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Inside insects, life is unchanged for 50 million years


For millions of years, life has hardly changed for bacteria that live inside aphids. The genomes of two Buchnera aphidicola bacteria are so similar that no evolutionary time seems to have passed since the species diverged from a common ancestor more than fifty million years ago. The genomes are the same size and have nearly identical architecture: No genes have been moved around, duplicated or acquired—and only a few lost—since the species separated, according to a new study.

Aphids host Buchnera aphidicola, a bacterium shown to exhibit extreme genomic stability.

Ivica Tamas and Lisa Klassen, both of the Evolutionary Biology Center at the University of Uppsala in Sweden, and colleagues compared the fully sequenced genomes of two Buchnera cousins that live inside different aphids. One bacterium lives inside the pea aphid Acyrthosiphon pisum, the other inside the greenbug Schizaphis graminum.

Buchnera aphidicola and aphids entered into a symbiotic relationship some 200 million years ago and have coevolved since. Their close relationship has allowed each species to streamline its genome and eliminate genes present in the other. The Buchnera bacterium now has a minimal genome and relies heavily on its host, while aphids no longer produce amino acids that are supplied by B. aphidicola.

The minimal nature of the bacterium's genome means that the DNA is less likely to change through recombination, and this may account for the genome's remarkable stability over time. "The number of possible genome variants that can be generated will decrease rapidly as the gene content and genome size are reduced," the researchers write in Science.

Studies of other bacteria associated with hosts have shown that these organisms tend to have relatively stable genomes, but B. aphidicola is the most extreme example reported so far. By way of comparison, the researchers note that closely related bacteria like E. coli and Salmonella have rates of genetic change 2,000 times larger than B. aphidicola.

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Tamas, I. et al. 50 Million years of genomic stasis in endosymbiotic bacteria. Science 296, 2376-2379 (June 28, 2002).
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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