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Pesticides Linked to Frog Mutations

February 28, 2006

 

According to two new related studies by scientists at the University of

California at Berkeley, commonly used pesticides disrupt the

development of sex organs in frogs, weaken their immune systems, delay

and stunt development, and otherwise contribute to declining frog

populations.

 

" If you look at one of these frogs, it's probably a hermaphrodite -

plus, it metamorphoses late, which means it is subject to its pool

drying up before it can become a frog, " said lead researcher Tyrone

Hayes, professor of integrative biology at U.C. Berkeley, and a

Pesticide Action Network associate. " It's also smaller, if it

metamorphoses at all, which increases the likelihood it will be eaten

and decreases its ability to eat. Plus, it's immuno-suppressed, and

more prone to die from infection. " The group observed that mixtures of

pesticides that accumulate in ponds near farms increased frog stress

hormone levels, creating holes in the thymus gland that likely causes

the impaired immune response.

 

" It's not the pesticides alone or introduced predators or ultraviolet

light or global warming that's causing this decline, but the

interaction between these on an animal that is pretty sensitive to its

environment, " said Hayes.

 

In research conducted four years ago, Hayes showed that atrazine, the

most common weed killer used on corn in the United States, disrupts the

sexual development of frogs by producing more hermaphrodites,

decreasing the size of their vocal organs (critical to mating success),

and causing a tenfold drop in testosterone in mature male frogs.

 

In one of the studies published online in Environmental Health

Perspectives, Hayes reported even stronger evidence that atrazine, a

powerful endocrine disruptor, both chemically castrates male frogs by

blocking the action of the male steroid androgen and by stimulating the

production of the female hormone estrogen.

 

" One week of exposure at the critical time is all that's required to

make these males look feminine, which probably interferes with mating, "

he said. While noting that some frogs seem to adapt to atrazine by

delaying development, presumably so that the critical developmental

period takes place when the herbicide is at its lowest, Hayes suspects

that not all frogs would adapt quickly enough to survive. Plus, delayed

maturation comes at the risk of having the pond turn into a puddle and

dry up before the frog completely metamorphoses.

 

In the other study also published online in Environmental Health

Perspectives, Hayes looked at the combined effect of various pesticides

on the health of frogs. His research group again examined atrazine as

well as three other herbicides, two fungicides and three insecticides

used on Midwestern cornfields. All nine were found in the scientists'

study area in Nebraska in pools of water beside cornfields early in the

growing season, when spraying typically occurs. Levels ranged from 0.1

parts per billion (ppb) to 10 or more ppb.

 

Hayes and his colleagues analyzed four years of data indicating that

while some of the pesticides, herbicides and fungicides used on corn

fields may not by themselves have a noticeable impact on frogs, in

combination they create significant effects. Among these are delayed

maturation (the tadpoles take longer to metamorphose into frogs),

retarded growth, and an increased susceptibility to meningitis caused

by normally benign bacteria.

 

When pesticides were combined, they had a stronger effect. All nine

compounds together at 0.1 ppb - one of the lower concentrations

measured in the field - lengthened the time to metamorphosis by 15

days, or about 25 to 30 percent. The mixture also caused a frog

mortality of 35 percent.

 

All nine compounds together also produced a startling effect: the

longer a tadpole took to mature into a frog, the smaller it was. It's

normally the other way around, Hayes said. Separately, six of the

pesticides did not affect this correlation, but three disrupted frog

metamorphosis to the degree that there was no relationship between time

and size. " In humans, this is like saying, 'The longer you are

pregnant, the smaller your baby will be,' which means the womb is no

longer a nurturing environment, " Hayes said.

 

" Estimating the ecological risk and the impact of pesticides on

amphibians using studies that examine single pesticides at high

concentrations only may lead to gross underestimations of the role of

pesticides in amphibian declines, " Hayes concluded.

 

Hayes' laboratory colleagues were UC Berkeley students Paola Case,

Sarah Chui, Duc Chung, Cathryn Haefele, Kelly Haston, Melissa Lee, Vien

Pheng Mai, Youssra Marjuoa, John Parker and Mable Tsui. Co-authors on

the atrazine paper were former UC Berkeley students A. Ali Stuart, Atif

Collins, Nigel Noriega, Aaron Vonk, Gwynne Johnston and Dzifa Kpodzo,

and current students Magdalena Mendoza and Roger Liu.

 

The work was supported by the National Science Foundation, Henry H.

Wheeler, the Park Water Co. and the Howard Hughes Biology Scholars'

Program.

 

Sources:

Hayes, Tyrone B., et al. 2006. " Pesticide mixtures, Endocrine

disruption, and amphibian declines: Are we underestimating the impact? "

Environmental Health Perspectives Online , published January 24th,

2006.

http://www.ehponline.org/members/2006/8051/8051.pdf

 

Hayes, Tyrone B., et al. 2006. " Characterization of atrazine-induced

gonadal malformations in African clawed frogs (Xenopus laevis) and

comparisons with effects of an androgen antagonist (cyproterone

acetate) and exogenous estrogen (estradiol 17Beta]): Support for the

demasculinization /feminization hypothesis. " Environmental Health

Perspectives Online , published January 24th, 2006.

http://www.ehponline.org/members/2006/8067/8067.pdf

Contact:Panna

 

 

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