Something fishy happens when the pesticide DDT gets into eggs—it can transform genetically male fish into apparent females. These altered males are fertile, able to lay eggs that produce young, according to a new study.
“We were really not expecting to see complete [sex] reversal at all,” says John S. Ramsdell, a toxicologist from the National Oceanic and Atmospheric Administration (NOAA) in Charleston, S.C. “This is a rather interesting biological observation.”
Ramsdell and his colleagues report their results in the March Environmental Health Perspectives.
Sex changes in fish are fairly common (SN: 10/21/95, p. 266: http://www.sciencenews.org/sn_edpik/ls_4.htm), and scientists have known that a type of DDT called o,p’-DDT weakly mimics the sex hormone estrogen and can cause sex changes in fish and other animals (SN: 7/15/95, p. 44). This is the first time, however, that scientists have seen normal reproduction in fish that are sex-reversed by chemicals, says Marius Brouwer, an environmental biochemist from the University of Southern Mississippi in Ocean Springs.
Adult fish tuck away DDT and other toxins in fat stores. When the fish spawn, they move fat from these stores into eggs to nourish developing embryos. In this way, Ramsdell says, mother fish effectively concentrate potentially harmful chemicals and then expose developing embryos to them.
Ramsdell’s colleague J. Stewart G. Edmunds imitated that process in the lab by injecting a drop of o,p’-DDT-laced oil into the yolks of eggs from medaka fish.
Usually, scientists can’t determine if any individual fish has changed sex, so they report whether the ratio of males to females in an entire population of fish has changed. The NOAA researchers used a trick to track the sexual fate of single fish. Genetic males produce a pigment that makes them orange, while genetic females are white. Fish that undergo a sex change retain the color appropriate to their original genetic programming as they develop other physical characteristics of the opposite sex.
The researchers found that DDT injections completely transformed six of seven genetically male fish into females. Although orange, the six had female body parts and laid eggs that hatched. The seventh appeared to be a normal male.
Injections of other common pollutants that could be carried by fat into eggs produced defects in the eyes, spine, and brain of the fish, Ramsdell says. “This route of exposure may be something that needs to be of greater concern to toxicologists,” he says.
The study confirms that contaminated fish can pass toxins on to the next generation through eggs, says reproductive endocrinologist Louis J. Guillette Jr. from the University of Florida in Gainesville.
The researchers dosed the eggs in their study with quantities of o,p’-DDT many times higher than would typically be found in wild fish. Nevertheless, this study shows that even a weak estrogen mimic such as o,p’-DDT can profoundly alter development, says Diana M. Papoulias, a fisheries biologist from the U.S. Geological Survey in Columbia, Mo. Papoulias has used the potent estrogen estradiol to induce sex reversals in medaka.
Mixtures of estrogen mimics found in pollutants could disrupt development in unexpected ways, adds Frederick S. vom Saal, a reproductive biologist from the University of Missouri-Columbia.