The world of insects can get pretty strange when it comes to the various exotic ways of survival and procreation for these tiny terrorists. For example, Aphidius ervi, a wasp, injects its eggs directly into aphids. When the eggs hatch, the larvae eat the aphid from the inside out.
Acyrthosiphon pisum, the pea aphid, has an unusual defense mechanism against this attack. Some aphids are infected with the Hamiltonella defensa bacteria and with the APSE (A.pisum secondary endosymbiont) virus at the same time. When combined, both micro-organisms work together in the aphid to produce a toxic protein which kills the wasp larvae over 90% of the time.
Unfortunately for the pea aphid, the virus frequently disappears from the scene outside lab conditions, leaving the aphid vulnerable.
In lab conditions, the alliance between bacterium and virus is a very stable one, but in the wild, H.defensa has a bad habit of spontaneously losing its all-important phage lodger. Even within a single aphid, only some bacteria have viral genes integrated into their own. No one knows why the virus should disappear so frequently. Perhaps it’s not an entirely welcome tenant and its presence carries some sort of cost that occasionally outweighs its benefits. Perhaps only phage-free H.defensa can be passed on from aphid to aphid.
The bacteria is essentially useless for defense without the virus.
Faced with an enemy that threatens them all, the virus, the bacterium and the aphid have formed an evolutionary alliance, with infection as its foundation.
Bacteriophages currently play an important role for studying genetics in the laboratory. Their ability to infect bacteria may allow them to play a future role in defeating multi drug resistant strains of disease causing bacteria.