Salmon puzzle: Why did males turn
female?
Science News: Volume No. 26 & 27
Every year, rivers of chinook--the Pacific's largest salmon--leave the
ocean for an upstream trek into the streams of their birth. When these
4-to-6-year-olds reach home, they spawn and die. Surprisingly, a new study
finds, most of the moms in one of Washington State's major spawning populations
appear to have begun their lives as males.
"This is clearly abnormal,"
notes James J. Nagler, a fish reproductive biologist at the University of Idaho
in Moscow. The findings that he and his colleagues reported late last week worry
several environmental scientists.
"Salmon are declining everywhere,"
observes Kelly Munkittrick, an ecotoxicologist with Environment Canada who's
based at the University of New Brunswick in Fredericton. He says the discovery
of sex reversed male chinook (Oncorhynchus tshawytcha) provides "another
potential piece of the puzzle as to why these animals are in so much
trouble."
During an analysis of adult salmon gonads performed last year,
Nagler identified 50 males and 50 fish that appeared to be female at each of
three spawning sites: hatcheries in Washington and Idaho and a river bend called
Hanford Reach near Richland, Wash. The last of those sites is the most important
fall spawning area for wild chinook in the main Columbia River.
Nagler
took small fin samples and then assayed the DNA for a chemical normally
associated with a male's Y chromosome. Hatcheries use this assay to determine
gender in young chinook.
No surprise, the males all tested positive.
However, 84 percent of apparent females from the Hanford Reach also tested
positive for the male-associated chemical, whereas none of the females spawning
at the hatcheries did. In the January ENVIRONMENTAL HEALTH PERSPECTIVES,
Nagler's team argues that some pollutant probably converted wild male embryos or
hatchlings into fish that look like and function as females.
Unfertilized
fish eggs normally carry one of the mother's two X chromosomes. Sperm can carry
either the father's X or his Y chromosome.
When hormone-mimicking
chemicals create chromosomally male fish that lay viable eggs (SN: 2/5/00, p.
87), some of these eggs carry a Y chromosome instead of an X. That increases the
odds that an offspring will get at least one Y chromosome and be male.
Furthermore, some offspring may get a Y from both mother and father and become
so-called supermales.
If supermales reproduce, Nagler says, any offspring
would invariably be male--skewing the sex ratio in a population. Eventually,
females could all but disappear. The scenario offers a new potential explanation
for the decline of many salmon populations, says Nagler.
The trigger for
the chinook's sex reversal remains a mystery. Though estrogen-mimicking
pesticides running off upstream croplands could play a role, Nagler has found no
reports of high concentrations in the Columbia River near Hanford Reach. Another
suspect, he notes, is the daily drop of several degrees in water temperature in
response to a nightly release of cold water from hydroelectric dams. For several
fish species, including another salmon, temperature variation has induced gender
changes in laboratory studies.
By not releasing fish until many weeks
after their gender becomes irreversible, hatcheries appear to protect fish from
such environmental influences, Nagler says. But why weren't all the Hanford
Reach males feminized? It's possible, Nagler says, that some came from a
hatchery. "At Hanford," he says, "hatchery fish could be masking the impact of
what's going on in wild fish."
As the first report of environmental
gender bending in wild Pacific salmon, "this is an extremely important paper,"
notes Don Campton, a regional fish geneticist with the U.S. Fish & Wildlife
Service in Longview, Wash. Further studies are needed to confirm the result and
find out whether it represents a one-season fluke. Nevertheless, he worries it
may also signal risks facing other fisheries.
Munkittrick agrees. He
argues that whatever triggered the chinook's sex reversal may threaten its
beleaguered kin, including Atlantic salmon, which he studies. "If we can get
access to similar tools," such as an assay for the Y chromosome of the Atlantic
species (Salmo salar), he told SCIENCE NEWS, "we're definitely going to
see if similar things are going on here." --J. Raloff