Parasitic wasps must lay their eggs in the right fly, or else

a fly on Gurania spinulosa / Luz Maria Huerto Santillan
Janet Fang 18 Mar 2014, 20:45
 
Scientists couldn’t figure out how so many species of flies and parasitic wasps could live in the same plants. They are, after all, a very specific host-parasite system: once the female wasp stabs the baby flies with her sharp genitals, the flies are doomed for a grisly death when the young wasps hatch, eat the flies’ insides, and burst out. If there's more than one wasp (or fly) species, wouldn’t they just outcompete (or out escape) the other? 
 
“We figured there would be just one wasp that would just hit every baby fly down the line in this niche," study researcher Marty Condon of Cornell College in Iowa tells Popular Mechanics. Now, a closer, molecular look at tropical, plant-eating flies and the parasitic wasps who lay eggs inside them reveals a stunning number of highly-specialized species who are all killing each other. 
 
Species tend to occupy a very particular ecological space containing the resources they need to survive and make babies, while simultaneously hiding from predators and beating competitors. Adapting to unique niches leads to species differentiation and ultimately diversity. It just doesn't make sense to have many species filling what appears to be the same niche.
 
To solve this very strange case of overlap, Condon and her colleagues studied thousands of insects whose larvae feed on the juicy parts of climbing squash flowers (genus Gurania) at a site in Peru. They collected 3,636 flowers yielding 1,478 fly pupae and used molecular methods to uncover just how many species are present, as well as how they interact. 
 
What they found was huge: 14 fly species and 18 wasp species, all occupying just 2 species of climbing squash. The Blepharoneura flies live off the plants, and the Bellopius wasps lay their eggs in the flies. Being a specialist should reduce the amount of overlap that you have with other species so you’re not fighting over the same stuff, “but we're finding lots and lots of fly species and lots and lots of parasitic wasp species in the same place,” study coauthor Andrew Forbes of the University of Iowa says in a press release
 
So they looked closely at fly larvae samples and found that unhatched wasps died if they were laid in the ‘wrong’ species of fly. Turns out, the wasps are extreme specialists: their offspring will only survive in one species of fly. “The prey is lethal to the predator,” Condon says in a press release. "My guess as to how the flies protect themselves against most of the wasps is through some type of immune reaction, but we don't exactly know," she adds.
 
All the species of wasps look essentially identical, and so do the flies. Even the wasps have trouble telling the flies apart, so what you end up with are predators attacking prey -- who turn out to be lethal to the predators’ offspring. “The reason so many species co-exist appears to be that each fly has a very specialized ability to escape all but one parasitic wasp species, and each wasp has a very specialized ability to kill one specific fly species,” Forbes explains. “Specialized interactions between species allow more species to live in the same place.”
 
The work was published in Science this week. 
 
 
Images: plant-eating fly by Luz Maria Huerto Santillan (top), parasitic wasp by Marty Condon (below)
 
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