An intimate and ancient partnership
Inside almost every aphid is another, much older organism: the bacterium Buchnera aphidicola. This partnership began between 160 and 280 million years ago, when an ancestor of modern aphids was infected by a free-living bacterium. Since that single event, the fate of the two organisms has been intertwined. Buchnera is now an obligate endosymbiont, meaning it cannot survive outside its host. In turn, aphids are completely dependent on Buchnera for survival. If the symbiont is experimentally removed, the aphid grows slowly and cannot reproduce.
The bacteria live exclusively within specialized aphid cells called bacteriocytes, which are themselves grouped into a larger organ-like structure, the bacteriome. A single adult aphid may contain over 5.6 million Buchnera cells. This relationship is maintained through strict maternal transmission. Mother aphids pass the bacteria to their developing embryos, ensuring each new generation is equipped with its essential partner. This vertical transmission has led to the complete co-speciation of host and symbiont; as aphids evolved into new species, so did the bacteria inside them, creating perfectly matched phylogenetic trees.
The incredible shrinking genome
Life inside a stable host cell is a comfortable existence. Protected from the outside world and provided with many necessary molecules by the aphid, Buchnera no longer needed many of its ancestral genes. Over millions of years, it has undergone one of the most extreme cases of genome reduction known. While its free-living relatives, like E. coli, have genomes around 4.6 megabases (Mb), the genome of Buchnera has shrunk dramatically. Modern strains have genomes ranging from just 412 to 650 kilobases (kb).
This process involved the loss of thousands of genes. A reconstruction of the ancestral genome suggests it contained over 2,400 open reading frames; modern Buchnera has only around 354 to 587. It has shed genes for DNA repair, cellular defense, and the production of its own cell wall components. What it has meticulously retained are the genes for a very specific purpose: synthesizing essential amino acids. Aphids feed on plant phloem, a diet high in sugar but poor in the ten essential amino acids they cannot make themselves. Buchnera acts as a living factory, producing these nutrients, such as tryptophan, leucine, and methionine, for its host. In some cases, the metabolic pathways are shared, with the aphid's own enzymes completing the final steps of amino acid synthesis that Buchnera can no longer perform. This has created a single, highly integrated metabolic system from two separate organisms.