PARALLELS AND CONTRASTS BETWEEN SYMBIOTIC BACTERIA AND BACTERIAL-DERIVED ORGANELLES - EVIDENCE FROM BUCHNERA, THE BACTERIAL SYMBIONT OF APHIDS

Bacteria of the genus Buchnera (gamma-Proteobacteria, allied with Esch erichia coli) occur exclusively in symbiosis with aphids. Buchner a is advantageous to aphids because it over-produces essential amino acids , which are made available to the insect tissues. The congruence of th e phylogenies of Buchnera and aphids, together with dating from the ap hid fossil record, suggests that Buchnera has been transmitted vertica lly down the aphid generations for 160-280 million years. Despite this long history in symbiosis, the Buchnera genome does not exhibit the r eduction and simplification characteristic of bacteria-derived organel les. The principal genetic reorganisation, relative to E. coli, relate s to amino acid over-production. Buchnera in advanced aphid families h as genes for enzymes in the synthesis of two essential amino acids, tr yptophan and leucine, amplified on plasmids. The Buchnera genome is al so characterized by high AT content and high rate of nonsynonymous sub stitutions in protein-coding genes. The latter may include deleterious mutations that accumulate because (as a consequence of vertical trans mission) Buchnera lacks the opportunity for recombination.