MINNEAPOLIS / ST. PAUL-For the rats of Hamelin, it was the Pied Piper's tune. For the destructive sea lamprey of the Great Lakes, it's a chemical attractant, or pheromone, released by lamprey larvae living in streambeds. Following the pheromone trail, adults are drawn to streams favorable for spawning. Researchers have long wanted to identify the pheromone so it could be synthesized and used to control the sea lamprey, which laid waste to Great Lakes fisheries of lake trout and other species in the mid-20th century. Now, a team of University of Minnesota researchers has identified the three major components of the pheromone and synthesized the principal one, a novel steroid akin to a shark steroid that possesses anticancer activity. This is the first migratory attractant to be identified in any fish. The work is the cover story for the November issue of Nature Chemical Biology and will be published online in the journal Sunday, Oct. 2.
The lamprey is one of the earliest relics of vertebrate evolution, dating back nearly 400 million years, before the evolution of jaws and bony skeletons. The species parasitizes other fish by attaching with their circular, toothy mouths and sucking the body juices. A single lamprey will feed for about a year, consuming on average 40 pounds of fish. In the Great Lakes, their prey have been commercially valuable species like lake trout and whitefish.
Currently, the Great Lakes Fishery Commission (GLFC) controls lamprey by means of a poison that kills lamprey larvae in streambeds. It also kills every invertebrate it comes in contact with, and sometimes fish. The lampricide is tanker-trucked to streams, some of them in populated areas, in an expensive, labor-intensive and unpopular undertaking. The GLFC is eager to use a synthetic form of the newly found pheromone to replace the poison by luring lamprey to traps and sterilizing the males, the researchers said. Using the pheromone would be environmentally friendly and less expensive.
"The GLFC has the goal of controlling lamprey with a new and better technique by 2010. This could be it," said Peter Sorensen, a professor of fisheries, wildlife and conservation biology who led the study with chemistry professor Thomas Hoye. "Also, lamprey are important to native peoples on the West Coast, who value it for food. This pheromone could help restore lamprey runs by attracting lamprey to suitable spawning beds."
When they stop feeding, lamprey seek out streams for spawning by following the pheromone trails. After arriving at the spawning grounds, a sex pheromone -- which was identified by Weiming Li, an earlier doctoral student of Sorensen -- attracts the females to males. After spawning, which takes a few weeks, the adults die.
It has taken Sorensen and his colleagues about 15 years to find, isolate and purify the pheromone so that Hoye and his colleagues could identify and synthesize it. The key component is a steroid with potency so great that lampreys would smell a single gram dissolved in 10 billion liters of water, enough to fill 5,000 Olympic-sized swimming pools. This level of potency tops that of all other fish attractants, including those of salmon.
To find the pheromone, the researchers extracted 8,000 liters of water from tanks holding 35,000 larvae. The yield was less than a milligram, or 35 millionths of an ounce. Much of the work in isolating and purifying the pheromone was performed by Sorensen's graduate student Jared Fine, while Vadims Dvornikovs, Christopher Jeffrey and Feng Shao in Hoye's lab were integral to synthesizing the key component, called PADS. The chemical structure of PADS is very similar to that of squalamine, a compound made by the "dogfish" shark. Squalamine has been reported to work against cancer by inhibiting the growth of blood vessels that feed tumors but not other blood vessels. The other two components of the pheromone are a second steroid and an already known, lamprey-specific bile acid derivative.
Until the pheromone can be synthesized in bulk, extracts of water from larval lamprey nurseries are being used to trap adults on an experimental basis in Michigan streams.
"Using these extracts has been shown to work," said Sorensen. "Capture rates are up six-fold." Most lamprey traps have been simple, rather ineffective devices, but now the attractants have opened up many new possibilities for attracting lampreys safely, easily and inexpensively into streams, where they might be captured. In theory, the cue could attract lampreys from many locations for miles around and could work for as long as several months.
The work was supported by GLFC, the University of Minnesota Agricultural Experiment Station and the National Institutes of Health.
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