SCIENCE NEWS This Week More Frog Trouble Herbicides may emasculate wild males

New studies of male frogs in the wild link trace exposures to common weed killers with partial sex reversal. The findings sug- gest one possible factor behind declining amphibian populations worldwide.

Atrazine, the nation’s most widely used herbicide, is nearly ubiquitous. Some U.S. rainwater carries up to 0.4 part per billion ofthe chemical. Earlier this year, scientists at the University of California, Berkeley reported bending the gender of male frogs by incubating tadpoles in as little as 0.1 ppb atrazine (SN: 4/20/02, p . 243). Affected animals grew extra testes and sometimes, ovaries.

Those researchers used the amphibian equivalent of a lab mouse, a frog native to South Africa. The Berkeleyteam now reports similarlaboratoryresults intwo U.S. species,

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DAD’S EGGS Ordinarily, a frog testis g pinches during development, and the lower 2 tkro-thirds disappears. However, atrazine- 2 exposed males can retain this lower portion, L which can produce eggs (bumps in the left

photo). Cross-sections of the gonads at $ arrow-marked levels appear at right. They p show dense, immature tissue (top image) and

heavy loads of eggs (middle and bottom 5 images). scale bar at bottom right indicates 9 250 micrometers for cross-section images.

the leopard frog (Rana pipiens) and the Pacifktree frog (Hyla regilk). It also studied wildleopafdfro~fromeightsitesintheMid- west and West. Half came from areas of high atrazine use, but all sites had measurable con- centrations in streams. Concentrations at the most heavily exposed site fluctuated over the year between 0.7 and 15.2 ppb.

At seven of those sites, 10 to 92 percent of the males had underdeveloped testes, Qrone Hayes and his colleagues report in the Oct. 31 Nature. Furthermore, por- tions of some of the frogs’ testes produced eggs, the researchers show in a more detailed report slated to appear in Envi- ronmental Health Perspectives. The sci- entists didn’t detect any effects of atrazine on female frogs.

The males’ malformations-where a testis appears male at the top but becomes increasingly female toward the bottom- have been witnessed only in frogs exposed to atrazine, Hayes told Science News.

“It’s impossible that those gonads are functioning normally,” herpetologist James Hanken of Hamard University’s Museum of Comparative Zoology concluded after seeing photos from the new study.

At a symposium hosted by Wane and Xavier Universities last month in New Orleans, Hayes described lab studies of tad- poles exposed to a blend of atrazine and metolachlor, another common herbicide. When exposed to concentrations seen in runoff from Nebraska cornfields, each, by itself, appeared to prolong metamorphosis very slightly. However, when delivered together, the pair increased the tadpoles’ metamorphosis time-normally 60 days from egg to frog-by 10 days.

Hayes points out that prolonging meta- morphosis increases an amphibian’s expo- sure to atrazine while the animal is most vulnerable-during development. In fact, he’s found that the most dramatic emascu- lating effects occur in atrazine-exposed lab frogs that develop most slowly. Hayes is now studying whether the affected frogs are even fertile.

What Hanken finds ”especially worri- some” in Hayes’ findings is that trace con- centrations of atrazine affect frogs’ testes. That’s troubling, says Hanken, because ”rainfall virtually everywhere now has atrazine.”

Joseph Kiesecker of Pennsylvania State University in State College also worries that these herbicides “can serve as a stressor that interacts with disease outbreaks in [foster- ing] amphibian declines.”

However, toxicologist Tim Pastoor, with the atrazine maker Syngenta in Greensboro, N.C., remains unpersuaded. He’s particularly troubledbyasurprising~~bythe Berke- ley group: that 25 ppb exposures of atrazine in the lab produced only one-third as many hermaphrodites as 0.1 ppb did. “It doesn’t appeal to common sense,” Pastoor says.

Hayes disagrees. Atrazine’s impacts in frogs appear to stem from its stimulation of aromatase, an enzyme that converts testos- terone to estrogen, he says. Normally, when gonads make too much estrogen, some of the excess enters the blood and signals the pituitary gland to shut down aromatase. However, Hayes speculates, low concentra- tions of atrazine probably stimulate the testes to make enough estrogen to feminize them but not enough to engage the pitu- itary‘s shutdown switch.

The big question, he and others main- tain, is why atrazine appears to trigger excessive aromatase activity in the first place. -J. RALOFF

Neural Shape-Up Brain anticipates object perception

Many neuroscientists suspect that in order to see, a person first sorts through edges, contours, and other basic visual features using a brain area called the primary visual cortex. Then so-called higher visual areas of the brain assemble these features into per- ceptions of shapes and objects.

However, according to a new investiga- tion, at least one of these higher areas shapes what we see from the get-go, and it does it by actually suppressing activity in the primary visual cortex.

This give and take of higher and lower parts of the visual system allows the brain to use predictions about the world, gener- ated from experience and presumably stored in neural structures, to perceive objects under often ambiguous circum- stances, propose neuroscientist Scott 0. Murray of the University of California, Davis and his coworkers.

“Higher visual areas anticipate the struc-

DOUBLE VISION In one experiment, volunteers watched this diamond with obscured corners move back and forth. The four visible segments sometimes appeared to move independently and sometimes as a group.

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