Author: Yejin Kang
Institution: Rice University
A common example of a parasitoid relationship, where an organism lies inside a single host organism for a significant part of its life history, is between the wasp and the aphid because parasitoid wasps lay eggs inside aphids. Aphids, commonly known as plant lice, are small plant-eating insects. When the eggs hatch, the larvae survive by consuming the aphid. It used to seem that the aphids had no defense mechanism against the parasitoid larvae, but scientists from the University of Arizona have discovered a method that allows the aphid to survive. Results of a study recently published in the August 2009 issue of Science show that virus-infected Hamiltonella defensa, a bacterium that resides inside aphids, can help confer survival.
Researchers compared aphids containing virus-infected H. defensa with uninfected H. defensa in a laboratory experiment. Eighty percent of the aphids containing the uninfected bacteria died while 90% of the aphids containing the virus-infected bacteria were able to survive. According to Kerry M. Oliver, this is a landmark finding because this is the first instance showing that virus- infected bacteria are actually beneficial. Surprisingly, the same virus is found in Escherichia coli in humans and actually makes the bacteria more harmful to human hosts.
Initially, the presence of bacteria was thought to be the resistance conferring mechanism since bacteria typically reside inside aphids in symbiotic relationships. For example, bacteria provide aphids with nutrients unavailable in nutrient-poor plant sap. Therefore, H. defensa were believed to be the source to confer wasp resistance. However, when the aphids were stored inside the lab, some strains of aphids maintained an H. defensa population yet became susceptible to wasp attacks.
Therefore, the scientists conducted further experiments to narrow down the source of wasp resistance to the Acyrthosiphon pisum secondary endosymbiont (APSE) virus. The potency of the virus comes from genes it carries that code for several toxins. Researchers also demonstrated that aphids, previously containing virus-infected bacteria but had lost the bacteria, also lost the ability of resistance as well. "In every instance where the virus was lost, protection was lost almost completely," said Kerry M. Oliver. The significance of the presence of virus in certain bacteria shows that the world functions at a minute level. "It really shows how complicated life is," Oliver said. "It's really a microbial world."