There is nothing friendly about parasites. One, a parasitic wasp (Aphidius ervi), uses pea aphids (Acythrosiphon pisum) as growth chambers for its young, which consume the aphids from the inside before emerging from their hosts’ mummified husks. To ensure the aphid's body feeds her offspring well, a mother wasp injects venom along with her egg. The mother's venom is a cocktail of molecules that cause the aphid to stop reproducing and instead produce more nutrients for the developing wasp. The most abundant molecule in the venom is the protein Ae-γ-GT, which has been shown to cause the degradation of the reproductive organs of aphids. Scientists have determined the protein's role by isolating it and injecting large quantities into aphids, but previously no one had checked the effects of Ae-γ-GT on an aphid infected by the other components of the venom and the developing wasp. Elia Russo, Ilaria Di Lelio, Andrea Becchimanzi and Francesco Pennacchio of the University of Naples, Italy, with Min Shi of Jiaxing Nanhu University, China, decided to change that.
The team identified young wasps that were pupating in mummified aphids and injected the insects with bits of RNA designed to target and deactivate the genes encoding Ae-γ-GT. When the treated wasps emerged as adults, they could then produce all the components of their venom except for the Ae-γ-GT protein. The researchers provided treated and untreated wasps with aphids to infect and tracked the development of the wasps’ offspring and the destruction of their hosts. They then compared the two groups to learn more about the role of Ae-γ-GT.
As the scientists expected, aphids infected by the treated wasps did not lose their reproductive capabilities, but the scientists were surprised to observe other differences as well. The aphids that had not been exposed to Ae-γ-GT grew much larger than their counterparts, fueled by an explosion in the number of beneficial bacteria in their bodies. Aphids, which subsist on sap, carry a special type of bacteria (Buchnera aphidicola) that produce essential amino acids that would otherwise be absent from their sugar charged diet. Those amino acids are also important for developing wasps, and while previous studies involving untreated wasps had found that aphids had more bacteria after infection, removing Ae-γ-GT from the equation supercharged the bacterial proliferation.
Within these bacteria laden aphids, the developing wasps grew larger as well and ultimately emerged as bigger adults. At first glance, bigger children might seem like a win for the treated wasps, but the researchers found that those big offspring also died younger and produced fewer offspring of their own in comparison to the children of the untreated wasps. In the long run, wasps that fail to produce Ae-γ-GT would be left with fewer descendants. The molecules that a mother wasp injects along with her egg interact with each other and it appears that Ae-γ-GT limits the bacterial growth caused by some other molecule in the venom. This function may be even more important to the health of the developing wasp than shutting off the aphid's reproductive system. Isolating Ae-γ-GT allowed scientists to begin to study it, but the protein is a part of a mixture, and a mixture cannot be fully understood by studying its parts in isolation.
