One of the first twits that I received this morning was from Discover Magazine and was related to a post entitled “Parasitic wasps vaccinate aphids by spreading anti-wasp bacteria”. Of course the combination of vaccines and aphid is unusual considering that the insect immune system is devoid of memory and vaccination is actually a sort of immune priming rather than a true vaccination.
The interest in this paper is also hampered considering that aphids have a peculiar immune system since, as recently reviewed by Poirié and Coustau (2011), the immune deficiency (IMD) signalling pathway was apparently non functional in aphids and no genes coding for peptidoglycan recognition proteins (PGRPs) and several well-conserved antimicrobial peptides, such as defensins and cecropins, have been predicted in the pea aphid Acyrthosiphon pisum.
Actually the authors of the paper entitled “Parasitoids as vectors of facultative bacterial endosymbionts in aphids” do not refer to vaccination, even if they described a very intriguing event due to parasitoids! An example is Aphidius colemani showed in the Jarmo Holopainen photo from PBase.
Gehrer and Vorburger demonstrated ”that parasitoids can transfer endosymbionts of Aphis fabae aphids between clones by sequentially stabbing infected and uninfected aphids—a previously undescribed route of horizontal transmission. The wasp’s ovipositor appears to act as a ‘dirty needle’ that can inoculate previously uninfected aphids. If the recipient aphid resists the parasitoid and survives the attack, this can result in a new, heritable infection. Considering the fact that many aphid parasitoids use multiple hosts, it is likely that they can transfer endosymbionts not just within but also between aphid species.”
But that’s not all… the two bacterial symbionts that have been transferred by wasps are Hamiltonella defensa and Regiella insecticola and both have a protective function in aphids. Indeed, the first is known to increase aphid resistance to parasitoids whereas the latter protect aphids against fungal pathogens. The wasp-mediated transfer of H. defensa is very intriguing from an evolutionary point of view since wasps are spreading a symbiont (H. defensa), which provides aphids with protection against parasitoids (including wasps), so that wasp-mediated transfer is detrimental to their own fitness. This result could seem strange or self-damaging, but actually, as perfectly exemplified by Gehrer and Vorburger, wasps are simply using ‘dirty needles’ so that they are not self-damaging but just paying a price for their dirty work!
Gehrer, L., & Vorburger, C. (2012). Parasitoids as vectors of facultative bacterial endosymbionts in aphids. Biology Letters DOI: 10.1098/rsbl.2012.0144