Eight legs, thousands of genes
Climb the stairs to the fifth floor of Science Building I and you’ll find yourself in a long hallway with laboratories on either side. In one of these labs, a small space lit by a bank of windows, the central work area is stacked with terrariums and clear containers – an arachnid housing development.
You’re in the laboratory of Susan Masta, assistant professor of biology at Portland State. Masta works with spiders and other arachnids, and enjoys speaking up for spiders and their kin.
“Spiders are common victims of lore and misconception,” says Masta, explaining that most are harmless. “They really get a bad rap.”
“Many are so small they can’t even get their little fangs into you to bite,” Masta explains. “Most spiders aren’t the least bit aggressive. In general they mind their own business. Humans are regarded as a waste of perfectly good venom – after all, you’re too big to immobilize.”
Masta’s laboratory – located on the 5th floor of Science Building I – is home to her own projects and also to the work of three undergrads, two graduates and one postdoctoral fellow.
While all spiders are arachnids, all arachnids are not spiders. The arachnids – distinguished by having eight jointed legs and two body parts (cephalothorax and abdomen) – include spiders, scorpions, ticks and mites, among others.
Masta describes her major work as determining the relationships between all arachnid groups. “We do this mostly by looking at DNA,” she says.
“I was interested in doing a genetic survey of a single species that was in the process of diverging into multiple species,” says Masta, who attended graduate school in Arizona.
“You can tell this is happening by finding a single species that begins to look different in different geographical locations, or that looks similar but has members that exhibit different behavioral patterns,” she explains.
Arachnids are an ideal focus for Masta’s work. They’re an ancient group, with fossil evidence dating them back at least 400 – 500 million years. They’re also a big group: Spiders alone account for some 38,000 species.
“It’s challenging to determine relationships among groups that old and that big,” Masta says. “Genome sequencing has made the process much more clear.”
Masta’s work involves sequencing the whole mitochondrial genome, looking for evidence of evolution and diversification.
“Because genomes are small, I can address basic questions more easily,” she continues, “like how a species changes over time. For instance, genes can move to different locations on the genome, or the sequencing itself can change. There can also be structural changes at the molecular level.”
“The work will help answer some important questions, like the basics of who’s related to who,” Masta says.
Masta collaborates with the Department of Energy-sponsored Joint Genome Institute in Walnut Creek, California.
A tour of Masta’s lab yields a number of different arachnids. On one end of the spectrum is a tiny pseudoscorpion a few millimeters long. On the other is an enormous Chilean Rose Tarantula the size of an adult human hand.
“The pseudoscorpion uses grasping pincers – chelicerae – to grab and pinch prey as it wanders by,” Masta remarks.
Other appendages – pedipalps – are covered with rasps and have poison glands at their tips. Many arachnids use venom as an additional means of immobilizing their future meal.
The tarantula’s glass-walled home is screened from direct light and includes two small caves.
“Tarantulas don’t see very well,” explains Stuart Longhorn, a postdoctoral fellow working with Masta. “They’re nocturnal – they hunt at night.”
“She’ll go up to a year without feeding,” Longhorn says. “She’ll eat when she needs to.”
Longhorn removes Mary Stuart Buttercup from her terrarium and hands her carefully to Masta. The large spider begins a slow amble up Masta’s arm. Strands of silk flutter behind.
“Silk-producing spiders spin soft bits of web as they move around,” Masta says. “It functions as a kind of drag line, something to catch them if they were to drop suddenly.”
“See there,” says Masta, pointing. “She’s feeling more secure now. She’s folded her spinarettes back up.”
The tarantula is covered with soft hairs, known as trichobothria. “The hairs respond to vibration,” explains Masta. “She doesn’t see very well, and the hairs help her interact with the world around her.”
As Mary Stuart Buttercup is coaxed gently back into her terrarium home, she clings to Masta’s sweater with Velcro-like feet. “Most spiders have hairs on their feet that are really sticky. She can walk upside down on a piece of glass,” Masta says proudly.
During the lab tour, Masta is careful to distinguish the lore versus the reality of spiders.
“Most arachnids are liquid feeders. They don’t necessarily drink blood – they drink the victim’s body fluids,” she explains.
While web-builders rely on the ability to trap and immobilize their prey, not all spiders produce silk, and therefore many don’t spin webs.
“Jumping spiders don’t spin webs. They jump their victims, bite them and inject a paralytic venom,” Masta says.
A spider’s longetivity is species-related. “Most spiders undergo a series of molts en route to adulthood,” Masta says.” Once they reach full size, they mate and then live a very short life.”
“Tarantulas are the oddball,” she continues. “They can reach 15-20 years of age.”
No doubt Mary Stuart Buttercup will be relieved to hear this.