For a tropical fish called the bluehead wrasse, sex isn’t always permanent. When a group of the fish loses its dominant male, the biggest female rapidly changes sex, taking on distinctive male coloring and producing mature sperm in as little as 8 days. Although this change is well documented, the molecular mechanisms that drive it have remained unclear. Now, an international group of researchers has offered a chemical explanation for the transformation after analyzing tissue samples from transitioning fish (Sci. Adv. 2019, DOI: 10.1126/sciadv.aaw7006).
The following is a transcript of this video.
Kerri Jansen (voice over): In coral reefs off the coast of Florida, a species of fish called the bluehead wrasse is helping scientists understand sexual development. Researchers are interested in a striking change the small yellow females undergo in their social groups. When the larger, dominant male in the group leaves, the largest female changes sex and takes the male’s place just days later.
Scientists have documented sex changes in many species of fish, but they still don’t have a complete picture of what molecular mechanisms drive the change. To help figure that out, researchers from Australia, New Zealand, and the US removed dominant males from groups of bluehead wrasse in the waters off Florida, then analyzed tissue samples from the remaining fish as the sexual transformation was in progress.
What they found is that the transitioning female starts behaving differently within hours after the male is removed, and the fish’s coloring becomes darker as it eventually develops the distinctive blue head of a male. To investigate how these changes correlate with changes to the fish’s genes, the researchers analyzed gonad cells from the transitioning fish. They found extensive shifts in gene activity and in the pattern of methyl groups attached to the fish’s DNA, a modification that can affect the function of certain genes. For example, genes associated with the production of female hormones turn off. As estrogen levels collapse, there is a gradual increase in genes responsible for producing androgens, hormones associated with male reproductive activity. Ovarian tissue also starts to degrade and is replaced by testicular tissue, which starts producing sperm within 8–10 days. The researchers think this genetic rewiring might get triggered by increased levels of cortisol, a stress hormone that spikes in female fish soon after the male disappears from the group.
The team hopes a better understanding of how cells get reprogrammed in this tropical fish might help scientists understand how genes are turned off and on in other animals and even humans.
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