Parasitic fig wasps of the genus Apocrypta lay their eggs inside unripe fruits of fig plants. Larvae that emerge from eggs of parasitic fig wasps feed on larvae of another species — the pollinating fig wasps. So, to ensure best nutrition and survival for its own eggs, a parasitic wasp female lays them near larvae of pollinating wasps already developing in the fig fruit.
To lay eggs inside, parasitic wasp females have to dig through the hard and woody skin of unripe figs several times in their lifespan. Evolution has gifted them with a long, flexible and slender egg-laying organ called the ovipositor, which they use to manage this feat. But there is more to an ovipositor that gives the wasps an edge. Using scanning electron and atomic force microscopes, researchers from the Indian Institute of Science (IISc) Bangalore, have found that the ovipositor has a zinc-enriched tip that’s serrated like a knife blade or a scissor.
Researcher and lead author of the work Namrata Gundiah says, “The wasps do not want the ovipositor tip to get worn out when it cuts through the fig substrate several times. To be able to do that the tip must be very hard. We found that the tip has a high amount of zinc and hypothesize that the presence of zinc increases its hardness.” Serrations enriched with zinc are used to cut through and remove fig material, she explains.
Dr. Gundiah and her co-worker compared the ovipositor of parasitic wasps with that of pollinating wasps – the species they parasitize. They found the serrated ovipositor tip of parasitic wasps to be almost thrice as hard as the ovipositor of pollinating wasps.
The ovipositor is not only softer but also shorter in pollinating wasps. Its tip is smooth with a “spoon-like structure,” they discovered, possibly because a pollinating wasp female need not repeatedly dig through unripe figs like its parasitic counterpart. It deposits eggs on flowers inside a fig through a natural cavity, called ostiole, which closes once eggs are laid. In the process the female loses her wings and dies, but pollinates the flowers with pollen she carried from the fig she grew up in.
“We saw a spoon-like morphology in the pollinating wasp, which needs to lay eggs on a very soft substrate – the flower. In contrast, the parasitic wasp has a hard drill-bit like tip in its extremely long and thin ovipositor,” she said.
In addition, the ovipositor tip of the two wasp species differs in the receptors they have. The thickness of the ovipositor tip is about 14 micro metres in a parasitic wasp and 9.5 micro metres in a pollinating wasp.
These structural and functional adaptations in ovipositors of parasitic and pollinating wasps have evolved in response to the different challenges they face while laying eggs, the researchers report in a paper published today (May 29) in The Journal of Experimental Biology.
(The author is a freelancer based in Bangalore)