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Buchnera: the genomic evolution of a bacterium
By Christina A. Pan

Researchers have mapped the genomic evolution of a bacterium found in aphids. This parasite, called Buchnera, has shared a mutually beneficial relationship with the insect for about 250 million years. Buchnera does not behave like other bacteria because it relies on aphids for functions such as protection and reproduction. In return, the aphid depends on the bacteria to supply necessary proteins it cannot obtain on its own.

Buchnera, found inside the host's cells, is surrounded by a membrane provided by the aphid.

A team of researchers led by Hajime Ishikawa of the University of Tokyo's Graduate School of Science sequenced the Buchnera genome and found that it lacks the genes normally found in other bacteria. To find out why these differences have occurred, Ishikawa and his colleagues plotted the evolutionary position of Buchnera among other bacteria with a similar set of genes. Their results indicate that Buchnera fell away from the bacterial lineage of R. prowazekii, leaving Buchnera's closest relatives as E. coli and H. influenzae, with which it shares some genes. The results of this study are published in the current issue of Nature.

Buchnera is a small genome, resulting from years of evolutionary dependence on aphids. According to Ishikawa, "the gene repertoire of the Buchnera genome is so specialized to intracellular life that it cannot survive outside the eukaryotic cell." For example, Buchnera lacks the genes to make proteins needed for membrane construction, and it is dependent on its host, the aphid, for shelter and protection against harsh environments and predators. Unable to reproduce on its own, the bacterium also relies on the aphid's reproduction process to pass its own descendants.

"This study is the first case where genomic evolution of a mutualistic organism is revealed at the genomic level, " says Ishikawa in Nature. "Further experimental data may give us an even deeper insight into the evolutionary significance of tight interspecies associations."

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Shigenobu, S. et al. Genome sequence of the endocellular bacterial symbiont of aphids Buchnera sp. APS. Nature 407, 81-86 (September 7, 2000).

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