Buchnera aphidicola a member of the Proteobacteria, is the primary endosymbiont of aphids (A. pisum). It is believed that Buchnera was once a free living gram negative ancestor similar to a modern Enterobacteriaceae such as Escherichia coli. Buchnera are 3 µm in diameter and have some of the key characteristics of their Enterobacteriaceae relatives such as a gram-negative cell wall. However, unlike most other gram-negative bacteria, Buchnera lack the genes to produce lipopolysaccharides (LPS) for their outer membrane. The long association with aphids and the limitation of crossover events due to strictly vertical transmission has seen the deletion of genes required for anaerobic respiration, the synthesis of amino-sugars, fatty acids, phospholipids, and complex carbohydrates. This has resulted not only in one of the smallest known genomes of any living organism, it is also one of the most genetically stable.
The symbiotic relationship with aphids began between 200 million and 150 million years ago, and has persisted through maternal transmission and co-speciation. Aphids have developed bacteriocyte cells to house Buchnera. It is estimated that a mature aphid may carry 5.6 × 106 Buchnera individuals. Buchnera have lost regulatory factors allowing continuous overproduction of tryptophan and other amino acids. Each bacteriocyte contains multiple vesicles, symbiosomes derived from the plasma membrane.
Buchnera was first named by Paul Baumann and his graduate student, and the first molecular characterization of a symbiotic bacterium was carried out by Baumann, using Buchnera. The initial studies on Buchnera later led to studies on symbionts of many groups of insects, pursued by numerous investigators, including Paul and Linda Baumann, Nancy Moran, Serap Aksoy, Roy Gross, who together investigated symbionts of aphids, tsetse flies, ants, leafhoppers, mealybugs, whiteflies, psyllids and others.
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