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The Spokesman-Review Newspaper
Spokane, Washington  Est. May 19, 1883

Salmon don’t swap gender

When scientists announced seven years ago that chinook salmon in the Columbia River appeared to be switching genders, it attracted a lot of attention.

For one thing, there was the simple strangeness of it – researchers said more than 80 percent of female fall-run chinook that spawned at the Hanford Reach in 1999 had started their lives as males. The news had potentially dire implications for the fish run, suggesting that future generations could become ever more heavily male and smaller.

Now the scientists are saying something new: Never mind.

It turns out that genetic “markers” the researchers were using in those studies don’t mark what scientists thought they did. And subsequent studies of Sacramento River fish that exhibited that marker produced offspring at the expected ratio – half male, half female.

“My conclusion right now, at least based on the information my lab and other labs have generated, is I don’t think there’s evidence for sex reversal,” said James Nagler, a University of Idaho scientist who led the research.

The story of the sex-changing chinook drew a lot of media coverage and concern from scientists and environmentalists. But Nagler was always circumspect about the findings, saying in a 2000 Spokesman-Review story, “We can speculate all day long about what this means, but we really don’t know yet.”

What his subsequent research showed was that science is still sorting through the emerging knowledge about genetics. Nagler and other researchers had been studying the fish using a genetic “marker” – a sequence of DNA – that was believed to indicate the presence of the male Y chromosome. But further study shows the marker itself doesn’t reside on the Y chromosome, but possibly moves around the salmon’s genome, and doesn’t seem to indicate a male fish at all.

The way the information and assumptions changed is a typical pathway for science – researchers establish hypotheses, gather data to see whether they’re correct, and then double- and triple-check it. Nagler said he didn’t initially expect to see so much sex reversal in his studies, and he’s not disappointed now that those initial findings didn’t hold up.

“I’m not disappointed that we didn’t find sex reversal,” he said Friday. “I’m kind of relieved that we didn’t. If we had, it would have suggested some pretty extreme things going on in the environment.”

The initial findings of the researchers – including Nagler and Washington State University’s Gary Thorgaard – weren’t as impossible as they might sound. Gender-bending in fish populations is well-established in lab environments, and researchers knew the presence of certain chemicals could cause the development of “intersex” fish, with characteristics of both males and females, or even total sex reversal.

Back in the late 1990s, Nagler wanted to find out whether such sex reversals occurred in the wild. Genetic research about the chinook salmon at the time had indicated the existence of the genetic markers appearing on the Y chromosome.

When Nagler’s team began examining fish in the 1999 Columbia River chinook run, they found a lot of females with the “male” marker. They hypothesized that such a sex reversal might be caused by heavy concentrations of chemicals in the river – the same chemicals that scientists used in the lab to reverse a fish’s gender.

“That was a surprise to us,” he said. “That wasn’t what I expected.”

But subsequent research by Nagler and others called into question the scientists’ assumptions about the marker. Using another such genetic marker that was believed to indicate the presence of a Y chromosome, they found that not only did some females have the male marker – but some male fish lacked it.

Scientists in California did some studies involving breeding Sacramento River fish who showed the markers to see what happened to subsequent generations.

If female fish did indeed carry the Y chromosome, that should have meant that future generations became more and more heavily male. Instead, the gender ratio of the offspring stayed normal.

“This was additional evidence supporting the idea that this marker was not linked to sex,” Nagler said.

Another indicator was the absence of “intersex” fish. Lab studies of gender changes in fish showed that chemicals which interfere with sex differentiation among fish could cause the development of fish with both male and female characteristics at low levels, and full gender reversal at high levels.

So if scientists found sex-reversed fish, they also ought to find some intersex fish, Nagler said. But “we never found that.”

He said he now believes that the genetic marker may be a sequence that moves around in the genome of different fish – though it’s still being studied.

Meanwhile, he and his colleagues are working to find a true genetic marker of fish sex, as well as seeking to learn more about the DNA sequence that appears to move around.

“Is that just something that’s normal in fishes, or is it unique to the chinook, or is there an environmental factor that is enhancing this movement of the DNA?” he said.