Inner Nature: Palynology

By Vidya Rajan, Columnist, The Times

Palynology is the study of tiny particles such as pollen, spores, dust and soil particles, microfossils, and other miniature items that are carried as inclusions in macro items such as rock layers, soil on a shoe, paint on a wall, and foodstuffs such as honey. The presence of pollen in honey provides a clue to the flowers from which bees obtain nectar, and is a key method for tracing honey origins and verifying specialty honey labels of expensive honey such as manuka honey. But there are a whole lot of other people who also use pollen for detection purposes: forensic pathologists, paleontologists, art historians, archaeologists, and drug enforcement agencies. Let’s unpack that staggering list.

Pollen is the sperm of many land plants. In water or in aqueous media, sperm can swim from the spermatogonia (the place where sperm is made, equivalent to the testes of animals) to the ovule (the egg of the plant). Since they have to travel, evolution has resulted in sperm shedding pretty much everything except the genetic material that they will contribute to their offspring. Waterborne sperm swim, so they do have a flagellum (tail) with which to travel towards the chemical signals that the egg produces. Plants that have waterborne sperm are mosses, ferns, and cycads, and these all need water for reproduction, explaining why they grow in wet places. But plants better adapted to land have had to get to the egg across dry spaces. A fragile sperm propelled by a tail just doesn’t cut it. A series of iterative evolutionary steps by which sperm lost their tail, but instead developed a protective coat made of a waterproof chemical called “sporopollenin”. Or should I say a collection of chemicals, because sporopollenin is a heterogeneous mixture of a collection of hydrophobic molecules such as long-chain fatty acids and carotenoids, and a slew of phenolics and other chemicals, all cross-linked together into a tough capsule, resistant to drying, digestion, and mechanical damage, but conveniently ornamented with apertures out of which the nuclei inside can emerge to fertilize the ovule.

I use the adjective “ornamented” deliberately in the last sentence. The reason that pollen fascinates a whole bunch of people (see list above) is not just its remarkable resistance to destruction, but also its structure, which is unique to different plant species (see the figure). This makes pollen a durable and specific identifier for a variety of purposes. Forensic pathologists can seek out pollen that may have hitched a ride on car tires or shoe treads of murder victims to assess if the body was moved from the site of the murder to a different disposal site by matching the pollen on the body and in the soil around. (A recent discovery of a structure called “Borg DNA” in wetland microorganisms also might prove to be a tracer, in the same way as pollen or DNA fingerprinting.)

Miscreants who adulterate drugs can also be tripped up by palynology. The comparison of crushed up drugs under the microscope can distinguish ones that were prepared under unsanitary conditions, which can lead to the inclusion of dust particles, fibers, and even microscopic mites, in the preparations. The US Federal Government’s Food and Drug Administration has inspectors who do random spot checks on drug manufacturing plants around the world and collect samples for forensic testing. Failure to comply with Good Manufacturing Practices can lead to being slapped with a “FDA Form 483” which puts the manufacturer on notice for failure to comply with hygiene or labeling standards, and can lead to the plant being shut down.

Dr. Vaughn Bryant, of Texas A&M University, is the godfather of melissopalynology, the study of pollen in honey. By diluting honey with water and filtering it through a fine-pore filter, pollen grains in the honey can be recovered, their structure traced to their source plant, and the composition compared to the label’s claims. It is a federal offense under the Fair Packaging and Labeling Act of 1967 to mis-label the contents of a container. It’s because of FPLA that beekeepers must put their information on the label of honey that they sell to customers. Any downstream problems can then be traced to the source. The alphabet-soup of federal agencies (USDA, FDA, CDC and so on) who concern themselves with keeping the public safe use these labels to trace back to potential contamination. Using melissopalynology, it has been found that some honey on the shelves of grocery stores is really sugar-water. Having been caught, the miscreants have evolved their methods too: some actually mix pollen grains into sugar-water! But nature is far more subtle – there are volatiles, DNA, isotopes, and other signatures that cannot be mixed into honey. With the right equipment, honey can be traced back to its terroir in a manner similar to fine wine.

Maybe we should identify local honey with our 9-digit Zip code. Honey With A Zip. What could be sweeter?

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