Scientists Unlock Mystery of Sex Changes in Fish

Clownfish in the Andaman Coral Reef. (Ritiks via Wikipedia)

(CN) – Hundreds of species of fish are able to completely reverse their sex several times during their lives, and researchers in New Zealand said in a study published Wednesday they have finally identified the genes that allow for this transformation.

The Caribbean bluehead wrasse, the kobudai and the clownfish – made popular by its central role in the Disney-Pixar film “Finding Nemo” – are some of the 500 species of fish that routinely change their sex, according to the study published in Science Advances.

Biological sex is not fixed at birth for all plant and animal species, University of Otago scientist and study co-author Erica Todd said in a statement Wednesday. But the science behind the genetic-level sex transformation in fish has long been a mystery.

“Most bluehead wrasses begin life as females, but can change sex sometime later to become males – a process that takes just 10 to 21 days from start to finish,” Todd said, adding the transformation typically occurs when a female fish replaces a dominant male fish that leaves its group.

Researchers used RNA-sequencing and epigenetic analyses to determine the specific genes that fish turn on and off in both the brain and gonad to facilitate the sex reversal.

They found that a chain reaction leading to “gonadal metamorphosis” begins when the gene aromatase – which is responsible for making the female hormone estrogen – is turned off.

“Our study reveals that sex change involves a complete genetic rewiring of the gonad,” Todd said. “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.”

Study co-author Oscar Ortega-Recalde said the research reveals sex reversal in fish involves “profound” changes to chemical markers cells place on DNA to control gene expression and body function.

“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,” Ortega-Recalde said.

Researchers induced sex changes in female fish by removing dominant males from their groups in patches of coral reef off the coast of Florida, according to the study.

The bodies of female fish began to transform within minutes after the males left their group. The female fish’s transformation begins with color changes, and then they begin to display male-like behaviors.

Eventually, a female fish’s ovaries will regress to a point where fully functioning testes grow in their place.

Researchers said further analysis of the speed with which bluehead wrasse’s ovaries transform into new testes may hold applications for aquaculture industries as well as tissue and organ engineering.

“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.

Contributors to the study include Otago’s Neil Gemmell and Tim Hore, and researchers from North Carolina State University.

The study was funded by the Royal Society of New Zealand Marsden Fund, the Rutherford Foundation, and the National Science Foundation.

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