Bluehead wrasse help reveal how some fish can change their sex

There are many species in the animal kingdom where sex is not fixed at birth. 

Nearly 500 species of can completely change their sex, including clownfish and the Caribbean bluehead wrasse, but how these biological changes occur at the genetic level is not well understood. 

Researchers from the University of Otago in New Zealand conducted a thorough genetic analysis of bluehead wrasse to identify the underlying mechanisms that trigger sex changes in females. 

A female wrasse can change their sex and become male in 10 to 21 days. 

“When a dominant male is lost from a social group, the largest female transforms into a fertile male in 10 days flat,” said Dr. Erica Todd, the lead author of the study. “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.” 

The researchers used high-throughput RNA-sequencing and epigenetic analyses (inheritable genes) to study the sex change process in females and found that specific genes turning off and on in the brain prompts the gonads into action. 

A gene called aromatase is turned off, which starts a chain reaction. Aromatase is the gene responsible for estrogen production. 

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

What triggers the aromatase gene to switch off is still not known, but the researchers theorize that the stress of losing a dominant male could signal the change. 

The study sheds light on the underlying genetic and chemical markers that make sex change possible in blue wrasse fish, and the research could have important implications in studying sexual development in other vertebrates, including humans. 

“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,” said Oscar Ortega-Recalde, the co-lead author of the study. “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.” 

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By Kay Vandette, Earth.com Staff Writer

Image Credit: Kevin Bryant