Not just a pesky allergen: pollen as a forensic tool

Photo by Dartmouth College Electron Microscope Facility (CC0)

On June 25, 2015 a pet dog discovered a trash bag on the shore of Deer Island, Boston. It contained the remains of a two-year-old child. Investigators christened her “Baby Doe,” brought in their own cadaver dogs, and began the quest to find Doe’s identity—and her killer’s. They quickly hit dead ends. There didn’t seem to be enough data to clinch the case, until the United States’ only full-time forensic palynologist, or pollen expert, arrived on the scene. Andrew Laurence, head palynologist at the U.S. Customs and Border Protection Agency, read the pollen signature clinging to the toddler’s hair and clothing to trace where she had lived. Though prolonged exposure to the damp had rendered her fingerprints unreadable, her pollen print was clear enough: two species of cedar trees that could not have grown wildly but were certainly present in the Boston Arboretum had left their mark on the body: she had to have lived close by. Having established the child was local, detectives were able to tighten their search and close in on an eventual lead. A neighbor’s tip led investigators to her mother, Rachelle Bond, and the discovery of Baby Doe’s name: Bella Neveah Amoroso Bond.  

Bella Bond’s case is a rare one: palynology (despite frequent appearances in crime dramas like the series Bones) is a seldom-wielded forensic tool, and represents a sparsely populated discipline in the United States. Elsewhere, however, investigators have employed the distinctive specialty since the 1950s to the chagrin of numerous criminals. In Austria in 1959, police confronted a suspected murderer with pollen evidence from his muddy boots. The offending articles carried a 20-million-year-old pollen fossil that had become part of a tiny region’s pollen signature. In 1996 a man in Brisbane, Australia, was traced to where he hid his wife’s body under some acacia bushes by the pollen grains left clinging to his clothes—even after he’d fastidiously washed them. Each region produces a precise pollen signature that varies predictably by season, making the powder the ideal tool to track bodies and objects to their origins in both time and space.  Precise though it is, palynology is also painstakingly specialized: experts must be able to identify plant species by the microscopic features of their pollen grains—shape, wall thickness, holes, structure, and size. Further muddying the field is the sheer number of plant species on the planet: according to an estimate by the Royal Botanic Gardens Kew in the U.K., that number may be as high as 400,000.

Now, however, advances in palynology may have brought the practice to the verge of mainstream forensics. Researchers are merging the delicate discipline with DNA analysis to trace pollen prints by their genetic signatures. The shift is symptomatic of a broader pattern in forensics: the discipline as a whole now inclines more toward machines and mathematics rather than what Dr. Greg Hampikian, director of the Idaho Innocence Project, calls “fuzzy human reasoning.”

Until recently, palynology relied on both the fuzzy reasoning and the advanced clarity offered by modern microscopes. Now, DNA analysis increases the resolution even further, right down to the scale of chromosomes. Genetic analyses have held the stage in forensic science for decades, but only recently advanced to the point of sorting and classifying mixed DNA—a vital faculty for analyzing pollen signatures, which contain minute amounts of material from many different plant species. The development of this new method, called DNA barcoding, adds another measureable item to the list of pollen’s identifying characteristics: a genetic code.

Before DNA barcoding could take the field by storm, however, it had to prove itself. Seeing the potential of the new tool to advance forensic studies, Dr. David Gangitano, a forensic geneticist and associate professor at Sam Houston State University, performed a proof-of-concept experiment along with his students to demonstrate the capabilities of DNA barcoding last year. The two-week-long study found no evidence of DNA degradation in the pine pollen grains used. “The DNA in pollen stays there for years. Hundreds of years,” Gangitano said. “There are studies in which they found DNA from fossils—pollen grains in the soils that have been there for thousands of years.”

So far, researchers have only tested DNA barcoding on pollen mixtures they’ve concocted themselves, according to Dr. Karen Bell, a lecturer at the University of Western Australia. Bell has studied the development of DNA barcoding in forensic palynology, and believes the technology is ready to enter the field. “I think the most important thing to know is just how useful it is—how many different fields of research can use pollen identification methods,” Bell said. “Thousands of people have the skills to do it—compared to identifying pollen under a microscope,” which only a very small number of people can do.

Among that select group is Dr. Vaughn Bryant, professor and director of the palynology department at Texas A&M University. Bryant was a pioneer of palynology in the United States, and remains one of the country’s few practitioners to this day. He notes that there are many reasons palynology is underused in the United States: the profession demands expertise in botany, plant ecology, and geography, many people are unwilling to train for such a specific line of work, many judges are unfamiliar with it, and, as one might expect, it suffers from restricted funding.  

Despite these limitations, Bryant believes palynology exemplifies a valuable forensic tool, although he’s skeptical about the utility of DNA analysis when it comes to pollen grains. “Some people ‘believe’ that DNA will be the answer but it will not,” he wrote in an e-mail. According to Bryant, pollen DNA is found only in cytoplasm—the solution of water, salt, and proteins that fills cells—and is vulnerable to rapid decay. “To do a forensic study one must examine all of the pollen in a sample—most of which does not have DNA,” Bryant wrote. “The idea of using DNA is more myth than reality.”

“Palynologists, they don’t want to change,” Gangitano said. “I understand—because they have their own discipline, they have their own rules, and if you’ve done it for 20 years, you don’t want to change.” While Gangitano’s method has yet to be used to solve a case, he believes it’s only a matter of time. “Pollen DNA is a powerful tool,” he said. “We’re evolving the views of forensic plant DNA.”

As palynology—and forensics as a whole—evolves into a collaborative human-and-machine endeavor, the focus turns to the roles expected of players on the new field. “I think where machines are becoming superior is the ability to apply large-scale, repetitive, mathematical simulations and to choose the best simulation to match the evidence,” Hampikian said. “We think machines have a different way of thinking that we’re only starting to appreciate—they took the lessons of the philosophers literally, and they applied very rich rubrics of thinking to come up with very pure answers. And now we’re starting to introduce fuzzy logic to machines, and more probabilistic thinking to the machines. So we’re coming to a time when machines are going to think more like people.”

“We’re not solving everything,” Hampikian said. “We’re solving some things. And every time you solve a problem in science you create new ones. There’s no way to avoid that.”

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