Many insect species harbor internal microorganisms that help them survive. The bacterium Buchnera aphidicola is a primary example, living exclusively inside aphids, a group of small, sap-sucking insects. This arrangement is an endosymbiosis, a mutually beneficial partnership where one organism lives within the cells of another. Buchnera is a primary endosymbiont, meaning the relationship is essential for the aphid’s survival and reproduction.
The Buchnera-Aphid Symbiotic Partnership
The bacteria are not scattered randomly within the aphid’s body but are housed inside specific host cells called bacteriocytes. These specialized cells are themselves organized into a distinct, organ-like structure known as a bacteriome, which is located in the aphid’s main body cavity. A single mature aphid can host millions of Buchnera cells within its bacteriome.
The partnership is maintained across generations through direct maternal transmission. The bacteria are passed from the mother’s bacteriocytes to her developing embryos, ensuring that nearly all aphids are born with their necessary bacterial partners. This vertical transmission makes the relationship obligate, as aphids cannot survive without Buchnera, and Buchnera cannot live outside its aphid host.
Essential Role in Aphid Nutrition
The primary function of Buchnera is to supplement the aphid’s diet. Aphids feed exclusively on plant phloem sap, a sugary liquid rich in carbohydrates but poor in nitrogen-based compounds, especially essential amino acids. Essential amino acids are protein building blocks that an animal cannot synthesize and must acquire from its diet. This dietary deficiency would otherwise be lethal to the aphid.
Buchnera solves this problem by taking up precursor molecules from the aphid and synthesizing the ten essential amino acids the aphid cannot produce. The host and symbiont have complementary metabolic capabilities; together, they can produce all the compounds needed for survival, a feat neither could accomplish alone. This nutritional provisioning allows aphids to thrive and reproduce rapidly on an otherwise unsustainable diet.
Evolutionary Journey and Genomic Features
The symbiotic relationship began between 160 and 280 million years ago, leading to a long history of co-evolution between the bacteria and their hosts. This association has led to significant changes in the bacterium’s genetic makeup, most notably genome reduction. Compared to its free-living ancestor, which was likely similar to modern E. coli, Buchnera has one of the smallest known bacterial genomes.
Genome reduction is the loss of genes no longer needed in the protected, stable environment of the host cell. Buchnera has shed genes responsible for functions provided by the host, including:
- Anaerobic respiration
- The synthesis of fatty acids and phospholipids
- The production of complex carbohydrates
Despite this shrinkage, the genes required for synthesizing amino acids for the aphid have been meticulously retained. This genomic streamlining reflects a complete commitment to a symbiotic lifestyle.
Broader Implications and Research Interest
The partnership between Buchnera and aphids has consequences for agriculture and ecological interactions. The nutritional support from Buchnera is a primary reason for the success of aphids, many of which are considered serious agricultural pests. By enabling aphids to thrive on plant sap, the symbiosis allows them to achieve large population sizes and cause widespread crop damage.
Buchnera can also play a role in how aphids transmit certain plant viruses. The bacteria produce a protein called symbionin that can bind to virus particles, protecting them as they circulate within the aphid’s body. This protection increases the likelihood that the virus will remain intact and be successfully transmitted to the next plant the aphid feeds on. Because of these factors, the Buchnera-aphid relationship is a research model for studying the evolution of symbiosis, host-microbe interactions, and genome reduction.