Two parasites, the love vine and the gall wasp, are both hosted by one species of oak tree, but on August 20 in the journal Current Biology, researchers at Rice University describe a new interaction between them. Rather than thriving at the expense of the oak alone, the vine leaches nutrients and moisture away from the wasp larvae developing in the tree. While the young insects mature, the oak grows tumors around them, but the vine seeks out these unusual growths to take out the next generation of wasps.
"I've been studying gall wasps and their interactions with their hosts and natural enemies from the southern tip of Florida to the southern edge of Texas for over a decade but had never observed this interaction between the wasps and parasitic vine," says first author Scott Egan, an evolutionary biologist at Rice University. "We have discovered a new interaction between plant and insect parasites when the two exist on a shared host. Most notably, the vine parasite is directly influencing the fitness and survival of the insect parasite."
Love vines attack many tree species across Florida, including live oaks, and use a specialized root structure to draw nutrients and water away from the host. Gall wasps lay their eggs into developing tissues of live oaks, like new stems or buds, and that egg-laying behavior causes the tree to grow tumor-like structures called galls around the hatching larvae. The larvae then parasitically feed on the tree from within the galls until they emerge as adults. Previously, the researchers thought that while the two parasites could share the landscape of a single oak tree, they would not directly interact.
However, when a wasp causes a gall to form on an oak where a love vine is living, the vine actively seeks out the unusual growth and parasitizes it the same way it does the oak itself--it attaches its suction-cup-like roots to the gall and sucks the nutrients and water away from the growing young wasp. But it doesn't kill the larva right away. The wasp continues to fully develop into an adult, but then dries out, is mummified, and dies because of this interaction.
"When the students in my lab brought me this unusual gall, we initially thought the growth was just a seed from the vine that we'd accidentally collected, but when we dissected it, we realized that it was actually a modified gall that still had the wasp inside, and when we went back into the field, we found them all over the place," says Egan.
Next, he wants to find out just how the vines find the galls. "We know that the vine's root structures are targeting the gall tissues specifically because they're attaching to galls located in regions of the tree the vine normally doesn't attach to," Egan says. "This suggests that the vine may have some kind of searching mechanism or that there's something special about the gall that's drawing them in."
Given the vast number of parasitic plant and gall wasp species across the planet, Egan finds it likely that this parasite-parasite interaction may be quite common and is interested in finding new examples of this interaction. But he is also interested in the agricultural and medical applications of his discovery.
"Understanding the interaction between these two parasites may help farmers to drive away either or both of these pests," he says. "And if this vine is detecting a tumor on the host plant, we want to know how, as this might have implications for cancer research."
This research was funded by Rice University.
Current Biology, Egan et al.: "Botanical parasitism of an insect by a parasitic plant" https://www.cell.com/current-biology/fulltext/S0960-9822(18)30815-7
Current Biology (@CurrentBiology), published by Cell Press, is a bimonthly journal that features papers across all areas of biology. Current Biology strives to foster communication across fields of biology, both by publishing important findings of general interest and through highly accessible front matter for non-specialists. Visit: http://www.