Mystery behind rapid sex change in fish revealed

Scientists have unlocked the secrets of sex change in fish

flock-of--standard--clownfish-sea-fish-shut A flock of standard clownfish | Shutterstock

Scientists have, for the first time, unlocked the mystery of how 500 species of fish—including the famous clownfish—change their gender in adulthood, often in response to environmental cues.

"Most blue-head wrasses begin life as females, but can change sex sometime later to become males—a process that takes just 1021 days from start to finish," said Erica Todd from the University of Otago in New Zealand.

"When a dominant male is lost from a social group, the largest female transforms into a fertile male in 10 days flat.

"Females begin this transformation within minutes, first changing colour and displaying male-like behaviours. Her ovaries then start to regress and fully functional testes grow in their place," she said.

This phenomenon occurs in about 500 species of fish including the clownfish and the kobudai as a natural part of their life cycle, according to the study published in the journal Science Advances.

The researchers by using the latest genetic approaches, high-throughput RNA-sequencing and epigenetic analyses, discovered when and how specific genes are turned off and on in the brain and gonad so that sex change can occur.

"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," she said.

This chain reaction begins when the female hormone (estrogen) making gene called aromatase is turned off. What triggers aromatase to turn off is unknown.

However, the stress of social change resulting from the loss of the existing dominant male may be an important signal in turning off the genetic pathway that maintains the ovary.

Oscar Ortega-Recalde from The University of Otago said 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," Ortega-Recalde said.

"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," he said.

"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," Todd said.

A blue-head wrasse's ovaries can be transformed into new testes at a remarkable speed which also opens up the possibility of applications in tissue and organ engineering, with potential benefits to medical science.