Spiders An Evolutionary Detective’s Best Friends
June 14, 2004
The array of test tubes and glass vials lining the shelves of a small, temperature-controlled chamber in the Biology-Psychology building hold hundreds of the world’s most reviled yet ecologically vital organisms: spiders. Most are no bigger than the spiders you might find in your home. But for Greta Binford, assistant professor of biology, these tiny creatures hold clues to a big mystery: How does evolution spawn new traits and shape the world’s vast biological diversity?
Binford, who arrived at Lewis & Clark College from the University of Arizona in 2003, hopes to answer this question by building an on-campus spider lab that runs the gamut from pen-and-paper observational research of feeding behavior to highly complex genetic analysis. Her own passionate quest for answers has excited students about the broad scope of evolutionary biology. She has enlisted undergraduates to help with laboratory experiments and with collecting eight-legged arachnids in other parts of the hemisphere.
Binford is keenly interested in how 400 million years of evolution shaped the vast diversity of spiders found all over the world. Today more than 38,000 described species of spiders crawl around the earth, differentiated by their shapes, eye patterns, spinerettes, venom glands, and other physical characteristics. Perhaps tens of thousands of spider kinds still await human discovery.
To understand what produced such a multitude, Binford is examining the evolution of spider venom, a complex mixture of toxins used primarily to immobilize prey. The exact composition of venom varies significantly from spider to spider, even among close relatives, and few researchers have analyzed its evolution.
“Spider venoms are rich pools of unique chemistry,” she explains. “If we can understand the pattern of variation, it helps us to better understand evolution and may even aid pharmaceutical research by predicting where to find novel chemicals with medicinal benefits.”
For her part, Binford is trying to pinpoint the evolutionary origin of one of spider venom’s better-known components, an enzyme called sphingomyelinase D, or SMD. In the entire animal kingdom, SMD is found only in the venom of theLoxosceles genus of spiders, commonly known as brown recluse spiders, and in the venom of some of their closest relatives, Sicarius.
Scientists have identified SMD as the chemical that triggers skin lesions and sometimes severe systemic reactions in humans bitten by brown recluse spiders. But it is unlikely that SMD evolved from the need for these spiders to poison people, Binford says, since they are shy and bite people only when their lives are endangered.
So why did these two genera of spiders end up with SMD? When in history did spiders first acquire the enzyme? And what about the enzyme’s effect on prey makes it advantageous to the spider? “There’s an evolutionary puzzle here,” says Binford. “I’d like to know where SMD originated, how it originated, and what it’s doing for the spider.”
Above, left: Venom is extracted from a Sicarius rugosa via electrostimulation. Above, right: Greta Binford (right), assistant professor of biology, points out the fangs of a preserved tarantula to Lindsay Rodgers ’07 (left) and Michael Janes ’06.
Most of us don’t give spiders much mind, and might describe them as creepy, ugly, or scary. But Binford, like most arachnologists, finds them captivating. They are carnivorous predators, and most are poisonous to insects, but very few have strong enough venom or big enough fangs to affect humans. In fact, they help us by eating insects, filling a vital role in regulating most ecosystems as well as agricultural economies. Medical and pharmaceutical researchers value their nerve-affecting venoms, and materials scientists envy their strong-as-steel silk.
Some spiders weave funnel-shaped webs. Some can leap 40 times their body length. Others feast by snaring tiny fish with their teeth. “One of my favorite spiders is the Scytodes,” raves Binford, whose enthusiasm for spiders is difficult to hide. “They’re spitting spiders. They spit toxic glue on their prey. Is that not cool?”
Binford has gotten a kick out of studying spiders since 1988, when she was a zoology major at Miami University in Ohio and accompanied a professor to Peru to document spider feeding behavior. Since then, she’s collected spiders throughout the hemisphere, including Mexico, Costa Rica, South Africa, and Hawaii. Some of these spiders now live in a walk-in chamber in the Biology-Psychology building that’s about the size of the average home bathroom. Since many of its residents hail from the desert Southwest, the room maintains an atmosphere that mimics what Binford describes as a pleasant day in Tucson, Arizona—albeit with higher-than-average humidity.
To an evolutionary biologist like Binford, spiders represent a potential wellspring of discovery, since they are both under-studied and underappreciated. More than 15 years after her first professional spider hunt, Binford says, “There are still big aspects of spider biology that we don’t know much about.”
One of those aspects is the evolutionary relationships among spiders, a sort of family tree known as a phylogeny. The spider phylogeny has thus far been arranged primarily according to morphology, the physical characteristics of spiders.
Students in Binford’s lab are examining DNA sequences to reconstruct the relationships among various kinds of spiders. Using this kind of gene-level analysis is often a more reliable way to chart evolutionary history than using morphology. This research should create a phylogenetic tree that is useful for answering many evolutionary questions about these spiders, because it will help scientists understand how certain traits were passed down and altered over time.
“The fact that we inherit characteristics from our ancestors explains most patterns of differences in characteristics among organisms,” says Binford, who taught a course entitled Phylogenetic Biology during spring semester. “When organisms share similar traits, it’s most likely because they’ve been inherited from a common ancestor.”
To trace SMD’s evolution, Binford is combining the latest molecular wizardry with almost primitive research methods.
Her guess is that SMD originated in the common ancestor ofLoxosceles and Sicarius. To test her hypothesis, Binford is comparing the genes of various species within those two families. She’s hoping to find hints to the gene’s date of origin as well as clues to its molecular evolution—in other words, she’d like to know how the molecular machinery turned on the SMD gene in the first place.
To answer her other question, why SMD is advantageous to the spider, she is observing how spiders with SMD-laced venom live in their natural habitat, how they catch prey in the lab, and how their prey responds to the poison.
Several students help Binford test her theories. Melissa Bodner ’04, Kate Baldwin ’05, and David Merin ’04 all spent three months last summer with Binford conducting research at the University of Arizona. Bodner and Baldwin spent the first two weeks collecting specimens of Loxosceles, Sicarius, and a close relative, Drymusa, in Costa Rica. Meanwhile, Merin collected Arizona brown spiders, part of the Loxosceles family, by turning over rocks in the foot-hills of the Santa Catalina Mountains in Tucson.
Baldwin then began working on deciphering the molecular evolution of the gene that produces the SMD enzyme. Her goal is to figure out how these SMD-producing genes vary across species, and to look for differences and similarities that could help determine when and why the genes changed.
Bodner and Merin spent much of their time feeding crickets to spiders, time-marking the spider’s behavior—from bite to paralysis to consumption—to gain insight into the venom’s effect. Binford says this kind of observational research provides context to her genetic inquiries, and could illuminate why SMD is present in certain branches of the spider’s family tree but not in others. “I want to understand the evolution of spider venom in the context of the evolution of spider biology,” says Binford.
All three students continued their work into the 2003-04 school year. Bodner’s research was the most revealing. By decoding the DNA of a gene from 17 species of spiders, she found evidence that the morphology-based understanding of relationships among four genera—Loxosceles, Sicarius, Drymusa, and Scytodes—may be wrong. “It looks like SMD may have originated in the most recent common ancestor of all of these genera and then was lost in Drymusa andScytodes,” explains Binford.
Although more research is needed to confirm the findings, Binford says these results have significant implications: They show how easy it is for evolution to turn on and off venom proteins over time, and they provide a better understanding of which species are potentially harmful to humans.
This summer, Bodner is examining the DNA of additional species to fill out more branches of her phylogenetic tree, while Baldwin is “turning on” what Binford and her research colleagues believe is the SMD gene in bacterium. This will produce copious amounts of the SMD protein to test on insects and to create antibodies for future research. (Merin, thanks to a connection made by Binford, will be in Ecuador studying caterpillars.
Binford has worked with students at Lewis & Clark more extensively than at any previous institution. “The quality of students here is so high,” she says. “I can give them tasks that they’re not daunted by and can work on independently. They’re enthusiastic, ripe with ideas, and just a lot of fun to work with.”
The students are appreciative of Binford’s long leash. “She gives us a tremendous amount of guidance and freedom to explore the things we’re interested in,” says Merin.
Binford and her students say a full-fledged spider lab will be an attractive lure for biology majors whose interests run the gamut from field biology to molecular genetics. On Binford’s wish list are more space to observe spider behavior and a high-quality video camera to record it, an infrared light to allow nighttime observations, and equipment that performs sophisticated protein separation techniques to enable more molecular analysis. “I want to expose students to a broad menu of options for them as scientists,” she says, “and see which one lights their passion.”
Dan Sadowsky is a freelance writer in Portland.