The humble aphid may appear unremarkable, but it is cleverly adapted to its plant-sucking way of life. Symbiotic bacteria greatly increase the ability of plant lice, also known as greenflies, to grow and reproduce, causing annoyance to gardeners and damage to crops worldwide.
Plant Sap Provides Insufficient Nutrients
In general, aphids disregard the flowers, fruits and vegetables that humans value so highly. These insects feed on plant sap – a sugary fluid that flows within the stems and leaves of plants in much the same manner as blood flows around the bodies of animals. Unlike blood, however, plant sap contains little in the way of vital nutrients. For example, certain “essential” amino acids (the building blocks of protein) are largely absent.
In common with other animals, aphids cannot produce essential amino acids within their own bodies, but as any rose-grower will testify, they can reproduce at a spectacular rate. How is this possible, if their diet does not supply all the nutrients they need for growth? The answer is a special relationship that has evolved between aphids and bacteria.
What is an Endosymbiosis?
A symbiosis is defined as an interaction between two different species that is beneficial to both. In an endosymbiosis, the entire body of one partner (the symbiont) is located inside the body of the other (the host).
When the symbiont is a micro-organism such as a bacterium, it may lose characteristics (e.g. the ability to move) that it needs for a free-living existence, and will die if it is separated from its host. For this reason, it can be difficult to demonstrate that both partners in an endosymbiosis benefit in terms of producing more offspring together than they would apart.
It has instead been suggested that endosymbiosis should be defined an association in which the host benefits from metabolic abilities possessed by the symbiont that the host lacks. In the case of aphids, endosymbiotic bacteria are thought to produce essential amino acids that the aphid uses to build proteins.
Endosymbioses Common in Insects
Aphids are not the only insects with endosymbionts. In fact, most insect species contain micro-organisms within their gut or floating free in their body fluids. Many species have been shown to depend on their symbionts for supplies of vitamins or enzymes.
Aphid endosymbionts are found inside special organs called mycetomes. When an aphid reproduces, the endosymbionts are passed on to the mycetomes of its offspring. Other species with mycetomes include lice, termites and beetles. When the endosymbionts are removed experimentally, these insects grow slowly and die prematurely, suggesting that they are highly dependent on the bacteria.
Aphids More Than Just Pest Insects
An aphid infestation is an annoyance to the gardener, but these apparently unremarkable insects are fascinating creatures in their own right. The study of aphid endosymbionts has cast light on insect evolution, and in the future may provide a way of controlling pest insects and the diseases they transmit.
Douglas AE, Prosser WA. Synthesis of the Essential Amino Acid Tryptophan in the Pea Aphid (Acyrthosiphon pisum) Symbiosis. J Insect Physiol 1992; 38: 565–8.
Douglas AE, Smith DC. Are Endosymbioses Mutualistic? Trends Ecol Evol 1989; 4: 350-2.