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.
A Shared Assembly Line
The metabolic relationship between the aphid and Buchnera is an integrated partnership; it is a fully integrated partnership. While Buchnera holds the genetic blueprints for producing the ten essential amino acids, its own pared-down genome is missing genes for a few key steps in the synthesis pathways. The aphid's genome has stepped in to fill the gaps. Genes coding for the missing enzymes are highly expressed within the bacteriocyte cells. This creates a shared metabolic assembly line where chemical intermediates are passed between the host cell's cytoplasm and the bacterium to complete the final amino acid products.
This division of labor is incredibly efficient but leaves both partners vulnerable. The aphid cannot survive without the amino acids from Buchnera, and experiments show that removing the symbiont causes the aphid to grow slowly and fail to reproduce. Likewise, Buchnera cannot be cultured outside of its host cell. The symbiosis is obligate. The fate of the bacterium is tied to the fate of the aphid lineage it inhabits, a co-evolutionary history that has continued in parallel for over 100 million years. The coordinates for this Point of Interest lead to the University of Arizona, where pioneering research by scientists like Nancy Moran has unraveled the genetic and evolutionary details of this ancient alliance.