The world`s most stable genome has been identified in aphid endosymbionts

Bacteria that reproduce inside aphids have not changed their genetic make-up for the last 50-70 million years. This makes the genomes of these bacteria the most stable of all organisms yet studied. This finding is presented by a team of scientists at Uppsala University, Sweden, in the latest issue of the scientific journal Science.

Under the leadership of Professor Siv Andersson, researchers Ivica Tama, Lisa Klasson, Björn Canbäck, Kristina Näslund, Ann-Sofie Eriksson, and Johan Sandström at the Department of Molecular Evolution, Center for Evolutionary Biology, in collaboration with Professor Nancy Moran in Tucson, Arizona, have described the entire genetic make-up of a bacterium that reproduces inside aphids, Buchnera (Sg) and compared it to that of a close relative, Buchnera (Ap).

These aphid endosymbionts, so called because they live in symbiosis with aphids, are closely related to common bacteria like Salmonella, but the adaptation to the aphids have entailed a drastic reduction in the size of the genome, which now consists of only 640,000 bases, about 14% of the genome of Salmonella species.

Aphid endosymbionts produce important amino acids that are not present in the plant sap that the aphids drink. The bacteria live in a special type of cell in the body of the aphids and are transmitted from one generation to the next by being packed into the eggs of the aphides. These bacteria are believed to have lived in symbiosis with aphids for at least 150 million years. They have now become so important that aphides can no longer live without their bacteria. If aphids treated with antibiotics, they becomes sterile — or die.

With the aid of available fossil data from aphids, it has been estimated that the aphids that harbor these two bacteria diverged from each other roughly 50-70 million years ago. Since these aphis symbionts have lived enclosed in the bodies of the aphids, this dating can also be used to determine when the bacterial endosymbionts diverged from each other. By measuring differences in the two genomes, the Uppsala scientists have been able to calculate for the first time exactly how many mutations have taken place in the genome of a bacterium in nature over a period spanning 50-70 million years. Surprisingly, it has now been shown that these tiny, isolated aphis bacteria have largely escaped the ravages of time. The biggest surprise is that the order of the genes has not changed over the past 50 million years.

This stability is in stark contrast to the genomes of Salmonella species, which change very rapidly in structure. It has been calculated that the genomic structure of Salmonella has been altered at a rate more than 2,000 times that of the aphid endosymbionts. The secret behind the extreme stability of the aphid endosymbionts probably lies in the fact that during the early process of degradation they eliminated the genes that are needed for cutting and pasting genetic material.

However, it is extremely unlikely that the aphids` stable minibacteria will ever return to a normal life outside the aphids. They are now completely controlled by the aphids, so much so that the question can be raised whether they should be seen as bacteria or rather as organs of aphides. But if that is the case, then this is the first organ that has its own genetic code!

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