Genetic secrets of sex-changing fish discovered

Around 500 species are fish are known to undergo one of the most startling transformations in the natural world – the complete reversal of sex. Now, a study carried out at the University of Otago on bluehead wrasses, a small-bodied fish that lives in coral reefs in the Caribbean, has found out how they do it.

Most bluehead wrasses begin life as females, but when the dominant male is lost from a social group, the largest female transforms into a fertile male in as little as 10 days. They begin this transformation within minutes of the male’s departure, first changing colour and displaying male-like behaviours. Their ovaries then start to regress and fully functional testes grow in their place.

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Using the latest genetic approaches, the researchers discovered that the change is the result of specific genes being ‘turned off’ in the brain and gonad. The transformation begins when aromatase, a gene responsible for making the female hormone oestrogen, is turned off. Exactly what triggers aromatase to turn off is not yet known.

“How this stunning transformation works at a genetic level has long been an enigma,” Dr Erica Todd says. “Our study reveals that sex change involves a complete genetic rewiring of the gonad. We find that genes needed to maintain the ovary are first turned off, and then a new genetic pathway is steadily turned on to promote testis formation.”

The amazing transformation also appears possible through changes in cellular “memory”.

“In fish and other vertebrates, including humans, cells use chemical markers on DNA that control gene expression and remember their specific function in the body. Our study is important because it shows that sex change involves profound changes in these chemical marks, for example at the aromatase gene, thus reprogramming cell memory in the gonad towards a male fate,” researcher Oscar Ortega-Recalde said.

As many genes important for sexual development in fish are also important in other animals, the team’s discovery has practical applications for humans.

“Understanding how fish can change sex may tell us more about how complex networks of genes interact to determine and maintain sex, not only in fish but in vertebrate animals generally,” Dr Todd said.

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