Scientists have identified the first examples of insect genes directly involved in the creation of galls — intricate shelters that thousands of species of flies, wasps and aphids make by manipulating the development of plant tissue —  shedding light on a process that is not yet fully understood. 

How insects produce galls has perplexed entomologists and developmental biologists for centuries. Some research has linked galls to certain hormones, but the specific genetic and biochemical pathways for gall formation remain largely unknown. The findings of a study published March 2 in Current Biology bring scientists one step closer to solving this great mystery of the insect world.

“This feels like one of the greatest unsolved problems in biology,” said lead author David Stern, a group leader at the Howard Hughes Medical Institute’s Janelia Research Campus. “How does an organism of one kingdom take control of the genome of an organism in another kingdom to completely reorganize its development to produce a home for itself?”

Insects use the galls they produce as a hiding place for development or reproduction, and they take on a form unique to each insect that creates it. Galls are so specialized that they can often be used to identify the insect that created them without ever collecting an actual specimen.

Some galls, like the common oak gall, are simple spheres. Others can take on intricate shapes, like the rose-shaped galls produced by Rabdophaga rosaria mites. In all cases, the changes that gall-making insects produce in plants are massive, completely overhauling the appearance of plant tissue — but only in the place the gall occurs.

“When we looked at the genome of the plant and compared gall tissue to healthy tissue, we found that two-thirds of the genes of the plant change their expression patterns in the gall,” Stern told The Academic Times. “Two-thirds is massive — a complete reprogramming of plant cell biology. It’s basically like the gall is a different organism under the thumb of the insect.”

Because many insects inject saliva into plants to stimulate gall formation, Stern and his team looked to the salivary glands of aphids, which produce cone-shaped galls on the leaves of witch hazel trees, to look for gall-producing genes.

The researchers identified a group of about 450 genes in the salivary glands that were only activated in aphids that produced galls. The team dubbed these genes BICYCLE genes, after a characteristic pattern of cysteine-tyrosine-cysteine, or CYC, amino acids found in the proteins produced by these genes.

Identifying these genes served as a starting point, but what was missing was a link to a specific component of a gall. To do this, Stern took inspiration from an observation he had made years earlier in the woods around the Janelia campus — that witch hazel galls come in two colors, red and green.

“Having two colors of gall is really a geneticist’s dream because it’s easy to make comparisons,” said Stern.“When we looked at all the gene variants in aphids producing these different-colored galls, we found that there were just a handful of variants that were very strongly linked to gall color, and they all occurred in one gene.”

Appropriately, the team called this gene the determinant of gall color gene. And as the final piece of the puzzle, the researchers discovered that this gene was one of the BICYCLE genes they had identified.

“This is the strongest evidence we have that this gene is really controlling the gall. I was shocked that the signal was so strong,” Stern said.

The team verified its findings by further comparing the genes of red and green gall tissue.

“This was another surprisingly strong signal,” said Stern. “We expected to find hundreds of genes that were different, but we only found eight. And all of them were involved in the biochemical pathways producing red pigments.”

The team is next interested in identifying the plant molecules targeted by proteins produced by BICYCLE proteins, finding more links between the genes and specific aspects of gall formation. But for now, the research represents a promising first step toward answering a question that for years has confounded scientists, as well as Stern himself.

“I’ve been thinking about these galls since I was a graduate student in Malaysia 30 years ago, and scientists have been fascinated by them for centuries,” Stern said. “I’ve always had these aphids in the back of my mind, and it’s a pleasure unlike anything I’ve ever experienced in science to make progress on this deep mystery. “

The paper, “A novel family of secreted insect proteins linked to plant gall development,” was published March 2 in Current Biology. The authors of the study were Aishwarya Korgaonkar, Clair Han, Andrew L. Lemire, Igor Siwanowicz and David L. Stern, Janelia Research Campus of the Howard Hughes Medical Institute; Djawed Bennouna and Rachel Kopec, The Ohio State University; Peter Andolfatto, Columbia University; and Shuji Shigenobu, National Institute for Basic Biology, Okazaki.