Japanese and American researchers have discovered species of fish that change sexes like chameleons change color, altering their genitalia and behavior to suit the social circumstances.
The most interesting is a tiny tropical fish found off the coast of Okinawa; the fish live in groups of one dominant male and several females.
If a larger male comes along, the dominant male changes into a subservient female. But if something happens to the new dominant male, the largest female becomes a male - even if it was a male once before.
Although scientists knew certain species of fish can change gender when the opposite sex is in short supply, it was believed that such changes are irreversible - one conversion and you are out of the game.
But Matthew Grober of the University of Idaho told a meeting of the Society for Neuroscience on Sunday that at least three species of fish have been identified that can change sex repeatedly when social circumstances require it, restructuring their genitalia and their brain in an average of four days.
Biologist Grober said he and his colleagues have been able, for the first time, to induce fish to undergo such sex changes in the laboratory, enabling scientists to study accompanying changes in brain structure.
The discovery is significant, he said, because the brain region involved in the sex change is the same region recently suggested to cause transsexuality in humans.
The fish Grober studied, a variant of the gobi called Trimma okinawae, was discovered by marine biologist Tokomi Sunobe of the Natural History Museum and Institute in Chiba, Japan.
Growing to no more than 2 inches in length, the fish have an unusual culture in which the lone male not only is the dominant fish in the group but also is the primary caregiver for the group’s offspring.
When the group is taken over by a larger male, however, the first male’s behavior changes completely. It abandons the young and behaves like a female, forgoing aggressive behavior toward other males and adopting the female role in courting.
Its testicles shrivel up and its vestigial ovary begins producing eggs. And its external penislike organ inverts so that it can grasp and hold onto eggs and sperm.
“The record for a change is two days,” which is much faster than the time required by fish that undergo an irreversible change.
Later, if the new dominant male is removed from the environment, the newly created female can change back to a male, with all the processes being reversed. A complete male-female-male cycle takes about 10 days, the researchers said.
Grober said he has no idea how many times this cycle can be repeated. He has just started an experiment in which “we’ll run a single fish through the cycle over and over again until it dies or stops cycling.”
By sacrificing fish immediately after the changes, Grober also has been able to study changes in the brain.
Grober said he has found changes in the ventral forebrain, specifically in cells that produce a hormone called arginine vasotocin, which is known to directly control sexspecific reproductive and parenting behaviors in a wide variety of vertebrates.
He said the cells are much larger in females than in males.
Just recently, researchers from the Netherlands reported that the size of a portion of the ventral forebrain in humans called the BSTc is larger in males than in females.
In males who believe they are females trapped in a man’s body, however, the region was even smaller than in women. Grober says that a similar difference in size may account for gender differences in the fish he is studying.
What is important, he said, is that “humans and fish have the same pieces of the engine that drives sexual differences.”
It is difficult to study the brain in humans, but “we can play with the engine in fish and find really interesting insights about human behavior.”
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