Ichy Friday: on the evolution of parasitoid Hymenoptera
"Nature is so much worse than science fiction"
That's a quote from one of the students in my introductory entomology class, from earlier this week. That student was teaching about the Ichneumonidae - a hyper-diverse family of Hymenoptera, well known for their "parasitoid" behaviour. Yeah, the things they are do are quite, well, 'ichy' - and certainly as horrible as any science fiction movie (parasitoid, by the way, essentially means a 'parasite' which consumes/exploits its host, eventually killing it). As one example, there's a species that lives in the tundra of the Yukon territory, seeking out wolf spider females carrying egg sacs. The wasp female lays eggs into the egg sac of the wolf spider and the wasp larvae emerges and consumes the spiderlings. Ichy. Our research has documented that in some parts of the Yukon, parasitism rates can be remarkably high (Bowden & Buddle 2012). Here's a photo (by Crystal Ernst) of the wasp peeking out where one would normally expect to see spiderlings.
The student in my entomology course also showed this video - it too illustrates the rather incredible, yet somewhat grotesque, habits of some parasitoid wasps (um, not for the faint of heart).
By all accounts, the evolution of parasitism in the Hymenoptera has been remarkably successful. The Ichneumonidae alone are estimated to contain upwards of 60,000 species --> probably the most diverse family of organisms on the planet (Grimaldi & Engel discuss their diversity and evolution in great detail - interested readers should head there). And the Ichneumonidae are only one family in the Hymenoptera which are considered parasitoids.
Ok, back to our entomology lecture: the following question was posed: "what is the evolutionary history of this highly successful parasitoid behaviour?". Great question! And one (as usual) for which I did not know the answer. Thankfully two twitter friends, Miles and Laura, came to the rescue. Whitfield's (2003) paper is probably the most complete treatment of this question, and the answer seems to be the following: (I can't say it better than Whitfield, so I'm quoting him directly, here)
An earlier hypothesis on the origin of parasitism in Hymenoptera by Handlirsch (1906–8) seems to have hit the mark, however. Handlirsch noted the apparent morphological similarity between Siricoidea (horntails and woodwasps) and the putatively most primitive parasitic Apocrita, and the fact that the latter were often parasites of the former or of other wood-boring insects. He proposed a series of stages by which a gradual transition from wood-boring to ectoparasitism of wood-borers might have taken place. It is of interest in this regard to note that one relatively uncommon group of woodwasps, the Orussoidea, appears to have an ectoparasitic mode of life while still retaining much of the sawfly/ woodwasp morphology. (from Whitfield 2003)
So, stated another way, something that looked kind of like a wood wasp laid its egg near to a wood-boring host insect, and its larvae would have munched on that host instead of on plant tissue, and would have done so as an ecto-parasite (i.e., living outside of the host).
As Gauld (1988) argues, a primitive wasp, laying eggs in concealed plant tissue, could be putting its egg at risk due to the presence of other organisms (e.g., its future host) within the same tissue, so there could be a selective advantage in that egg being very, very close to that other arthropods... perhaps attached.... Gauld (1988) also argues that the switch from relatively semi-fluid plant tissues to semi-fluid insect tissues is a pretty easy one, from a physiological standpoint. So, voila: somewhere back in the Jurassic, natural selection favoured this parasitoid behaviour.
As a final note to this ichy post, the rather gruesome lifestyle of Hymenoptera parasitoids did not go unnoticed by Darwin: here's the quote:
"There seems to me too much misery in the world. I cannot persuade myself that a beneficent & omnipotent God would have designedly created the Ichneumonidæ with the express intention of their feeding within the living bodies of caterpillars..."
Charles Darwin, 1860
Bowden, J.J. and C.M. Buddle (2012). Egg sac parasitism of Arctic wolf spiders (Araneae: Lycosidae) from northwestern North America Journal of Arachnology DOI: 10.1636/P11-50.1
Gauld (1988). Evolutionary patterns of host utilization by Ichneumonoid parasitoids. Biol. J. Linnean Soc 35(4): 351-377.
Grimaldi, D and M. Engel 2005. Evolution of the Insects. Cambridge.
Whitfield JB (2003). Phylogenetic insights into the evolution of parasitism in hymenoptera. Advances in parasitology, 54, 69-100 PMID: 14711084