New professor turns to arachnids to solve evolutionary puzzles

Prashant Sharma, a new assistant professor in the Department of Zoology, is deciphering the scorpion's tail and the evolution of venom.

Back to News
Prashant Sharma Slideshow 645X415
Prashant Sharma Scorpion Slideshow 645X415

Once he decided to study arthropods instead of going to medical school, Prashant Sharma never looked back. Not even when he was kayaking into a pitch-black cave in central Laos last year, armed with camping gear, headlamps and vials to collect as many different species of pseudochactid scorpions, a mysterious lineage unique to Southeast and Central Asia, as he could possibly find.

"It was spectacular," recalls the University of Wisconsin-Madison assistant professor of zoology, who's among the 24 new faculty members joining the College of Letters & Science this fall. "Of this particular species, there were only four or five specimens known to science. But we found over 25 specimens in two hours!"

Why go looking for scorpions in the dark? For what they can tell us about the process of evolution.

Evolutionary biologists are turning to some of the smallest creatures in the animal kingdom for insights about the richness of species diversity.

We took inflatable kayaks into the mouth of Tham Nam Lot Xe Bangfai Cave. After the rapids, the waters are calm, deep, clear, and devoid of light.

Because they do not fly or swim long distances and are astoundingly diverse across their lineages, live arachnids like scorpions make ideal research models.  How did some species make it across the Pacific Ocean? Why is "biogeographical signal" — the correspondence between species and their geographic distributions — a better predictor of their relationships than anatomy? Questions like these lead scientists to explore the effects of "geological dynamism," or large-scale changes including sea-level fluctuations and continental drift, on Earth's life forms.

Other questions — How does a developing spider form its fang? And why do scorpions have complex, segmented tails, not seen in their closest relatives? — inspire ever-more sophisticated research into the genetic databank that encodes the body plan.

Sharma is particularly interested in the scorpion's tail and the evolution of venom.

"If you look at the genetic code of the arthropod common ancestor, the scorpion tail is an impossibility," says Sharma. "Scorpions should not have enough genes to create their tails — yet they do."

During his recent three-year postdoctoral appointment at the American Museum of Natural History in New York, Sharma studied the Hox family of genes, which control whether any animal, from worms to humans, will develop different body regions, such as mouthparts, wings or tails. What he found may have potentially solved the mystery of the scorpion's tail — and provided scientists with a useful framework for all sorts of other evolutionary questions.

But before he could even get started, there was a problem: No one had created a reliable scorpion phylogenetic tree — a branching diagram showing the inferred evolutionary relationships among species and lineages, based on their physical or genetic characteristics.

"Scientists had had to make all these inferences, over the years, about which scorpion groups were primitive, how they were related — but it was all pretty qualitative," he says. "We just didn't have enough molecular data to make a phylogenetic tree."

That meant collecting live scorpions, which meant putting a team of scorpion hunters together to poke around in places that were dark and bat-ridden. Sharma volunteered to collect in Southeast Asia and Australia. Others fanned out across the Americas and Europe. The team agreed to find all targets within 12 months.

Torrential rain, village-wide power outages, and unpredictable (or non-existent) transportation were all in a day's work for Sharma, as was the curled-up python he stumbled over in a cave in Australia. And then there were the scorpions, some of them "relatively large."

"You just pick them up with forceps," says Sharma. "It's really no different from collecting anything else."

Once the field team had completed its task, Sharma's team of postdoctoral researchers constructed a reliable phylogenetic tree of scorpions based on data from thousands of genes. Then they began lab work to explore the genetic blueprint of the scorpions' tails. And last summer, they published findings that help solve the mystery of the scorpion's unique tail.

"[Biologists] had inferred that there were 10 Hox genes present in the arthropod common ancestor — meaning the arthropod can only pattern so many segments," says Sharma. "But scorpions have always defied this — their body plan is very unusual.

"We found that they have at least 19 Hox genes, and what we were able to show was that the original Hox genes are duplicated. Some duplicated copies have acquired a new expression boundary during embryonic development in the region of the scorpion tail."

New expression boundaries for Hox genes means the acquisition of new functions and their recruitment to pattern different parts of the body, creating anatomical diversity.

With their phylogenetic scorpion tree, Sharma and his team were able to show that the Hox duplication had occurred at least as deep in the tree as the common ancestor of scorpions, if not earlier. Why do Hox genes duplicate in certain species, and not in others? Scientists aren't sure, says Sharma, but such duplications leave clear signatures on the shape of animal diversity.

Discoveries in evolutionary biology tend to blossom and multiply into new fields of inquiry. So insights from Sharma's work on arthropods might end up informing gene therapy, disease studies, epidemiology and many other disciplines. Here at UW-Madison, evolutionary biologists work alongside ecologists, geneticists, and cellular biologists, not to mention neuroscientists, geophysicists and even philosophers.

For example, the complete phylogenetic framework constructed by Sharma's team for scorpions can be applied to a wide range of evolutionary phenomena.

"You can ask the same question of many groups — What makes some lineages more diverse than others? What role does geography play in the evolution of diversity?" he says. "And this framework will apply."

But Sharma and his students will stick to scorpions, sea spiders, and other creepy-crawlies for the near future.

"For the sheer thrill of discovery, there is nothing like studying the evolutionary biology of arthropods," he says.