Caddisflies are among nature's master underwater builders, capable of spinning sticky silk that they use to form protective cases and webs in freshwater streams.
Scientists like the University of Utah's Russell Stewart have long studied this bioadhesive material in the hopes of using it as a chemical model for creating a synthetic version for use in the human body in medical applications. Now the genetics of caddisflies' evolutionary superpower is coming into focus, providing science with new clues for developing bioadhesives.
In his most recent study, Stewart worked with Brigham Young University biologists to zero in on a net-spinning species native to Utah called Arctopsyche grandis. The scientists focused on H-fibroin, a gene that produces the main protein in caddisfly silk with an eye toward determining how much this silk gene varies among individuals living in two nearby, but separate wild populations found in streams near BYU's Provo campus.
"It's an incredibly detailed look at how nature does polymer chemistry. We looked at the main silk protein from 18 individual caddisflies from the same species in two populations very close together. The heterogeneity in those genes was remarkable," said Stewart, an emeritus professor of biomedical engineering who has also conducted pioneering research into the natural glues created by marine sandcastle worms.
The results are reported in the journal Molecular Biology and Evolution.
Looking to nature for bio-inspired materials
Adhesives that work underwater would be incredibly useful, but such substances are notoriously hard to manufacture. Thanks to natural selection, however, many critters figured out millions of years ago how to produce substances to make stuff stick together.
These aquatic organisms are remarkable because their adhesives adhere underwater-something human-made adhesives and fibers struggle to do. Understanding how evolution naturally modifies caddisfly silk while preserving performance could lead to the development of new bio-inspired materials for medicine, engineering or underwater technologies.
"We started to look into the genes involved in silk production because of emerging techniques, especially now with genetic engineering, to make prototypes for bio-inspired materials," said co-author Paul Frandsen, a BYU evolutionary biologist. "The first step is to try to understand gene function. Russell's early work was really foundational to understanding exactly how the protein was folding and how it was adapted to life underwater."
Now an associate professor in BYU's Department of Plant & Wildlife Sciences, Frandsen came across Stewart's research-which was covered by The New York Times in 2010-while a graduate student and has since become a research collaborator.
In the years since, a University of Utah startup Stewart co-founded, Fluidx Medical Technology, has developed an embolic agent based on a synthetic version of sandcastle worm glue. That product, which forms targeted embolisms to cut off blood flow to a specific spot in the patient, has cleared clinical trials and is seeking approvals from the Food and Drug Administration.
Two different animals, same bio-adhesive superpower
Sandcastle worms and caddisfly are much different organisms inhabiting different underwater environments, but, in a clear example of convergent evolution, they both evolved the ability to produce underwater adhesives millions of years ago to build protective structures around their soft bodies.
"These animals are not related whatsoever, but we found the same chemistry in their adhesives, although the format is different," Stewart said. "The caddisfly pulls the glue out into a sticky fiber, like tape, whereas the sandcastle worm dabs the fluid adhesive onto the surfaces to be bonded."
Caddisflies hail from one of the most diverse orders in the animal kingdom, called Trichoptera-which is Latin for "hairy wing–a close relative to moths and silkworms. Caddisflies diverged 270 million years ago from the common ancestor they shared with silkworms.
"Moths make silk that works in air and caddisflies diverged to make silk in water," Stewart said.
Up to 17,000 species of caddisfly exist today, typically in freshwater streams, but the can survive almost everywhere including on land and in marine environments.
"Insects are extremely successful in freshwater and terrestrial ecosystems, but they are almost never found in the sea. However, one of the very few examples is a small group of caddisflies," Frandsen said.
Most caddisflies spend the first year of life in a larval state underwater, which is where their silk comes in. With each stage of larval development, they use the bioadhesive in different ways to survive.
"They build their cases out of materials that they gathered adventitiously from the stream where they live," Stewart said. "Different species use different materials. Some build their cases out of rocks or leaves or small sticks."
A genetic paradox
Sometimes the cases are portable. For some, the cases remain in fixed spots. Some even use the silk to create webs to catch food drifting in the stream. This insect is a favorite meal for trout, so fly fishermen often use lures that mimic the caddisfly species found in the streams they are fishing.
For Stewart and Frandsen's latest study, their team collected 18 A. grandis specimens from American Fork and Diamond Fork canyons in the Wasatch Mountains. Separated by about 40 miles, both streams drain into Utah Lake.
After sequencing the genomes of 18 caddisflies and analyzing 34 copies of the silk gene, the research team found a surprising amount of diversity in this small group. These insects carried 24 different versions of the gene with some versions resulting in silk fibers that were up to 25% longer or shorter than others. Although silk genes have been compared between species, this was the first study comparing silk genes within a single species of a natural population.
The findings suggest that the caddisfly silk gene evolves quickly and can tolerate a lot of variation without disrupting the silk's function, according to Frandsen. At the same time, the study found clear limits on what kinds of changes are possible, suggesting that certain features of the silk protein are essential for building strong, working capture nets.
"It's a bit of a paradox because the protein is ultra-diverse, but the features that make it diverse are very constrained," Frandsen said. "I've studied many types of genes, across many species of animals. This is one of the most interesting evolutionary stories I've seen, and it does have implications for designing a bioadhesive material. You have to take into account what this variation is promoting in terms of its material properties in the silk."
The results of the study will be critical for creating biosynthetic versions of the silk for technological applications.
This study was published June 3 in Molecular Biology and Evolution, under the title "On the dynamic caddisfly silk H-fibroin gene: a population study in a net-spinning species." The underlying research was funded by the National Science Foundation with additional support from the Deutsche Forschungsgemeinschaft and the Alexander von Humboldt Foundation. Co-authors include BYU students Ashlyn Powell and Gabriela Jijon, BYU faculty member Samantha Standring, and scientists with the American Museum of Natural History and Germany's Senckenberg Natural History Museum.
