University of Delaware
Office of Public Relations
The Messenger
Vol. 5, No. 4/1996
'Fingerprinting' an oyster?
     When spring arrived in 1888, dozens of schooners burst out
of local creeks and into the Delaware Bay in quest of "white
gold"-the American oyster (Crassostrea virginica). In the heyday
of the bay's oyster fishery, catches soared above 3 million
     During the next several decades, however, the oyster fishery
began declining from overfishing and habitat degradation,
although it remained a valuable industry. Then, in the late
1950s, the fishery was attacked by a single-celled parasite
called MSX and later by another parasite, Dermo. It has never
recovered. Last year, only about 32,000 bushels of oysters were
landed in Delaware Bay.
     To revive the Mid-Atlantic oyster fishery, scientists and
resource managers must find a way to combat the disease. In Sea
Grant research at the UD, marine biologist Patrick Gaffney is
working with a team of mid-Atlantic scientists to give oysters
the weapons they need- disease-resistant genes.
     "If we can develop disease-resistant oysters, our hope is
that they could be farmed by commercial mariculturists and be
used to help replenish depleted waters in the Delaware and
Chesapeake bays," Gaffney says. "While I don't think it's
realistic to expect the oyster industry to ever be what it was
years ago, disease-resistant oysters could help the industry
regain some lost ground."
     MSX and Dermo are harmless to humans but lethal to local
oysters. The parasites establish themselves on the oyster's gills
as the animal filters food from the water and then invade its
circulatory system. Many oysters die within eight weeks after
becoming infected. Since both diseases thrive in warm, high-
salinity waters, drought spells devastation for the oysters.
     To increase the oyster's resistance to disease, Gaffney and
his colleagues are working on two basic strategies.
     One approach is to create a disease-resistant hybrid by
breeding the American oyster (Crassostrea virginica) with the
Pacific oyster (Crassostrea gigas), a hardy, fast-growing species
with resistance to MSX and Dermo.
     The other approach is to explore the genetic diversity
within the native American oyster, which is found from Nova
Scotia to the Gulf states, to determine if any individuals in the
species can resist disease and thus be used in breeding programs.
     In each case, Gaffney is drawing on the revolutionary tools
of human genetic research to begin clearing a path through a
jungle of oyster genes to the gene or genes for disease
     For example, using the polymerase chain reaction (PCR), a
groundbreaking DNA fingerprinting technique borrowed from human
genetics research, Gaffney and master's student Jeff Wakefield
discovered that there are actually several genetic "types" within
the American oyster-meaning that a South Carolina oyster is
genetically different from a Long Island oyster and so
forth-valuable information for scientists working to preserve
species diversity and seek out disease-resistant lines.
     Gaffney is one of a growing number of biologists using PCR
in marine science applications. This molecular technique, widely
publicized in the forensics of the O. J. Simpson trial, enlists
the enzyme DNA polymerase to select a segment of DNA taken from a
subject's blood, tissue or hair and then rapidly replicate it,
yielding a large, readily analyzed sample of DNA.
     Once PCR has been run on the subjects they wish to compare,
scientists begin analyzing the differences between them, that is,
the differences in sequence of the DNA subunits. While automated
DNA sequencers can decode each product, Gaffney says, the time
and expense are considerable. His goal is to develop faster,
cheaper methods for screening genetic variation in oysters.
     One technique he now is using involves loading samples of
PCR-amplified DNA onto an acrylicamide gel containing a gradient
of a chemical denaturant. As an electrical current is applied,
the DNA molecules in each sample migrate through the gel until
they begin to denature (unwind), which causes migration to cease.
The variations in stopping points quickly reveal how different
the genetic compositions of the samples are. Using this technique
and others, Gaffney can begin creating DNA markers to help
scientists search among thousands of oyster genes for the handful
that may confer disease resistance.
     "We're fortunate in the marine sciences that we can draw on
the revolutionary technology of human genetics and the biomedical
industry," Gaffney notes. "Yet, there are a lot of allied
disciplines in which concepts and tools of molecular genetics are
becoming more and more useful."
     Gaffney says he also believes molecular genetics will be
critical to conservation efforts in the future. "As more species
become endangered and become propagated artificially at zoos,
understanding their genetics will become vital to preserving the
species," he says.
     "Already in the marine realm, scientists have been working
to understand the population structure of sea turtles and marine
mammals. I think molecular genetics will only continue to grow as
marine conservation biology develops, he says."
                                                -Tracey Bryant