Like a spider web, the evolution of spider silk proteins looks pretty complex. New research sheds some light on the evolution of these stretchy, sticky and tough proteins.
Everyone knows Spider-Man’s main (and only?) talent is shooting sticky liquid from his “web-shooters”. Often his webs take the form of a rope that is perfect for swinging through the streets of New York, but they can also be shaped like a net, amorphous globs or whip-like strands. The silk that real spiders weave is just as versatile. Depending on the application, it can have different mechanical properties. The silk they produce can range from sticky and stretchy threads for capturing prey to tough strands that form the web’s outer rim, or even aerial nets to capture flying prey!
Both spiders and Spider-Man can weave silk with different properties. Deinopis can weave nets and cast them to capture aerial prey!Source.
Spider silk is made of proteins called ‘spidroins’. Spidroins are huge proteins (> 3000 amino acids) and contain large stretches of repeated amino acids. Small amino acids like alanine, glycine and serine are repeated almost endlessly. The spidroins to fold into regular and tight structures thanks to this repetition. By mixing and matching different repeating ‘blocks’, the silk will get different properties.
The tight interactions between repeatable stretches give spider silk its mechanical properties. Source.
The repeating amino acids also cause major headaches for spider researchers everywhere, since the enzymes that are used for sequencing DNA get confused in the face of so much repetition. What’s more, since many spidroins evolved by the scrambling and rearrangement of entire repeated blocks, it’s very difficult to determine the course of spider silk evolution (scientists depend on steadily accumulating changes to reconstruct what happened in evolutionary history). If you consider that the earliest silk spinning spiders started diverging as much as 300 million years ago and that many silk proteins were duplicated and scrambled again, you’ve got a clear picture on how difficult the puzzle that is called spider silk evolution really is.
Luckily, the regions that flank the repeated region (the C-terminus and N-terminus) are evolving more steadily, making them more useful for figuring out what happened in spider silk evolution. So far, the C-terminus has always been used in these kind phylogenetic analyses because it is easier to sequence. But Jessica Garb and colleagues decided to also sequence and analyze the N-termini of many spidroins, since it is around 50% bigger*. You can find the the inferred phylogeny of the spidroins below.
Bayesian spidroin phylogeny. Inferred gene duplications are black circles on the branches, coloured crosses are inferred losses motifs and coloured squiggly lines are inferred gains.
The evolution of spider spidroins seems to be mainly associated with the evolution of their associated silk glands. As you can see in the tree above, the TuSp1, Flag and MiSp spidroins which are all expressed in separate glands cluster together. An exception are the MaSp spidroins, which are sprinkled all over the tree (top, middle and bottom). The authors suggest that several rounds of duplication and loss of a certain gland type could lead us to believe that while both gland and silk look similar to us, they’re actually not homologous (related by common descent).
The team also reports the discovery of an N-terminal domain in the group of Mygalomorphea (tarantulas and such) that has high sequenc similarity to N-terminal domains in the Areanomorphea (your garden spider). At the latest, these groups of spiders diverged 240 million years ago, indicating that the silk that spiders weave now is similar to the first silk that was first spun in the Triassic, a time when the earliest crocodiles and turtles started walking this earth!
Spiders: spinning silk since 240 Mya BC.
If you want to read more on transgenic silk production in silk worms and goat milk (!), check out this blogpost by Christina Agapakis on the Oscillator.
* While the inclusion of the N-termini in the analysis did not generate significantly different trees, the resolution with which some difficult branches could be resolved increased.
Garb JE, Ayoub NA, & Hayashi CY (2010). Untangling spider silk evolution with spidroin terminal domains. BMC evolutionary biology, 10 (1) PMID: 20696068
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“Everyone knows Spider-Man’s main (and only?) talent is shooting sticky liquid from his “web-shooters””
He can also climb surfaces with the tiny little hairs that have sprouted all over his skin, improved strength, ‘spider sense’, acrobatic skill. I’d be careful with saying his only skill is the web stuff. I’m not even a fan and I can think of those. Internet nerd rage can be vicious :)
On the actual post though…
Is the web made and stored or made to order so to speak? I’d never thought to find out myself.
Spiders usually have a reservoir of unspun silk ready. It is stored as a gel-like substance and needs some processing before it can be used in webs. This ‘raw silk’ is pulled through spinning ducts where water is removed and the silk proteins are aligned and interlinked, generating the strong silk fibers. I honestly don’t know if Spiderman uses similar techniques ;)