Parallels and contrasts between symbiotic bacteria and bacterial-derived organelles: evidence from Buchnera, the bacterial symbiont of aphids
Bacteria of the genus Buchnera (γ-Proteobacteria, allied with Escherichia coli) occur exclusively in symbiosis with aphids. Buchnera is advantageous to aphids because it over-produces essential amino acids, which are made available to the insect tissues. The congruence of the phylogenies of Buchnera and aphids, together with dating from the aphid fossil record, suggests that Buchnera has been transmitted vertically down the aphid generations for 160–280 million years. Despite this long history in symbiosis, the Buchnera genome does not exhibit the reduction and simplification characteristic of bacteria-derived organelles. The principal genetic reorganisation, relative to E. coli, relates to amino acid over-production. Buchnera in advanced aphid families has genes for enzymes in the synthesis of two essential amino acids, tryptophan and leucine, amplified on plasmids. The Buchnera genome is also characterized by high AT content and high rate of nonsynonymous substitutions in protein-coding genes. The latter may include deleterious mutations that accumulate because (as a consequence of vertical transmission) Buchnera lacks the opportunity for recombination.