A Symbiosis Started with the Dinosaurs
Deep inside the gut of nearly every aphid on Earth is a biological story that began 160 to 280 million years ago. A free-living bacterium, an ancestor of today's Buchnera aphidicola, infected the ancestor of all modern aphids. This was not a hostile takeover. Instead, it was the beginning of an unbreakable partnership. The aphid's diet of plant sap is high in sugar but critically poor in essential amino acids—the building blocks of protein that animals cannot synthesize themselves. Buchnera solved this problem. It retained the genetic machinery to produce these nutrients, supplying its host with the molecules necessary for life.
In return for providing this essential service, the bacterium received a safe home inside specialized aphid cells called bacteriocytes. An adult pea aphid might contain as many as 90 bacteriocytes, housing over 10 million individual Buchnera cells. Confined to this isolated environment and passed directly from mother to embryo, Buchnera no longer needed many of the genes required for a free-living existence. The result was one of the most extreme cases of genome reduction known. The genome of Buchnera aphidicola is now only about 412 to 650 kilobases (kb) long, containing around 500-600 protein-coding genes. For comparison, the genome of its close relative Escherichia coli is about 4,600 kb and contains over 4,000 genes. Buchnera has shed genes for DNA repair, anaerobic respiration, and building its own cell membrane lipids, becoming completely dependent on its host.