Symbionts

Symbiosis

The Host

aphid

Aphids are plant pests, capable of causing major agricultural damage. They feed on plants by piercing them with syringe-like mouth parts and sucking the sap out of the phloem, resulting in a diet that is rich in carbohydrates and deficient in amino acids. Some of these amino acids cannot be synthesized by the insect but are supplied by the intracellular symbionts Buchnera aphidicola. The interaction of the two partners dates back 150 to 250 million years and both have become so dependent on each other that under natural conditions they cannot exist without each other.

The Symbionts Supply the Host with Essential Amino Acids

Because plant sap contains little protein and aphids cannot produce ten amino acids (called essential amino acids), it was thought that if the aphids feed exclusively on plants then the required amino acids must come from their symbiotic bacteria.

This hypthesis was investigated using several different approaches. On a growth medium without amino acids, aphids can grow and reproduce. However, if an antibiotic that kills bacteria is added to the medium, the aphids fail to grow and reproduce. This suggests that bacterial symbionts are essential in supplying the amino acids. This was further supported using labelled sulfur or nitrogen compounds that showed amino acids containing these labels appearing in the symbionts and being provided to the host. The symbionts genome also reflects this biosynthetic activity. Buchnera aphidicola, the symbiont of Schizaphis graminum, carries on a plasmid the two genes trpEG which are important in tryptophan synthesis. Each bacterium contains three or four plasmids that contain four tandem repeats of these genes, resulting in 12 to 16 copies of trpEG.

The Host Supplies Symbiont Nutrients

The intracellular location of the symbionts requires that the host supplies the bacteria with energy, carbon, and nitrogen. One amino acid, gluatmine, is very abundant in the phloem. It was shown that glutamine is ingested by the aphid and transported to the cells in which the symbionts are housed (bacteriocytes). The bacteriocytes take up glutamine, convert it to glutamic acid, that in turn is taken up by the bacterial symbiont. The nitrogen from glutamic acid is then used to synthesize the other amino acids which are ultimately utilized by the host animal. This cycling of amino acids permits the growth and reproduction of aphids.

Location of the Symbionts

symbiont in host cell The symbionts are found inside host cells (intracellular) that are called bacteriocytes (also called mycetocytes). Each symbiont is surrounded by a membrane derived from the host cell that forms a vesicle called the symbiosome. The amino acids produced by the bacterial symbiont are thought to be released and taken up by the host cells. Digestion or destruction of the symbionts does not usually occur except during specific developmental stages. Being intracellular symbionts, the bacteria rely on the host to ensure transmission to the next generation. In several aspects, these intracellular bacteria resemble cellular organelles. The study of this system, as well as other symbioses with intracellular symbionts, may aid in understanding how mitochondria and plastids became an integral part of eukaryotic cells.

Genome Sequence of the Symbiont

The sequence of the symbiont's genome was determined. Its analysis revealed that bacteria carried the genes required for the biosynthesis of amino acids that the host could not synthesize but lacked the genes needed for the biosynthesis of non-essential amino acids. The symbiont also lacked many other genes that are commonly found in free-living or facultative intracellular bacteria. This suggests that the symbionts and host have coevolved to such an extend that they can only live in each others presence. The availability of the sequence will enable researchers to address a wide range of questions using powerful genomic approaches.

Evolution of Bacterial Symbionts and Aphids

The association between Buchnera and aphids is a mutualism, which was probably established 150 to 250 million years ago. At that time, an ancestor of Buchnera infected an aphid ancestor. An important aspect in the evolution of this symbiosis is the vertical transmission of the symbionts (from parent to offspring), indicating coevolution of the bacteria and host. This coevolution is apparent in the similarity between the branching patterns of the aphid phylogenetic tree and those of the symbionts.

phylogenetic tree

Investigators

Paul Baumann's group (University of California, Davis) identified the bacterial symbiont using 16S rRNA gene sequences and used molecular techniques to isolate genes and plasmids from the symbiont. These techniques are essential in studying this system because the symbionts cannot be cultured outside the host.

Angela E. Douglas is at the University of York, York, U.K. Her group provided the foundation for understanding the nutritional contribution of the symbiont to the host.

Nancy A. Moran, an evolutionary biologist, investigates the evolution of the aphid symbiosis.