December 18, 2012
A few things I notice in this wondrous little film by "Jean-Michel" making the rounds of the social networks that I wouldn't have noticed before beginning work with Cay on Spider Silk:
There is a spiral scaffold already in place before this spider begins laying down the final capture spiral (the main action here). I know now that that spiral scaffold consists of a silk that is different from the final capture spiral silk. (And, of course--although, I didn't know this "of course" before Spider Silk--the frame lines and radial lines consist of yet another different silk.)
The web isn't really symmetrical. Notice the spider reversing course and making extra passes at the bottom of the web.
When the spider gets back to the center of the web, the hub, it eats the silk that made up the hub.
If you'd like to see more examples of web construction, check out Samuel Zschokke's amazing site.
November 25, 2012
Last summer, London community radio chat show host Neil Denny made his way across the United States interviewing scientists and science writers for his Little Atoms Road Trip series of podcasts. On his last day of touring, he stopped by my house to talk about Spider Silk. The podcast went up last Thursday.
It was an honor to be included in such a distinguished list of interviewees. I highly recommend listening to the whole series--you'll learn a lot.
September 28, 2012
I’ve been waiting for years to learn something about mesothele silk genes and silk proteins. The 90 or so species of mesothele make up the oldest surviving spider lineage. They now live only in Southeast Asia. As we explain in Spider Silk, mesotheles are sometimes considered living fossils (a term biologists don’t like nowadays; it gives the false impression that the organism hasn’t evolved at all since some distant point in the past) because they display characteristics so similar to the characteristics found in a 290-million-year-old fossil mesothele and so dissimilar from more “modern” spiders. For one thing, unlike other spiders, their abdomens are segmented, an outward display of spiders’ descent from segmented ancestors. If we want a full picture of spider silk evolution, mesothele silk genes and proteins are an obvious place to focus.
James Starrett, Jessica Garb, Amanda Kuelbs, Ugochi Azubuike, and Cheryl Hayashi have, for the first time, looked at the sequence of mesothele silk proteins by synthesizing cDNAs from the silk glands of Liphistius malayanus, a mesothele. They also looked at the sequences of silk proteins of six species of mygalomorph spider—these are the spiders in the second major spider lineage, which includes the tarantulas.
Neither mesotheles nor mygalomorphs spin the kinds of webs most non-arachnologists think of when we think “web.” They are incapable, for example, of spinning the kind of silk araneomorph spiders (the third major lineage) use as dragline or rapelling rope. (This largely accounts for silk researchers’ relative neglect of mesotheles and mygalomorphs: there’s no obvious industrial or other money-making application of this research). Instead, these spiders use silk to create linings for their burrows, or above-ground tubes such as pursewebs, or sheets that spread away from the spider’s hiding place to create a prey detector.
The research team also looked at the sequence of silk proteins produced by Hypochilus thorelli, a lampshade spider, one of the earliest-evolved still-surviving araneomorph spiders. Unlike the mesotheles and mygalomorphs, Hypochilus produces different kinds of silk strands specialized for different tasks. The researchers state that the motive for their study is the belief that “characterizing silk transcripts in mesotheles, mygalomorphs, and a basal araneomorph lineage allows for a better understanding of the evolutionary transition from substrate-borne, general-use silk fibers to aerial webs with task specific fibers spun by orb-weavers.” In other words, ancient spiders may have produced multiple silk proteins, but they seem to have used them interchangeably. Later in spider evolution, spiders use specific silks for specific jobs. Can silk protein sequences tell us how spiders got from there to here?
Previous to this research, two different classes of silk protein had been identified: spidroins, or spider fibroins, which make up the bulk of spider silk fibers, and egg case proteins (ECPs). ECPs were identified only in 2005 (whereas spidroins have been studied for decades) and only in Latrodectus hesperus, the Western black widow. (Given how few spider families have been sampled, though, it’s quite possible other spiders also produce them.)
Back in the early 1990s, Joachim Haupt and Jacqueline Kovoor conducted histology and histochemical studies on mesothele silk glands that showed that these spiders produced more than one kind of fibrous protein. What makes Starrett’s team’s findings so interesting is not just that they confirm Haupt and Kovoor’s study, but also that they found such a large variety of recognizable silk proteins in the mesothele: a spidroin plus six ECP-like proteins.
This appears to confirm the hypothesis that spidroins evolved very early in spider evolutionary history. Arachnologists have long assumed that the “original” spider produced just one spidroin, and that the genes scripting later-evolved spidroins probably evolved from the gene scripting that original spidroin. This study found just one spidroin in a mesothele, which makes that assumption seem more likely.
At the same time, the presence of the ECP-like proteins in addition to the spidroin shows that silk proteins started to diversify quite early in spider evolutionary history. These mesothele ECP-like proteins, like the mesothele spidroin, contain little repetition compared to the araneomorph black widow ECPs. But the selection pressures on mesothele silks are quite different from the selection pressures on black widow silks. Mesotheles live underground in burrows, and their eggs are laid and covered in silk underground. Mesothele silk proteins may be selected to cope with dampness or dangers such as mold or interfacing with soil or underground predators and parasites, whereas black widow silks may be selected to cope with sunlight, aridity, attack by above-ground predators and parasites, and wind forces, among other challenges.
The big question is: Why did these silk proteins start to diversify in the first place? It’s possible that mesotheles are not quite so indiscriminate in their use of different silk proteins as we think. In black widows, the ECPs knit with a specialized spidroin called tubuliform silk to form egg casing. Neither mesotheles nor mygalomorphs produce tubuliform silk. The research team believes that, because both ECPs and the ECP-like proteins are rich in the amino acid cysteine, and because cysteine plays a role in the knitting together of black widow ECPs and tubiliform spidroin, the mesothele ECP-like proteins may knit with that spider’s spidroin. Does the mesothele tailor its protein mix to lay down, for example, an egg blanket versus a trap door?
That isn’t an easy question to answer. Araneomorphs’ specialized silks are produced in specialized glands, and they can be observed as they emerge from distinct spinnerets. Mesothele glands don’t seem much different from each other. And closely observing a mesothele laying down silk requires unusual dedication: they undertake housekeeping chores in the dark, stop when exposed to light, and also, unlike mygalomorphs and araneomorphs, their spinnerets are inconveniently (for observers, not for the spiders) placed under their abdomens instead of exposed out at the end. So whether he or she chooses dissection and RNA extraction or focused observation or both, some researcher is going to have to contract an obsession with this question before we get an answer.
It’s also possible that the early diversification of these proteins is related to the different amounts of energy required for their synthesis. Spiders not entirely dependent on any one protein in their heavy dependence on silk may have been able to weather fluctuations in prey capture more easily.
What is clear is that this early diversification of silk proteins shows once again that silk genes are evolvable; that is, they have a propensity for evolution—they can duplicate and change slightly but still be useful. Eventually, under the right circumstances, accumulated changes may result in proteins suited to specific tasks.
The research team’s analysis of the mesothele spidroin, the newly found mygalomorph spidroins, the newly found Hypochilus spidroin, and previously known spidroins supports this scenario. They believe that their analysis shows that the common ancestor of the opisthotheles (the combined mygalomorphs and araneomorphs) had a minimum of five related spidroin genes, meaning that this gene duplication occurred before the mygalomorphs and araneomorphs went their separate ways. Mygalomorphs, like mesotheles, have pretty uniform silk glands. So this would mean that spidroins diversified before glands evolved to be specialized.
ECPs and ECP-like silk proteins are probably also much more diverse—it strains belief that they are produced only by mesotheles and black widows, which are pretty distant relatives. It’s much more likely that, due to the way the cDNA synthesis process works, it’s possible the team sampled their mygalomorph and Hypochilus specimens at a time when the genes scripting ECP-likes or ECPs weren’t being expressed. We’ll only know for sure when genome sequences—rather than just cDNA sequences—are available for various spiders. Then we’ll also be able to see what, if any, evolutionary connections there are between ECPs and spidroins.
Besides these basic findings, this study found a number of other interesting details that should be of interest to silk specialists. And like all good research, it points the way to a number of further research questions, which the authors point out: We know that the number of repeats has an influence on silk fiber formation and mechanical properties, but what influence does the length of repeats have? Do the silks of different mygalomorphs have different mechanical properties, and how are those properties related to the molecular architecture of their silk proteins? And finally, once the mechanical properties and functional properties of mesothele and different mygalomorph silks are understood, how have those properties been influenced by the natural selection pressures exerted by life underground?
Once you get tangled up in spider silk research, there’s really no end to its fascination.
September 18, 2012
Here in the Northeast of the United States, we're in the thick of prime spider silk- and web-spotting season. By now, spiders who live only for a year are mature and either have already or are about to lay eggs. Which means they've been fattening up in anticipation. Which also means they've been laying out bigger and better traps for their prey and setting up nurseries. (Which also means that if you pay attention to news outlets that have to fill space, you're likely to read about all sorts of supposedly unusual surges in spider populations; these articles appear every September.)
On a walk around Fresh Pond in Cambridge, MA, this past weekend, we came across this large funnel web built in the shade, against a stump.
There were all sorts of other funnel and sheet webs scattered amongst the leaf litter and pasted to the hurricane fence around the pond. Orb webs are marvelous, but if you want to see the vast variety of constructions spiders make with their silks, squat down on your haunches and start scanning the shadier portions of the undergrowth and banks of ponds and streams. The close-up below shows this master builder to be most likely an agelenid, and likely a grass spider (Agelenopsis).
August 21, 2012
By now you've probably heard of the exciting discovery of Trogloraptor marchingtoni, a species of cave spider that warrants naming a whole new family of spiders. There's a nice BBC interview with Charles E. Griswold of the Arachnology Lab at the California Academy of Sciences, who, with Tracy Audisio and Joel M. Ledford, announced the discovery. (The last paragraph in the BBC article is inaccurate, though; there are many surviving species of goblin spider.)
While most news stories seem drawn to the newly discovered spider's claws, I'm more intrigued by the fact that Charles and his colleagues have determined that members of the family Trogloraptoridae are likely "the most primitive living members of the Dysderoidea," a superfamily that evolved relatively early in the araneomorph phase of spider evolution. The araneomorphs are the "true spiders," the spiders who can spin major ampullate silk. As we learn more about this spider and its behavior, we may learn more about the early evolution of the araneomorphs, and, of course, the early evolution of the production and use of silk by araneomorphs.
All the Griswold papers I've read have been very clearly written. The Trogloraptor paper, published in ZooKeys, is no exception and isn't behind a paywall. My fellow non-arachnologists, I urge you to take a look at the "Taxonomy" section of the paper. You may not understand all, or even any, of the terms mentioned. But you'll get an appreciation for all the many details taxonomists have to examine and consider before they boldly declare they've discovered a new family. Make sure to hover your cursor over the figure numbers to take a look at the anatomical structures described.
And the "Conclusions" section is a treat. If it doesn't make you want to rush off to explore the Pacific Northwest, I advise having your blood oxygen levels checked.
(We are grateful to Charles for reading and commenting on sections of Spider Silk while it was still in manuscript.)
May 7, 2012
I'm guessing that that these gorgeous little spiderlings--clustered in the nursery their mother set up in the slot in one of our backyard chairs--are the grandchildren of one of the Argiope babies featured in the very first Spider Silk blog post, posted nearly two years ago. Exactly which one is anyone's guess. Which says something about natural selection.
I love their shadows in this one.
Photos copyright Peter Loftus
May 2, 2012
We're grateful to receive such a thorough--and positive--review of Spider Silk from Rachael A. Carmen in the latest issue of Evolution: Education and Outreach. This review may especially interest those of you who teach evolution. Most of the review is behind a paywall, but you can read the first page for free here.
April 28, 2012
In what looks to be the last word on the tarantula foot-silk controversy, Fernando Perez-Miles and David Ortiz-Villatoro take a firm stand with the title of their new paper in the Journal of Experimental Biology: "Tarantulas do not shoot silk from their legs." In our last post, we outlined why Rainer Foelix and his team contend that the supposed silk spigots described by research teams supporting the pro-foot-silk position are most likely chemoreceptor hairs. Now, Perez-Miles and Ortiz-Villatoro also dispute the earlier findings of foot-silk; they contend that the silk those researchers found is actually spinneret silk.
Back in 2009, Perez-Miles, Ortiz-Villatoro and two other colleagues challenged the original claim that the zebra tarantula produces foot silk, reporting that when they sealed the tarantula's spinnerets with paraffin before having it cling to vertical pieces of glass, they couldn't find any silk. When yet another team of researchers claimed to have found foot-silk left behind on glass by four different kinds of tarantula, they suggested that the Perez-Miles team might not have shaken the glass enough. In other words, maybe the spiders didn't feel the need to produce foot silk.
Now, Perez-Miles and Ortiz-Villatoro have more closely repeated the procedure of previous foot-silk finders, with four tarantula species. When they left the spinnerets unsealed, they found silk threads in the "footprints" left behind on the glass. When they sealed the spinnerets, they found none. Their conclusion: the footprints became contaminated with spinneret silk, which easily travels from spinneret to leg because it's so light.
In combination with the facts that no one can find silk glands in the spiders' feet and that the "spigots" have more in common with chemoreceptors than with spinneret spigots, this latest piece of research makes it seem highly unlikely that anyone will find spiders whose feet produce silk.
March 10, 2012
Last summer, a team of researchers reported they had indeed found that tarantulas secrete silk from their feet. This appeared to settle a controversy that had started in 2006 when a different team reported finding foot silk. The original finding was bound to be controversial: no one had ever before observed the secretion of silk from the feet of any kind of spider. In 2009, a team investigating the first claim couldn't find evidence of foot silk. Last summer's report, which included a micrograph that seemed to show a blob of silk protein forming at the tip of a purported foot silk spigot, appeared to validate the 2006 report. (For a discussion of these three reports and of the questions concerning silk evolution they raise, see this earlier Spider Silk blog post.)
Well, there is now more evidence that tarantulas actually DON'T produce foot silk. Rainer Foelix is a leading spider anatomist and author of the must-have Biology of Spiders. When he examined the micrographs included in last summer's report, the alleged foot silk spigots looked like chemoreceptor hairs he had studied intensively in the 1970s. Kathryn Knight summarizes what Foelix did next. He, Bastian Rast, and Anne M. Peattie report their full findings in the April 1, 2012, issue of the Journal of Experimental Biology.
Foelix, Rast, and Peattie explicity address the questions concerning silk evolution that these earlier studies raised for us. They note that--even if the silk allegedly secreted through the foot "spigots" really is silk--all the tarantulas tested tend to stay close to the ground. It's not clear, then, what survival advantage foot silk would give them, particularly when the tiny amounts produced would add little cling compared to the spiders' already clingy adhesive setae, or hairs.
Most interesting to us, the team compared the feet of the tarantulas in their study to the feet of Liphistius desultor, a mesothele. As readers of Spider Silk know, mesotheles make up the oldest extant branch of the spider family tree. They live in burrows, rarely venturing more than a few inches beyond the burrow's trap door. Foelix, Rast, and Peattie state that Liphistius has no adhesive hairs on its feet. But it does have the same hairs that earlier researchers identified as silk spigots but that Foelix et al. are pretty convinced are chemoreceptors. The tiny amounts of "silk" produced from the hairs in question wouldn't allow the mesotheles to climb, even if they wanted to.
Next step in deciding whether the hairs in question on tarantula feet are silk spigots or chemoreceptors: testing them for sensory innervation with transmission electron micrography. Down the road, we wonder whether anyone will find any evolutionary connection at all--given the evolutionary relationship between limbs and spinnerets--between the proteinaceous fluid that apparently oozes from these hairs (which are also found on the spinnerets and all extremities) and the protein silk that is secreted through spinneret spigots.
As usual, the best research leads to more questions.
February 12, 2012
Happy Darwin Day!
If you google "darwin day" on blogs, you'll find a variety of science and other writers' personal appreciations of Darwin's genius--highly recommended. Here, we're of course partial to spiders. Darwin was more of a beetle man, but if you follow this link to last year's Darwin Day post, you can read of his noteworthy encounter with spider silk.