You can take a fish out of toxic water, but its epigenetic mutations will persist for at least two generations.
A research team led by scientists at Washington State University analyzed the epigenetics (molecular factors and processes that determine whether genes are turned on or off) of a group of fish, Poecilia mexicana, or Molly de Atlantic, which live in sources naturally rich in hydrogen sulfide, which is normally toxic to most organisms.
The researchers took a sample of fish from the poisonous water and reared them in freshwater. They found that the grandchildren of the sulphide-adapted fish had more epigenetic markings in common with their wild and poisonous water-living grandparents than other Atlantic molly that had always lived in freshwater.
“After two generations under laboratory conditions, the fish generally retained their same epigenetic markings, which was really unexpected,” said Joanna Kelley, associate professor of evolutionary genomics at WSU and corresponding author of the study published in Proceedings of the National Academy of Sciences. “In an evolutionary context, the study shows that these epigenetic marks are quite stable.”
Hydrogen sulfide is found in nature and as a by-product of many human activities, such as papermaking, wastewater treatment, and gas exploration. For most animals, including humans, it is generally toxic at relatively low levels and lethal at high levels. Yet some populations of Atlantic molly have adapted to live in sources rich in hydrogen sulfide, while other groups of the same species have remained in freshwater. For Kelley and her colleagues, these fish presented a natural experiment to help answer questions about how evolutionary adaptations occur.
For this study, Kelley collaborated with environmental epigenetics WSU and reproductive biologist Michael Skinner, to perform epigenetic analysis. The researchers reared a sample of sulphide and unsulfur fish in freshwater environments. When the fish produced two generations of offspring, they measured their epigenetic similarities, specifically DNA methylation regions, a type of chemical modification that can regulate gene expression, by turning a gene on or off, without altering the DNA sequence chain itself.
They found that the offspring of the sulphide fish had an 80% overlap in DNA methylation regions with the grandparent fish, even though it had been raised in freshwater. This compares to only a 20% overlap with unsulfurized fish that have always lived in freshwater.
“This is really one of the first examples where we took an organism out of its normal environment and put it in a different environment and showed that epigenetics continue to be inherited in the future,” said Skinner, also corresponding author of the article.
The study also adds to evidence from previous animal research showing that exposure to toxins has epigenetic effects that can last for generations. Skinner’s lab has conducted studies on other wildlife, including Darwin Finch in the Galapagos and the New Zealand mud snail, as well as laboratory animals, showing that epigenetic changes resulting from environmental toxins are transmitted. .
âIt’s not a one-off, unique event for a species. It impacts everything, including humans,â Skinner said. “Although this is an animal model, it is a demonstration of how an environmental toxicant can actually alter epigenetics, and it becomes programmed for subsequent generations.”
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Joanna L. Kelley et al., âEpigenetic inheritance of DNA methylation changes in fish living in sources rich in hydrogen sulfideâ, PNAS (2021). www.pnas.org/cgi/doi/10.1073/pnas.2014929118
Provided by Washington State University
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