31 October 2002
Used Herbicide May Cause Frog Mutation
DAVID P. HAMILTON
one of the most common herbicides in the U.S. turning frogs into
hermaphrodites in the wild?
data from a field study conducted by Tyrone Hayes, a professor of
integrative biology at the University of California at Berkeley,
suggest that low residues of the herbicide atrazine in agricultural
runoff may be seriously disrupting the sexual development of leopard
frogs across the central and western U.S.
research report, to be published Thursday in Nature, adds fresh
evidence to a controversy over whether atrazine is an "endocrine
disruptor," a chemical that can interfere with reproduction
and development by altering hormonal balances in a variety of organisms.
group of researchers funded by Syngenta Crop Protection Inc., a
maker of atrazine and a unit of Syngenta AG of Switzerland, immediately
denounced the Hayes study, charging that it drew sweeping conclusions
from limited and inconsistent data. Atrazine is undergoing a regularly
scheduled "re-registration" review of its health and ecological
effects at the Environmental Protection Agency.
if atrazine eventually proves to be an endocrine disruptor in amphibians,
it may not pose a risk to humans. Frogs are especially vulnerable
to water-borne chemicals because they develop to maturity while
immersed in water. Humans, by contrast, appear to rapidly purge
atrazine from the body in their urine. Still, frogs share some important
reproductive traits with humans. Early pregnancy tests, in fact,
involved exposing female frogs to a woman's urine because a human
hormone associated with pregnancy can cause the frogs to ovulate.
July, Dr. Hayes and several students, accompanied by an 18-wheel
freezer truck, set out to collect samples of leopard frogs and ground
water from Utah to Iowa. At each location, the team gathered 100
frogs, immediately killing them and preserving them in ethanol,
as well as 100 milliliters of nearby ground water, which they froze.
they returned, the researchers sent the water samples to two outside
laboratories to analyze atrazine concentrations. Then they examined
the frogs' gonads, determining that 10% to 92% of male frogs from
seven of the eight sample sites were actually hermaphrodites whose
testes produced egg cells.
team also found that the highest rate of sexual deformities came
from the North Platte River area in Wyoming, far from direct agricultural
runoff. Water-sample tests revealed a low but measurable level of
0.4 part per billion of atrazine there. By contrast, at a cornfield
in Polk County, Iowa -- the site with the highest level of atrazine
exposure -- fewer than 20% of male frogs had developmental problems.
field results lend support to Dr. Hayes's earlier lab studies. After
applying measured doses of atrazine to aquariums in which frogs
grew up from tadpoles, his group found that atrazine concentrations
as low as 0.1 part per billion were sufficient to produce reproductive
endocrine disruptors also have more significant developmental effects
at lower concentrations. In the case of leopard frogs, Dr. Hayes
suggests that higher atrazine levels may short-circuit developmental
circuitry in the pituitary gland, shutting down reproductive-organ
development before hermaphroditism can develop.
implications are disastrous," says David Sassoon, a molecular
developmental biologist with the Mt. Sinai School of Medicine in
New York. "Something like this has been suspected for quite
some time. It has just been difficult to figure out what it is."
other researchers caution that the Hayes report doesn't prove that
atrazine is solely responsible for the developmental problems in
amphibians. Joe Thornton, the author of a book on endocrine disruptors,
says the Nature study should be replicated on a larger scale at
different sites before the findings can be considered conclusive.
Nevertheless, he said, "We now have enough information to be
very concerned about atrazine's effects on wildlife, and possibly
scientists levy harsher criticisms, charging that Dr. Hayes overlooked
the fact that hermaphroditism in juvenile leopard frogs is common
in the wild. The Atrazine Endocrine Ecological Risk Assessment Panel,
a group of eight researchers funded by Syngenta via a consulting
firm, released a detailed two-page critique of the study.
Hayes once served on that panel. He resigned more than a year ago,
citing disagreements with the panel and Syngenta over the direction
of his research.
to David P. Hamilton at firstname.lastname@example.org