By: Paul Cavalho
Imagine a mama bear grabbing you and shoving its fertilized egg inside you. The baby bear starts to develop and slowly eat you from the inside until you give birth. Although it might look cute, you probably want to kill it for what is has done to you. Just when you think this traumatic experience is over, you immediately get a virus and now you’re paralyzed. Your feet are planted over this bear cub, protecting it from other creatures that attempt to prey on it. Finally, the cub can fend for itself and waltzes off without a thank you. You are free at last.
Wow what a horrific thought…unless you’re Chuck Norris, then of course you would be shoving your baby inside the bear. On a serious note, welcome to the trials and tribulations of the spotted lady beetle, lady bird or ladybug whichever you prefer.
Ladybugs (Coleomegilla maculata) are hosts to a parasitoid wasp Dinocampus coccinellae, both pictured here.
Female wasps deposit their eggs inside the ladybug through an organ called an ovipositor. A single wasp larva will develop inside a ladybug over the course of 20 days, feeding on nutrients inside the host. Just after this larval development period, the larva becomes a prepupa which will exit via the segmented abdomen of the ladybug. The prepupa spins a cocoon between the ladybugs legs and the ladybug exhibits a change in behavior. That’s right…the ladybug is still alive, but seems to be almost paralyzed. The ladybug grasps the cocoon but still displays random movements, especially when it’s disturbed. This is known as bodyguard manipulation.
The most amazing part of this behavioral manipulation is that the ladybug no longer has the parasitoid wasp inside its body. Also, the effects are strangely timed with that of the adult wasp breaking out of the cocoon. Once the adult wasp flies away, some ladybugs fully recover and even go on to reproduce – more little ladybugs, not more parasitoid wasps.
Other species of parasitoid wasps have been known to use polydnaviruses – viruses which the whole virus genome is embedded within the wasp’s genome. Polydnaviruses are usually injected into the host simultaneously with the eggs, and affect the host’s immune system immediately. This type of virus appears to be absent from the D. coccinellae genome!
So how are these wasps manipulating their hosts?
A group of scientists observing this phenomenon discovered a new type of virus inside the head region of parasitized ladybugs; they named it Dinocampus coccinellae paralysis virus (DcPV). The researchers hypothesized that DcPV is associated with Dinocampus coccinellae and infects the nervous system of the ladybugs, contributing to behavioral manipulation after the prepupa emerges from the ladybugs abdomen.
The researchers found DcPV in all adult wasps and larvae tested, but this virus’s genome was not embedded within the D. coccinellae genome. This information suggests DcPV is living inside the wasp symbiotically. Electron microscopy of cells in wasp ovaries and oviducts revealed small compartments or vesicles inside cells housing the virus in a very orderly fashion as seen below. They also found that the adult wasps harbored more non-replicating forms of the virus, but replicating DcPV was more prevalent in larva that was cut out of ladybugs during parasitism.
Ladybugs were exposed to parasitism by D. coccinellae in a lab setting, and the heads and abdomens were sampled at 5, 13 and 20 days after the wasp deposited its eggs. At 20 days after, just before prepupa crawl out of the ladybugs , DcPV levels were much higher than at 5 and 13 days after, suggesting DcPV plays a role in manipulating host behavior.
The results from these studies suggest DcPV is associated with D. coccinellae and plays an important role in bodyguard manipulation, but researchers say there is much more research to be completed before a definitive conclusion can be made about the role of DcPV.
And now I will apologize for putting that initial image in your head, I hope you can recover.
Dheilly, N.M., F. Maure, M. Ravallec, R. Galinier, J. Doyon, D. Duval, L. Leger, A.N. Volkoff, D. Misse, S. Nidelet, V. Demolombe, J. Brodeur, B. Gourbal, F. Thomas, and G. Mitta. 2015. Who is the puppet master? Replication of a parasitic wasp-associated virus correlates with host behaviour manipulation. Proceedings of the Royal Society B 282:20142773.
Koyama, S., C.E. Thomas, and T. Mamoru. 2013. Relationship between the size of the parasitoid wasp Dinocampus coccinellae (Hymenoptera: Braconidae) and host ladybird species (Coleoptera: Coccinellidae). Trends in Entomology 9:39-43.
Maure, F., J. Doyon, F. Thomas, and J. Brodeur. 2014. Host behaviour manipulation as an evolutionary route towards attenuation of parasitoid virulence. Journal of Evolutionary Biology 27:2871-2875.