2023-10-23 13:31:18
In 1981, scientists discovered that female fish exposed to high temperatures developed testes instead of ovaries. Since then, more than 1,100 studies on different animal species, including 400 on freshwater fish, have produced similar results.
This raises several questions.
Why does this phenomenon occur? How can we explain it? Does this harm fish populations in the long term? Our research has shown that a key factor explaining this phenomenon is the overproduction of stress hormones caused by higher temperatures.
No time to adapt
The reproductive organs of fish are very malleable to environmental changes, because unlike mammals, they have simpler structures. So even slight changes in water conditions can have a direct and significant impact on the metabolism and physiology of fish.
Thousands of dead fish float in Lake Piranha due to severe drought in Amazonas state in Manacapuru, Brazil. Global warming is displacing suitable fish habitat at a rate to which most are unable to adapt. (AP Photo/Edmar Barros)
Fish use this plasticity to their advantage by using environmental cues to modify their reproductive success according to seasonal conditions. For example, several species of fish, such as lake sturgeon, breed in the spring due to warmer water temperatures.
However, sudden environmental changes caused by climate change, for example, significantly affect fish populations and push some of them to move towards more suitable breeding habitats.
Temperature can change female fish to male
The study of how female fish become male (or masculinized) due to temperature change has led to a significant breakthrough. When fish are exposed to temperatures outside of their normal range, they become stressed and exhibit high levels ofstress hormone called cortisol. This is the case for several species of fish, such as Silverside argentinthe medaka et zebrafish.
Interestingly, the same enzyme that generates cortisol is also responsible for producing the most potent male hormone in fish, called cortisol. 11-ketotestosterone. The role of this male hormone is to trigger the development of male sexual characteristics in fish.
If fish experience stress – i.e. increased cortisol – from high temperatures, this can disrupt the hormonal balance of fish larvae and lead to the development of testicles. Overproduction of androgens causes more males to develop than females under high temperatures.
Not just stress
In 2019our research group has demonstrated that blocking stress receptors through gene editing tools completely suppressed high temperature-induced masculinization of fish. These results reveal, for the first time, that the brain acts as a driver of masculinization induced by heat stress.
In our new study published in Cellular and Molecular Life Sciences in 2023, we further demonstrated that thyroid hormones, in addition to stress hormones, are involved in fish masculinization. Once again, using gene editing, we were able to block stress receptors and demonstrate that the thyroid hormone pathway is affected when fish experience stress.
The larvae of the Centrarchid fish species. Warming temperatures cause fish larvae to disproportionately develop male sexual organs. (Wikimedia), CC BY
It was observed that when the production of cortisol and thyroid hormones was suppressed by the combined use of certain chemical drugs, no females were masculinized. Understanding the molecular mechanisms that explain fish sex determination allows us to predict how temperature induced by climate change may affect fish populations in the future.
The role of pollution
Several environmental contaminants, such as pesticides and plasticizers, are known to disrupt hormonal balance in animals. These contaminants – known as endocrine disruptors – can lead to abnormal development of sexual organs in fish.
With climate change, environmental factors that affect sexual development are now a major issue. Lately, temperatures have fluctuated considerably beyond the acceptable range for most fish species. Such changes cause high temperatures, acidification and hypoxia which can alter the natural sex ratios of fish in favor of males, and even generate populations of exclusively male fish.
In rivers and lakes, interannual events such as El Niño or El Niña can also be modified by climate change, which can cause significant periods of flooding or drought. This can exacerbate the stress placed on the fish. Furthermore, too few females in a fish population can cause its collapse, with disastrous consequences for biodiversity for all.
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