The Secrets Of Fishes' Sex Changes Revealed

The male bluehead wrasse defends the smaller yellow females of his harem. If he is removed the largest female changes behavior, then color, and then develops male reproductive organs. Kevin Bryant

More than 500 species of fish are known to change sex, but the genetic and metabolic processes involved have been poorly understood. Now, a new study reveals the changes in the female-to-male direction in unprecedented detail.

Male bluehead wrasses have a harem of females. “When the dominant male is lost the largest female transforms into a fertile male in 10 days flat. Females begin this transformation within minutes, first changing color and displaying male-like behaviors. Her ovaries then start to regress and fully functional testes grow in their place,” Dr Erica Todd of the University of Otago, New Zealand, said in a statement

Becoming the lone male among females is so evolutionarily advantageous that mid-sized females sometimes jump to a different harem to move closer to the front of the queue.

In Science Advances, Todd and co-authors describe measuring the chemical markers on DNA that control which genes are turned on and off during bluehead wrasses' sex changes.

"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," Todd said. It all begins with the suppression of the aromatase gene, sparking a cascade of responses.

Todd told IFLScience this is the first time these gene expression and epigenetic changes have been observed as animals transition. Global gene expression changes have been tracked previously in species, like the clownfish, that shift in the other direction from male to female.

Todd added: “Many genes that are important for sexual development in fish are also important in humans, so understanding how fish can change sex may teach us about how these complex networks evolved, and how they maintain development in other animals.” Co-author Professor Jenny Graves, 2017 winner of Australia's most prestigious science award, noted the commonality with genes controlling temperature-dependent sex reversal in dragon lizards.

Some sex-changing species, including the barramundi and some flounders, are heavily farmed, and Todd said aquaculturalists “want to be able to control their broodstock”, something her work may assist with.

Of course, there are still many questions unanswered, including why fishes' sex is so flexible that the capacity to change it in adulthood has evolved independently many times. Todd told IFLScience she has ideas about this, noting that fish have more simple external reproductive anatomy than mammals, so less reorganization is required. Nevertheless, there is plenty going on inside, including the degeneration of healthy ovaries to be replaced by testicular tissues, and she admits it is not clear why fish are able to do this so easily.

It's also a bit mysterious how the removal of a dominant male triggers this process in wrasses, nor why it is only ever the largest female who makes the change – even though others in the harem may be only slightly smaller.

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