Household Pollutants Disrupting Fish Genes

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Unlike most animals, many fish produce two forms of a gene responsible for the enzyme aromatase, which in turn makes estrogen. One form of aromatase is in the ovaries; the other is in the brain.

The study results suggest that direct chemical disruption of the brain aromatase gene, which directly affects production of brain estrogen, may be a key mechanism behind the endocrine-related disturbance of normal reproduction and development in fish.

The researchers first found that the differential expression of the brain aromatase gene was associated with sex differentiation. "It became clear that compounds that affect this gene will thereby affect sex and sexual behavior in fish," said Trant.

In other words, the researchers found that the endocrine-disrupting compounds can regulate the aromatase gene in the brain, affecting more than a fish's sex.

"What is dangerous is that in-between stuff," said Trant. "You might get males who do not display the correct behavior. In order to mate with a female, he may have to court her, build a nest, chase, or show some dominance. So, even if the concentration of these disrupting compounds in the water are not sufficient to completely reverse their sexual physiology, small adjustments in their behaviors would be equally fruitless."

Broader Implications

A growing number of scientists theorize that endocrine-disrupting compounds in the water behave chemically like hormones in fish tissues and cells.

When compounds such as polychlorinated biphenyls, dioxins, certain plasticizers, and some detergent additives are in streams or rivers, groups of fish, birds, frogs, and other animals are sometimes found to be all male, or all female, or are partially both sexes in their genitalia.

Historically, scientists have suspected actual estrogens or chemicals that mimic estrogens in pollution as the causes of the "gender-bending" effects on fish.

Estrogen (or estrogen-like) molecules dock onto a structure called an estrogen receptor in the cells of the liver, ovaries, fat, breast, brain, bone, and many other target tissues in man. The activated receptor initiates a series of changes into action related to sexual physiology. Many of the pollutants, such as PCB's, some pesticides, and petroleum products in the Chesapeake waters, are recognized as estrogen molecules by fish and human cells.

"That's why scientists have focused there," said Trant. "But, this is worse than we thought before. This is not simply toxicology. It is interfering with the reproduction of the adults, and potentially skewing sex ratios of the populations."

The research team reported that the aromatase gene expression in zebrafish was changed by multiple classes of pollutants such as estrogen mimics (or xenoestrogens, such as surfactants in detergents and pharmaceuticals), arylhydrocarbons (PAH's and benzo(a)pyrine), peroxisome proliferators (pharmaceuticals and plasticizers), and herbicides (atrazine).

"For people looking for a magic bullet of why productivity of the Chesapeake Bay is down, this is not it," said Trant. "There are probably many causes. But this is certainly affecting the reproductive health of animals that spawn here and the developmental health of animals that are raised in the Chesapeake Bay."

Scientists have just been looking too narrowly at estrogen mimics, said the researchers. They added that it's almost certain that the multiple compounds are affecting all the fish in the Bay and beyond, not just zebrafish in the laboratory.

Liquid Planet Resources

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