Researchers say they have found that a genetic marker associated with male salmon isn't actually a reliable way to distinguish male from female.

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Don’t write off male chinook salmon in the Columbia River just yet.

Six years ago, scientists reported that 84 percent of the female fall-run chinook they examined in the Hanford Reach appeared to have begun their lives as males. It was a startling finding that raised fears that the Columbia River’s ailing salmon were the latest victims of estrogen-mimicking chemicals that can switch the gender of fish, perhaps affecting the future ability of the population to reproduce.

Now it seems that the Hanford Reach salmon didn’t undergo sex changes after all.

Researchers say they have found that a genetic marker associated with male salmon isn’t actually a reliable way to distinguish male from female.

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The marker is a sequence of DNA that’s repeated 300 times in a male chinook’s Y chromosome. It was believed to be a genetic fingerprint of a male, so it caused confusion when it turned up in the chromosomes of female chinook.

Scientists from the University of Idaho, Washington State University and Battelle Pacific Northwest National Laboratory initially suggested that something in the environment — perhaps excessively warm water or the estrogenlike chemicals — had turned boy fish into girl fish.

Not long afterward, scientists at the University of California, Davis, found female chinook in the Sacramento and San Joaquin river basins with the same supposedly male marker.

The idea that salmon had been feminized by synthetic chemicals wasn’t outrageous. Other male fish exposed to feminizing chemicals in both the lab and in some heavily polluted waterways have developed so-called “intersex” characteristics — developing both male and female sexual characteristics — or have been transformed into females.

But after further study of salmon from the Columbia and Sacramento rivers, scientists have backed away from the chemical-sex-change hypothesis. If there were sex-reversed fish in the Columbia River, University of Idaho zoologist James Nagler wondered, why hadn’t anyone also found intersex fish, which are produced by lower concentrations of estrogen-like chemicals?

The concentrations of potentially feminizing pesticides and herbicides measured by the U.S. Geological Survey in the Columbia weren’t at levels associated with sex reversal elsewhere, said Nagler, who led the initial study of chinook salmon.

A more likely explanation, Nagler said, is that “these pieces of DNA that we were measuring were randomly moving around in the genome.”

California researchers, meanwhile, did a breeding experiment to settle whether there had been a sex change. They fertilized the eggs of questionable females with the sperm of incontrovertible males. If the females had begun life as males, one-quarter of their offspring would develop female sexual organs, three-quarters would develop male.

But half the offspring were male, half female. So chemicals hadn’t caused a gender reversal.

That came as a relief to Kevin Williamson, the Northwest Fisheries Science Center geneticist whose salmon-breeding experiment at UC Davis debunked the sex-reversal hypothesis he had espoused earlier.

Contradicting his earlier work, he said, was “one of the great things you can do as a scientist. You prove yourself wrong. That sounds odd, but that’s how science works.”

Keith Ervin: 206-464-2105 or

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