THE most widely used herbicide in the US, and another chemical found in many household cleaning products, skew the sexual development of zebrafish. But, surprisingly, they do it by affecting their brains, not their gonads.
This adds to a growing body of evidence suggesting that pollutants don鈥檛 have to mimic oestrogen to have a 鈥済ender-bending鈥� effect, and that many more chemicals than we thought could be triggering the increasingly widespread feminisation and masculinisation of fish in rivers around the world.
Until now, the received wisdom has been that gender-bending chemicals work by docking with the cell receptor normally reserved for oestrogen. But John Trant and his team at the University of Maryland Biotechnology Institute have discovered that common pollutants affect sexual development in a different way, by disrupting a brain enzyme called aromatase that converts testosterone into oestrogen.
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Trant exposed juvenile zebrafish in the lab to a dozen common pollutants at levels typically found in US waterways. Some compounds increased the amount of aromatase, some decreased it. Two had a large impact 鈥� the herbicide atrazine, and nonylphenol, a surfactant found in household products.
With nonylphenol 鈥渨e saw 270-fold increases in brain aromatase in undifferentiated larval fish, at the point just before they become males or females.鈥� Atrazine boosted levels of the enzyme by a factor of 200, Trant told a Society for the Study of Reproduction meeting in Baltimore last month.
He suspects that changing levels of aromatase alters the amounts of testosterone and oestrogen in the fish鈥檚 brains, in turn influencing their sexual development. Chris Kirk, who studies gender-bending chemicals at the University of Birmingham, agrees. 鈥淎romatase is an important enzyme in oestrogen synthesis in fish and people,鈥� he says. 鈥淚f you disrupt it early in life, it鈥檚 quite plausible that it could have an impact on sexual development.鈥�
To measure the enzyme levels, Trant had to kill his zebrafish larvae before they had a chance to become male or female. But he now plans to repeat the experiment over three months, giving the fish enough time to reach sexual maturity. That should allow him to spot whether changes in aromatase levels in the fish correlate with sex changes seen in the wild.
