How Males Become Females
|IOL February 28, 2007|
Frogs that started life as male tadpoles were changed in an experiment into females by oestrogen-like pollutants similar to those found in the environment, according to a new study.|
The results may shed light on at least one reason that up to a third of frog species around the world are threatened with extinction, suggests the study, set to appear in the journal Environmental Toxicology and Chemistry in May.
In a laboratory at Uppsala University in Sweden, two species of frogs were exposed to levels of oestrogen similar to those detected in natural bodies of water in Europe, the United States and Canada.
The results were startling: whereas the percentage of females in two control groups was under 50 percent - not unusual among frogs - the sex ratio in three pairs of groups maturing in water dosed with different levels of oestrogen were significantly skewed.
Even tadpoles exposed to the weakest concentration of the hormone were, in one of two groups, twice as likely to become females.
The population of the two groups receiving the heaviest dose of oestrogen became 95 percent female in one case, and 100 percent in the other.
"The results are quite alarming," said co-author Cecilia Berg, a research in environmental toxicology. "We see these dramatic changes by exposing the frogs to a single substance. In nature there could be lots of other compounds acting together."
Earlier studies in the United States, Berg explained, linked a similar sex-reversal of Rana pipiens male frogs - one of the two species used in the experiment - in the wild to a pesticide that produced oestrogen-like compounds.
"Pesticides and other industrial chemicals have the ability to act like oestrogen in the body," Berg said. "That is what inspired us to do the experiment," she said referring to her collaborator and lead author of the article, Irina Pettersson, also a researcher at Uppsala.
The other species examined was the European common frog, Rana temporaria.
Some of sex-altered males became fully functioning females, but other had ovaries but no oviducts, making them sterile, Berg explained.
The study does not measure the potential impact of pollutant-driven sex change for frog species, but the implications, said Berg, are disquieting.
"Obviously if all the frogs become female it could have a detrimental effect on the population," she said.
The only immediate remedy, she continued, would be to improve sewage treatment in areas where frogs and other amphibians might be affected to filter out oestrogen concentrations coming from contraceptive pills and from industrial pollutants.