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.
October 7, 2011
[This post contains later amendments that may be instructive to other science writers. See the later posts on purported tarantula foot silk to see why.]
Three papers published this summer might at first seem unrelated. But read together, they pull the entire arc of spider silk evolution into sharper focus. Two papers indirectly address the evolutionary origins of spider silk production. The other demonstrates that the evolution of silk proteins has been central to spider evolution even after the extraordinary proliferation of silks that made the vertical orb web possible.
First, F. Claire Rind and colleagues reported that at least some tarantulas (which belong to the family Theraphosidae) do indeed secrete silk from their feet. This report appears to settle a controversy that first broke out in 2006, when a team of researchers led by Stanislav Gorb announced that they had persuaded a Costa Rica zebra tarantula, Aphonopelma seemanni, to walk on a nearly vertical surface covered with glass microscope slides. The researchers claimed that as the tarantula started to slip, it left behind “footprints” made up of miniscule silk fibers. If this observation held true, it could have important implications concerning the origin of spider silk production. Like all spiders, tarantulas secrete silk through abdominal spinnerets, small appendages ending in multiple spigots that are the outlets for the abdominal silk glands. Genetic studies have shown that spider spinnerets are the evolutionary descendants of the gill branches of ancient arthropod limbs. If spiders secreted silk from their limbs as well as through their spinnerets, this fact might not only cement the limb-spinneret connection but also suggest new hypotheses for the earliest origins and survival value of spider silk.
The Gorb report left some questions open, however. (more…)
July 13, 2011
The newest version of The World Spider Catalog, by Norman Platnick of the American Museum of Natural History, is now online. This amazing document lists every species of spider described so far and includes counts of species in each spider family, a summary list of fossil spiders, and a massive bibliography. As of June 21, arachnologists have identified 42,473 spider species, but I wouldn’t be surprised if a few more have been identified since then.
January 26, 2011
Carl Zimmer has a wonderful article at the New York Times on the confirmation of Vladimir Nabokov's once-ridiculed idea about the evolution of his favorite group of butterflies, the Polyomattus blues. In 1945, when Nabokov published his idea that the butterflies had made their way from Asia, across the Bering Strait, and down to Chile in five different waves, there was no way to test his hypothesis. Today, thanks to research methods built on the discovery in 1953 of the structure of DNA, it's possible to construct evolutionary trees by isolating and comparing genes from closely related organisms. And that's just what a team led by Naomi Pierce at Harvard has done with these beautiful insects. Zimmer's article explains why their findings confirm Nabokov's hypothesis.
Pierce's team used some of the most advanced tools available to biologists. But their research fundamentally relies on Nabokov's painstaking efforts to develop an accurate classification of the different species within the group--classifications that the team re-examined and found reliable. To me, this is an example of the most interesting kind of biological research: the melding of advanced investigation of molecules with old-fashioned close and patient observation of organisms, both fueled by passionate creativity.
Zimmer's blog post has links to his Times article, to the Pierce paper, and to Nabokov's monograph.
January 4, 2011
We wrote Spider Silk for two reasons. First, we wanted to share the wonders of spider silk and reveal how it allows spiders to take advantage of their surroundings in surprising ways. But second, we believe spiders and spider silk can show nonbiologists, in concrete detail, how the theory of evolution explains the mind-boggling variety of living beings on Earth.
We found Jerry Coyne and H. Allen Orr's book Speciation invaluable as we wrestled with how best to present some of this information. For nonbiologists, the questions of what exactly defines a species and how species become and remain distinct from each other are often perplexing--just as they were for Charles Darwin and Alfred Russel Wallace before they independently arrived at their brilliant insight. Yesterday and today, over at his Why Evolution Is True site, Coyne explains why the "biological species concept" (BSC) is the most useful way to think about species:
"What is the origin of species? Under the BSC, that question becomes equivalent to 'What is the origin of reproductive isolating barriers between closely related species?' And that is a much more tractable question."
Highly recommended reading: Part 1 and Part 2.
September 17, 2010
Between reality TV and the 24-hour news cycle, it's hard to be surprised anymore by anything much humans do. Spiders, on the other hand, get up to all sorts of things that I doubt most of us would believe without documentary evidence. A case in point: In the newest issue of the always-interesting Journal of Arachnology (published by the American Arachnological Society), arachnologists Matjaz Kuntner and Ingi Agnarsson provide the first description of a remarkable species of spider from Madagascar. Even if you're not up to reading a whole paper like this, make sure to take a look at the extraordinary photographs included.
Kuntner and Agnarsson's newly described spider belongs to the little-studied genus Caerostris, commonly known as bark spiders because they often look like lumpy pieces of tree bark. They've named it C. darwini, or Darwin's bark spider, in honor of the 150th anniversary of the publication of On the Origin of Species. (more…)