FLAVOBACTERIA AS INTRACELLULAR SYMBIONTS IN COCKROACHES

Animal cells are the sole habitat for a variety of bacteria. Molecular sequence data have been used to position a number of these intracellu lar microorganisms in the overall scheme of eubacterial evolution. Mos t of them have been classified as proteobacteria or chlamydiae. Here w e present molecular evidence placing an intracellular symbiont among t he flavobacteria-bacteroides. This microorganism inhabits specialized cells in the cockroach fat body and has been described as a mutualisti c endosymbiont of uncertain phylogenetic position. The small subunit r ibosomal DNA of these bacteria was analysed after polymerase chain rea ction amplification to investigate their phylogeny. The endosymbionts of five species of cockroaches were found to make up a coherent group with no close relatives within the eubacterial phylum defined by the f lavobacteria. In addition, the relationships among the endosymbionts, as revealed by DNA sequence data, appeared to be congruent with the ho st taxonomic relationships. Based on the host fossil record, a tentati ve calibration of the nucleotide substitution rate for the cockroach f lavobacteria gave results congruent with those obtained for the aphid endosymbiotic proteobacteria.