UMBC postdoctoral fellow Sarah Stellwagen and co-author Rebecca Renberg of the Army Research Lab have released the first complete sequences of two genes that allow spiders to produce glue – a sticky, modified version of spider silk containing the spiders keeps booty in the net. The results appeared in Genes, Genomes, Genetics .
The innovative method they used might pave the way for others to sequence more silks and sizing genes, which are difficult to sequence due to their length and repeating structure. A better understanding of these genes could bring scientists closer to the next major advance in biomaterials.
Spider silk is what makes spider webs, and for years it has been touted as the next big thing in biomaterials with its unusual tensile strength combined with its flexibility. There are more than 45,000 known species of spiders, each producing between one and seven types of silk. Despite many subsequences, less is known about the complete genetic structure of spider silk: only about 20 complete genes were sequenced. "Twenty pale compared to what's out there," says Stellwagen.
It has also proved difficult to produce spider silk in large quantities. Spiders transform liquid silk blobs into solid, spindle-shaped fibers in a complex process. Scientists can produce the liquid, but "we can not repeat the process of transition from liquid to solid on an industrial scale," says Stellwagen.
However, spider glue is a liquid both inside and outside the spider. While the glue "has its own challenges," says Stellwagen, this difference could make it easier to make spider glue in the lab than silk.
Stellwagen sees great potential for spider glue applications as organic pest control. After all, she says, "This stuff has evolved to catch insects."
For example, farmers could spray the glue along a barn wall to protect their livestock from insects that bite or cause disease, and then rinse it off without infesting them worrying about polluting the waters with dangerous pesticides. They could similarly use glue to protect crops from pests. It could also be used in areas where mosquito-borne diseases are prevalent. "It could just be fun to play with," says Stellwagen.
A "monster of a gene"
Before Stellwagen and Renberg's work, funded by the Army Research Lab, was about the longest sequence of silk genes 20,000 base pairs. When she started this project, Stellwagen expected that she would quickly sequence the glue genes and then continue, building on what she had learned from the sequence. Instead, she and Renberg needed two years to complete the sequence.
"It was ultimately this monstrosity of a gene that is more than twice the size of the largest silk gene ever," says Stellwagen. It was a long, hard road to the day she found Renberg in the lab and said, "I think our gene is 42,000 bases long, so I think we ended it." And in the end, it took the risk of state-of-the-art technology, which ultimately provided the complete sequence.
Not only was the gene exceptionally long, but like spider silk genes it has many repeats of the same sequence on bases ̵
However, when your gene repeats, you need a single sequence or "reading" extending from the repetitive region beyond the end to know how many repetitions there are. If your repetitive section is long, as studied in the glue grips Stellwagen and Renberg, the chance of getting the required reading with next-generation methods is low.
Fortunately, third generation sequencing techniques are now available. Third generation sequencing results in longer reads, but less. Only by repeating the experiment several times will you have the chance to get the readings you need to determine the number of repeats and finally define the entire sequence of the gene. "It's a challenge," says Stellwagen. "You pick a needle from a haystack."
But it worked. After Stellwagen and Renberg had not gone to the computer for two years and seen no positive results, they finally got the required readings to define the entire sequence of the gene.
Stellwagen is already thinking about the next steps. "How do silks look from other species after we have a protocol for the discovery of full-length silk genes?" ask her.
"I'm really excited that I finally solved the puzzle because it was just that hard," says Stellwagen. While it was a much bigger challenge than she had expected, "We ended up learning a lot, and I'm glad to bring that out to the next person trying to solve a ridiculous gene."