According to the National Human Genome Research Institute, about 300 different species have had their entire genomes sequenced. Us, obviously, but also rats, puffer fish, fruit flies, sea squirts, roundworms, chickens, dogs, yeast, honey bees, gorillas, chimpanzees, sea urchins, a bunch of bacteria, and many assorted other birds, plants, animals, and fungi.
New to the list is the orb-weaver spider Nephila clavipes. Analysis of this spider’s genome hints at how spider silk evolved, helping us to understand the whole system better and bringing us that much closer to our ultimate goal of one day making super-strong spider silk to achieve our own ends. (Mwhahahaha… )
Orb weavers, the kind that weave circular webs, comprise the third largest family of spiders: about 3,000 species. Each female orb weaver can produce different kinds of silk in her different kinds of silk glands. The silk used for draglines, bridges, and web radii has great tensile strength. The silk used for prey wrapping and egg-case insulation is strong yet flexible. The silk used for prey capture is sticky and viscous.
Spider silks can be “stronger than steel and tougher than Kevlon,” in the words of the University of Pennsylvania researchers who just reported the new genome, “yet are much lighter weight than these manmade materials.” They can conduct electricity, are resilient to temperature fluctuations, have antibiotic properties, and are undetectable by our immune system. Hence our desire to figure out how to make them, for future medical and industrial uses.
Spider silks are made of proteins called “spidroins,” for spider fibroins. The orb weaver’s genome provides the first compilation of all the spidroins in a given species, and it offers some surprises. First off, the genome includes eight as-yet-unreported spidroins for a total of 28.
Historically, as spidroins were discovered, they were named after the particular silk gland in which they were first found. But looking at gene expression revealed that each silk gland produced spidroins for more than one type of silk, and sometimes spidroins showed up in glands distinct from their namesakes.
One of the novel spidroins was expressed exclusively in venom glands, suggesting that it may have a function “beyond silk-related applications.” Much like spider silk, spider venom is a complicated mixture of proteins. The production of both are ancient and defining characteristics of orb-weaving spiders.
Spidroins have a shared beginning and end, but vary in the middle. The middle of the protein is comprised of pieces from a set of 400 different motifs. Each spidroin contains a different combination of these motifs mixed and matched in different orders and with different frequencies, and this variation confers their specific and different physical properties. This observation lends credence to the idea that the spidroin genes evolved by means of tandem duplication events, wherein these motifs got copied.
And the beginnings and ends of the molecules, while similar enough to stay recognizable, are more variable than most other genes. These two properties—a hypervariable middle section sandwiched between two moderately variable regions—make for a highly plastic protein molecule that can evolve fairly rapidly. The researchers suggest that the spidroin gene family is still evolving, meaning we’d probably learn more by looking through the genomes of some more spiders.