Burreson's first clues came from scientists studying oysters from Korea and Japan. In 1971 Fred Kern was analyzing oysters at the Bureau of Commercial Fisheries laboratory in Oxford, Maryland when he saw parasites in Korean oysters that looked like MSX. The clue was a similarity based on a shape, a shape seen through a microscope, something akin to a witness watching a police lineup full of suspects who are all about the same size — but are all wearing masks.

In 1991, Carolyn Friedman had the same reaction while she was checking Japanese oysters with the California Department of Fish and Game. In both cases the MSX-like parasite turned up in Crassostrea gigas, a fast-growing Japanese oyster that has been profitably grown since the 1930s in California, Oregon, and Washington State. By the mid 1990s, the question of MSX's origins had been in the cold case files for nearly four decades. Scientists had clues, but no way to follow up, no way to unmask any suspects.

A breakthrough was coming, however, and it began late at night in 1983 on a lonely road in the hills of northern California. Kary Mullis, a lab technician who dreamed of being a novelist, was driving to his cabin in the woods, mulling over ways to analyze mutations in DNA when — in a flash of insight — he saw a new way of making copies of DNA. As he drove he became so excited, he later wrote, that he woke up his girlfriend to tell her. He also began dreaming about a Nobel Prize.

His insight led eventually to an inventive technique called PCR, short for polymerase chain reaction, a technique that allows scientists to multiply one small piece of DNA into millions of pieces. PCR became the hottest tool in molecular biology, opening the door to all kinds of new experiments and applications and investigations. Doctors could use PCR for diagnosing diseases, chemists for creating new drugs, police for doing DNA fingerprinting. It was a dream come true for biologists — and for Kary Mullis: in 1993, at age 49, he won his Nobel Prize.

For Burreson, the biologist-turned-detective, it was the dream tool for cracking the long-standing mystery of MSX. His first step was to hire Nancy Stokes, a scientist trained not in marine science but in the new DNA-based tools of molecular biology. With the PCR technique they could finally do more than peer at parasites through a microscope: they could try unmasking them by examining their DNA.

Their approach was elegant in concept: compare the DNA of the Chesapeake parasite against the DNA of the Japanese parasite. Was the killer in the Chesapeake a parasite from Japan?

Sitting down at his high, black lab bench, Burreson picks up an unshucked oyster and inserts a long, thin needle through the shell, slowly plunging it into the adductor muscle. Much like a doctor during an office visit, he's taking a blood sample from a sick oyster, one of his James River oysters infected with MSX. The oyster, a perfect patient, doesn't flinch. The blood comes up the needle as a clear liquid.

This is the first step in getting a pure DNA sample of MSX. Floating in the colorless blood are blob-like plasmodia. When Burreson squirts his blood sample into a petri dish holding a saline solution, the plasmodia continue to float while the blood cells settle out and stick to the glass. He pours his solution into another dish, and the blood cells settle out again. By pouring and settling several times, he gradually discards most of the oyster cells and concentrates the MSX cells. "That was the hardest part," says Burreson, "getting the pure MSX DNA without a lot of oyster DNA contaminating it."

The rest was hardly easy. With these samples Burreson and Stokes first had to create a DNA fingerprint for MSX. And PCR was their key tool because it gave them a lot of DNA to work with.

Using cycles of heating and cooling plus a key enzyme, PCR can split a DNA strand in two and then create two separate copies. By running the process multiple times — the heating, the cooling, the enzymes — scientists are, in effect, setting off a chain reaction. They are copying the copies, then copying the copies of the copies until they have quickly "amplified" one piece of MSX DNA into millions of pieces.

With all those DNA copies, Burreson and Stokes went looking for some of the signature genes that define MSX as MSX. After picking one sector of one key gene, they were able to outline a 564 base pair sequence that was unique to the parasite, as unique as a fingerprint to a criminal.

Their work, for the first time, unmasked MSX, the Chesapeake oyster killer. It identified the parasite not by its body shape as seen under a microscope, but by a diagram of its underlying DNA, its genetic fingerprint.

Like any detective worth his badge, the biologist took his new tools and started rounding up the usual suspects and fingerprinting them.

In this case the suspects were Japanese oysters that came from the far side of the country and the other side of the world. Burreson gathered tissue slides or spat or living oysters that originated in Matushima Bay in Japan, in Geoje Bay in Korea, and in Drakes Estero in Marin County, California.

Thanks to his work with PCR, Burreson now had a molecular probe that would seek out and stick itself to any DNA from MSX in any samples of oyster tissue. When his probe turned up MSX in the tissue of all his non-native suspects, the last mask was lifted: MSX, the killer parasite, was carried in oysters from Japan, Korea, and California.

For a final proof, Burreson and Stokes sequenced a section of DNA from the parasite in Japanese oysters. When they compared it with their earlier DNA fingerprint from the Chesapeake parasite, they found a near-perfect match, a 99.8 percent match that would convict a culprit in any court. "That is conclusive evidence that the parasite in Crassostrea gigas is, in fact, MSX," says Burreson. "They are the same organism."

His final verdict: the Japanese oyster was the culprit. It sometimes carried MSX, a parasite that seldom sickened gigas but was lethal to oysters in both Delaware and Chesapeake bays. "MSX was like smallpox coming in with the Europeans," says Burreson, "and the native Americans were wiped out, because they were na•ve to it. They hadn't seen it."

An answer like this only leads, of course, to a new question: who brought Japanese oysters — and MSX — into the Chesapeake?

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