be congruent with the palaeontological data on the host s's evolution. These results support previous claims that the original infection occurred in an ancestor common to cockroaches and termites. A loss of endosymbionts should subsequently have occurred in all term ite lineages, except that which gave rise to M. darwiniensis.