Aphids survive on infected bacteria

The phrase 'beneficial virus' sounds like an irrational thing. But for pea aphids being attacked by parasitic wasps, carrying infected bacteria is the difference between life and a slow death, according to a new study.

The hornets lay eggs inside aphids, and bee larvae feed on aphids that live from the inside out.

Lead author Kerry M. Oliver said: 'A parasitic death is a terrible outcome. Just like in the movie 'Alien' when an object grows inside you and goes out and eats you. ' In the experiments, about 80% of aphids carrying non-infectious Hamiltonella defensa died of wasps.

However, most aphids carrying H. defensa contain viruses that survive bee attacks.

Research is the first evidence that a virus infecting bacteria can help, not harm the host.

Oliver and colleagues conducted research at the University of Arizona, Tucson.

The researchers also tested the aphid strains that bacteria once infected but are no longer contagious.

'In all cases where the virus no longer exists, the protection is almost completely gone,' said Oliver, now a professor at the University of Georgie in Athens.

The virus, called APSE, contains genes that code for toxins that researchers think are involved in defense against wasps.

In contrast, being infected by viruses containing toxin codes often causes pathogenic bacteria such as E. coli to be more harmful, not less, for human hosts.

Biologists call the APSE virus DNA part that encodes toxins as 'flexible gene components'. The virus can move this flexible genetic component between single bacteria and between different bacterial species.

The flexible gene component can combine with the recipient's DNA, making them capable of producing toxins.

Converting DNA from one species to another through a flexible genetic component is quite different from that of parents transmitting genetic material to their children. In animals, DNA fragments often do not jump from a mature individual's genetic material to the genetic material of another adult individual.

Moran said: 'The most interesting thing for me is that you have the choice and adaptation to the resistance of wasps that appear in one species, and suddenly, it can appear in another species'. .

Pea aphids are agricultural pests and Aphidius ervi, wasps that the researchers tested, were used to control aphids.

Their research also revealed that after biological control of aphids with wasps is sometimes successful and sometimes unsuccessful.

'Our research shows that it depends on the virus that bacteria have.'

Oliver and colleagues at UA, Patrick H. Degnan, Martha S. Hunter and Nancy A. Moran, will publish their findings in Science.

Picture 1 of Aphids survive on infected bacteria Parasitic wasps, Aphidius ervi, chased aphid aphids so they could lay eggs inside them.(Photo: Kerry M. Oliver)

Moran, professor of evolutionary biology and biology at UA Regents, studied the role of internal symbiotic bacteria on bean aphids Acyrthosiphon pisum for more than 15 years.

Aphids and other pests that eat sap usually contain some of these bacteria. Some, called major symbionts, provide aphids with essential nutrients not found in sap.

Aphids cannot survive without these biological objects, and public objects cannot live outside aphids.

The relationship between aphids and major symbiosis is so close that bacteria live inside specialized cells inside aphids.

In addition, aphids often contain other bacteria called secondary symbionts. These symbionts are necessary for the ability to survive and reproduce under certain conditions, such as the presence of an enemy.

Oliver, working with Hunter and Moran, discovered that aphids carry a minor symbiont Hamiltonella defensa that is resistant to wasps.

But when aphids were kept in the laboratory for generations without contact with wasps, some strains lost their ability to resist bee attacks.

In fact, those aphids still contain H. defensa, but these bacteria have lost the APSE virus.

To eliminate the possibility that genetic differences between aphids or bacteria are the cause of low resistance to wasps, researchers have done another experiment.

The team compared aphids that carry H. defensa with the APSE virus with the same strain of aphids that contain the same H. defensa strain but not the virus.

When exposed to bees, about 90% of aphids with infectious viruses survived. Meanwhile, aphids without viruses often have catastrophic outcomes.

Oliver said: 'This shows the complexity of life'.

The National Science Foundation, the US Department of Agriculture and the National Institutes of Health have funded research.