Mutualism of the Month: Aphids and a protective bacteriophage

Every month we showcase a relationship between two or more species or groups of species that can be considered a mutualism: a relationship where both members benefit. This month’s mutualism is the relationship between a group of sap-sucking insects and a bacteriophage.

A typical bacteriophage. Bacteriophages attach to their host with their tail fibers and inject their genetic material stored in their head. Photo source.

A typical bacteriophage. Bacteriophages attach to their host with their tail fibers and inject their genetic material stored in their head. Photo source.

Before getting into the relationship, it’s important to understand what a bacteriophage is. A bacteriophage is a virus that infects bacteria, replicates within the bacteria, and then lyses (explodes) the bacteria sending their newly formed virions out to infect more bacteria (the name bacteriophage roughly translates from Greek to devourer of bacteria). Bacteriophages inject their genetic material into the body of a bacteria. There, the genetic material is replicated by the infected bacteria. After the bacteria have replicated the “desired” amount of virus, a lysing agent is produced (coded within the genetic material of the bacteriophage). This lysing agent destroys the cell membrane of the bacteria releasing the bacteriophage into the surroundings where they can infect more bacteria. Other bacteriophages incorporate their genetic material directly into the genetic material of the bacteria and lay dormant. The bacteria continue replicating and the virus is replicated along with the bacteria’s genetic material, sometimes for thousands or more generations. Once host health deteriorates, the bacteria is forced to replicate the viruses’ parts and once they get to a critical mass the viruses lyse the cell and infect more bacteria (think rats leaving a sinking ship). Check out the YouTube video below for a hilarious explanation of the two “life”-cycles of bacteriophages.

Some bacteria provide benefits to their host bacterium by adding new functions during dormancy (this is called lysogenic conversion). For example, some bacteria become more virulent (toxic) when infected with bacteriophages. This is the case with the bacteria Hamiltonella defensa, which infects the pea aphid Acyrthosiphon pisum.

An adult pea aphid sucking the sap out of a plant. Alongside are several nymphs (younger stage), Photo source.

An adult pea aphid sucking the sap out of a plant. Alongside are several nymphs (younger stage), Photo source.

Pea aphids are parasitic insects, sucking the sap out of legumes (peas and beans family). One particularly strong control of aphids is parasitic wasps, which lay eggs within the aphids. For another Mutualism of the Month involving parasitic wasps, click here. The wasps inject a single egg into each aphid with a small amount of venom that stunts the growth of the aphid’s own ovaries, stopping the host from “wasting” energy on its own reproduction so there's more food for the wasp larvae. The larvae of the wasps grow up, and like the bacteriophage, burst out of their host when development is complete, killing the host aphid in the process. The bacterium H. defensa has been shown to protect the aphids from infection by killing developing wasp larvae or embryos in the aphid’s body cavity. Through reducing the mortality after wasp injection, the pea aphids relies heavily on its symbiotic bacteria for protection, which is transferred from parent to young or during sex. In return, the bacteria are provided with a home, full of nutrients, where they don't have to compete with the billions of other bacteria.

A bacteriophage, similar to the that infects Hamiltonella defensa. Photo source.

A bacteriophage, similar to the that infects Hamiltonella defensa. Photo source.

Further studies have shown that the power of H. defensa to protect the pea aphid vary and until recently it was unknown as to why. It turns out that there is a bacteriophage that infects the bacteria, which inserts its genetic material into the bacteria’s DNA. Through lysogenic conversion the bacteria becomes more toxic to the wasp larvae. Uninfected bacteria confer little protection to the aphids, but some variants infected by the bacteriophage can be nearly 100% effective at dispatching wasps.

While this relationship is spectacular enough, scientists have discovered yet another dimension to this relationship. The parasitic wasps have developed a way of detecting which aphids are infected by H. defensa and which are defenseless. Those aphids that have the protective bacterium are more likely to receive multiple eggs. The wasps seem to boost their odds by increasing the number of larvae. How wasps detect infected aphids is still unknown, but there are several postulations. Perhaps, during the laying of the first egg, the aphid’s internal body chemistry is sampled. Infected aphids have been found to release less alarm pheromones than their non-infected conspecifics; maybe the lack of pheromones tips off the wasps.

A parasitic wasp injecting an aphid with an egg. The egg will grow up into a larvae, consume the insides of the aphid as it grows and burst out once it is mature. Very "Alien"-esque. Photo source.

A parasitic wasp injecting an aphid with an egg. The egg will grow up into a larvae, consume the insides of the aphid as it grows and burst out once it is mature. Very "Alien"-esque. Photo source.

This type of mutualistic relationship with a bacteria protecting its host from parasite infection is one in an expanding field. Look forward to seeing more relationships similar to this! To read a scientific article summarizing the relationship check out this paper from the journal: FEMS Microbiology Ecology.

Don’t forget to check out more mutualisms of the month and all the other great articles on FTDM! Stay hungry!

Check out more mutualisms of the month below!