Turning up the heat turns female fish male: Study

Dr Liew (left) and Prof Orban found that heating water zebrafish are in not only created females that turned into males, but also caused the development of females that expressed male genes. Their findings could have implications for fish populations and the supply of fish.
PHOTO: The Straits Times

Local scientists have discovered that subjecting juvenile zebrafish to elevated temperatures - by heating up the water they are in - forces the females to turn into males, lending support to the theory that global warming may be affecting fish populations.

The research by Temasek Life Sciences Laboratory (TLL), in collaboration with the Institute of Marine Sciences in Barcelona, reinforces earlier studies involving other types of fish which argued that rising sea and ocean temperatures could be skewing the gender ratio of fish towards males.

This could have implications for fish populations and the supply of fish.

While there have been earlier studies, the latest one is the first to document that the sex change remains even at 90 days after the fish are treated with heat.

Curiously, the heat not only created females that turned into males, called neo males, but also caused the development of females that expressed male genes.

A paper on the findings was published this week in scientific journal Proceedings Of The National Academy Of Sciences Of The United States Of America.

On the female fish with male-like gene expression profiles, Dr Liew Woei Chang, a research fellow at TLL and first author of the paper, said: "They are not going to function like a female.

So even if you have many female fish, under heat stress, they might not be fertile, and that could have an impact on the fish population."

In the experiments, more than 800 juvenile zebrafish from six families - each with slight variations in their genetic make-up - were subjected to gradual increases in temperature up to 36 deg C - 8 deg C higher than the control - between 18 and 32 days after fertilisation.

Overall, around 80 per cent of the female fish developed male reproductive organs or abnormal ovaries.

However, Professor Laszlo Orban, senior principal investigator at TLL who led the study, noted that the temperature increases used in the experiments were far more extreme than that experienced in nature, where global sea surface temperature has risen by approximately 1 deg C over 140 years.

"The question is, what happens when you try to increase the temperature by just 2 deg C, but expose the fish not just for two weeks but throughout its lifetime, and then do the same to its offspring and later generations," he said.

The team will be looking to answer that, as well as study if wild zebrafish are affected similarly by temperature increases.

Professor Gong Zhiyuan from the National University of Singapore's (NUS) department of biological sciences said the study's significance extends far beyond the laboratory zebrafish model.

"It should also help us understand the molecular mechanisms of temperature-dependent sex determination in many species in nature," he said.

Besides fish, reptiles can also change their gender as a result of heat.

Professor Chou Loke Ming, an adjunct research professor at NUS' Tropical Marine Science Institute, said the finding provides further evidence of the environmental impact on genes.

Investigations on how temperature change, as well as pollution and climate change, affects various species are needed to predict what biodiversity will be like in a warmer world, he said.

"We live in a world with a changing environment fully attributed to human activity.

"There will be huge impacts on biodiversity, and we are not able to envisage the enormity of the impact if we don't understand the underlying mechanisms."


This article was first published on Jan 28, 2017.
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