“This work supports the hypothesis that exposure to some environmental chemicals during sensitive periods of development can cause significant health problems for those organisms later in life – and affect their offspring and, possibly, their offspring’s offspring,” says Dr. Gerald LeBlanc, a professor of environmental and molecular toxicology at NC State and lead author of a paper on the work. “We were looking at a model organism, identified an important pathway for environmental sex determination, and found that there are chemicals that can hijack that pathway.”
Environmental cues normally determine the sex, male or female, of Daphnia offspring, and researchers have been working to understand the mechanisms involved. As part of that work, LeBlanc’s team had previously identified a hormone called methyl farnesoate (Mf) that Daphnia produce under certain environmental conditions.
The researchers have now found that the hormone binds with a protein receptor called the Mf receptor, which can regulate gene transcription and appears to be tied to the production of male offspring.
In experiments, the researchers exposed Daphnia to varying levels of an insecticide called pyriproxyfen, which mimics the Mf hormone. The pyriproxyfen exposure resulted in Daphnia producing more male offspring and fewer offspring in total, with higher doses exacerbating both effects.
“At high concentrations, we were getting only male offspring, which is not good,” LeBlanc says. “Producing fewer offspring, specifically fewer female offspring, could significantly limit population numbers for Daphnia.”
And low exposure concentrations had significant impacts as well. At pyriproxyfen concentrations as low as 71 nanograms per liter, or 71 parts per trillion, the Daphnia would still produce some female offspring. But those females suffered long-term reproductive health effects, producing significantly smaller numbers of offspring – despite the fact that they had not been exposed to pyriproxyfen since birth.
“We now want to know specifically which genes are involved in this sex determination process,” LeBlanc says. “And, ecologically, it would be important to know the impact of changes in population dynamics for this species. Daphnia are a keystone species – an important food source for juvenile fish and other organisms.”
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