All cockroaches examined so far have been found to harbour a bacterial endosymbiont in specialized cells of the fat body, whereas Mastotermes darwiniensis is the only termite currently known to harbour an intracellular symbiont. The localization and mode of transmission of these bacteria are surprisingly similar, but so far no data have been published on their phylogenetic relationships. To address this issue, molecular sequence data were obtained from the genes encoding the small subunit ribosomal RNA of the M. darwiniensis endosymbiont, and compared with those obtained from endosymbionts of seven species of cockroaches. Molecular phylogenetic analysis unambiguously placed all these bacteria among the flavobacteria-bacteroides, indicating that the endosymbiont of M. darwiniensis is the sister group to the cockroach endosymbionts examined. Additionally, nucleotide divergence between the endosymbionts appears to be congruent with the palaeontological data on the hosts'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 termite lineages, except that which gave rise to M. darwiniensis.

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The purpose of the research was to study the morphological alterations that were produced in the symbiotic equilibrium of individuals of Leucophaea maderae, Periplaneta americana and Nauphoeta cinerea heat-treated at + 39° C. The bacteria localized in the bacteriophorous vacuoles in the bacteriocytes of the samples treated underwent a series of structural modifications that led to protoplast formation. This phenomenon is similar to that which occurs in free-living bacteria. The protoplasts were then further digested into the bacteriophorous vacuoles which had become cytolysosomes, to the point that they became residual bodies. When the process of endocellular lysis involved the whole symbiotic complement, the nucleus and the cytoplasmatic organelles of the bacteriocyte began to degenerate. These alterations were irreversible and could lead to the death of the cell. As a result, the number of bacteriocytes diminished. Heat treatment, which had lethal effects on the bacteria, altered the equilibrium which was established during the process of coevolution between the symbiotic prokaryote and the host cell. The bacteriocyte, deprived of its symbiotic complement, did not survive the loss of its morphofunctional equilibrium.

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