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Some of the female chinook salmon who spawn along a stretch of the Columbia River in Washington State hold a secret: They began life as males.
Researchers with the University of Idaho and Washington State University analyzed DNA from the salmon that laid eggs and died in the Hanford Reach, the only free-flowing stretch of the Columbia east of Bonneville Dam. They found that instead of the normal XX chromosomes, four-fifths of the females tested were XY, indicating they began as males and developed as females.
Sex reversal, which occurs in a fish's embryo stage, is not uncommon. In some types of fish farming it is practiced routinely, by exposing the embryos to hormones.
But the Idaho and Washington State researchers, whose work appeared in Environmental Health Perspectives, said their results were the first evidence of sex reversal occurring in the wild (although they cautioned that their sample was relatively small).
This stretch of the river passes the Hanford Site, the government- run nuclear reservation where much of the nation's plutonium was processed, but the researchers ruled out radiation as a cause of the sex reversal. Instead, they suggest that pesticides or other environmental contaminants that can mimic the action of hormones may be to blame.
And they say the sex reversal may help explain some of the difficulties Columbia salmon have had in reproducing. These females could produce YY males, which would then be capable only of producing male offspring, upsetting the sex balance in a fish population.
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