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Buchnera aphidicola, a member of the Proteobacteria, is the primary endosymbiont of aphids, and has been studied in the pea aphid Acyrthosiphon pisum, Baizongia pistacea, Schizaphis graminum, and Cinara cedri . Buchnera is believed to have had a free-living, Gram-negative ancestor similar to a modern Enterobacteriaceae, such as Escherichia coli. Buchnera is 3 µm in diameter and has some of the key characteristics of their Enterobacteriaceae relatives, such as a Gram-negative cell wall. However, unlike most other Gram-negative bacteria, Buchnera lacks the genes to produce lipopolysaccharides for its 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, but also one of the most genetically stable.

Genome
The genome of many strains of Buchnera aphidicola have been sequenced. The sequenced strains have between 422,434 to 655,727 base pairs, a GC content between 20.2% and 26.4%, between 35 and 37 RNAs, one chromosome, and one or two plasmids. The relatively small size of the genome is likely due to degenerative evolution. This is common among obligate symbionts of insects, especially insects that eat plant sap that is deficient in essential amino acids. The genes lost in Buchnera aphidicola are mostly concentrated in gene transfer and recombination, which means that different Buchnera strains have essentially evolved independently from a common ancestor. Differences between strains could affect the aphid’s ability to adapt to evolving host-plant resistance. Therefore, a deeper understanding of coevolution between Buchnera and aphids could help us find ways to protect host plants from aphid infestation in the future.

Most genes in sequenced Buchnera genomes that can be assigned a function are related to amino acid and protein metabolism. This is directly related to Buchnera’s function and location within an aphid. Buchnera live in bacteriocytes, which together form a bacteriome within the gut of the aphid. Biosynthetic pathways are formed between the Buchnera and the aphid to allow transfer of amino acids, proteins, and other nutrients essential to each. Some of the genes that Buchnera have lost are used in phospholipid biosynthesis, suggesting that the bacteriome is necessary for its survival.

Symbiosis between Buchnera and aphids
The symbiotic relationship with aphids began between 160 million and 280 million years ago, and has persisted through maternal transmission and cospeciation. Aphids have developed bacteriocyte cells to house Buchnera. A mature aphid may carry an estimated 5.6 × 106 Buchnera cells. Buchnera has lost regulatory factors, allowing continuous overproduction of tryptophan and other amino acids. Each bacteriocyte contains multiple vesicles, symbiosomes derived from the cell membrane.

One interesting characteristic of the Buchnera--aphid relationship is seen in instances of horizontal gene transfer in the aphids, which are mostly expressed within the bacteriocytes. Although the aphids acquire microbial genes, they are not genes from Buchnera. Instead, they have been found to be related to other bacterial lineages that are not currently found in the aphids. This suggests that aphids were in earlier (on an evolutionary time scale) endosymbiotic relationships with other bacteria, which could shape the aphid’s specific cooperation with its endosymbionts today.

Buchnera and plant viruses
Buchnera also increases the transmission of plant viruses by producing symbionin, a protein that binds to the viral coat and protects it inside the aphid. This makes it more likely that the virion will survive and be able to infect another plant when the aphid next feeds.

History
Buchnera was named after Paul Buchner(de:) 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, and Roy Gross, who together investigated symbionts of aphids, tsetse flies, ants, leafhoppers, mealybugs, whiteflies, psyllids, and others.