Redefining Nemo: Transsexualism No Big Deal At Sea

January 11, 2006

Release from: Faye Flam
Knight Ridder Newspapers

Imagine a world in which any female who achieved a position of authority automatically metamorphosed into a male. Or a female-dominated world in which a male seizing power would lose his testicles and become the queen.

Such goings-on may suggest life on a faraway galaxy, but it happens here on this planet, under the surface of the sea.

"Most of the fish you see on a coral reef are sex-changing," says Robert Warner, an evolutionary biologist at the University of California. "Groupers, wrasses, parrotfish, damselfish, many of the gobies can change their sex."

Some fish spontaneously change behavior, reproductive organs and often shape and coloration, a mysterious transformation scientists are only now beginning to understand.

Research suggests that in humans, too, social cues like marriage and parenthood can alter our sex-hormone levels.

But for us, gender is set by the presence or absence of a gene normally on the Y chromosome and is relatively hard to change. For fish, genetic and environmental factors determine sex. That makes sex change much easier for fish.

Their protean nature also renders them more vulnerable to pollution, especially from chemicals that resemble sex hormones. Scientists are seeing fish with abnormal sexual development, features of both sexes or groups with too many of one sex, says Warner. A recent study found that 11 of 82 male fish caught off California had grown ovarian tissues.

Scientists say fish living near heavily populated areas are also exposed to human hormones excreted in our urine.

Still, there's a big evolutionary advantage to being able to change your sex, as some fish generally do, biologists say. It comes in especially handy in social structures where males dominate a group of many females. Like a sultan with a harem, the dominant male can sire hundreds of offspring.

In evolution, success means passing down your genes to as many surviving offspring as possible, so if you're a nondominant mammal, not achieving dominance can mean evolutionary oblivion.

But if you're a fish, and you can't beat the guy on top, you can still pass down some genes by joining the harem.

In the brilliant orange-and-white clownfish, transsexualism goes the other way. If someone made a true-to-nature sequel about the clownfish Nemo, he might mature to become Nellie.

Clownfish groups are headed by a dominant female who rules over a subordinate male and a number of juvenile, asexual satellites. If the male dies, little Nemo could become a true male and replace him. If the female then died, Nemo would change sex.

Georgia State University biologist Matthew Grober and colleagues recently showed that such changes are kicked off by a chemical change in the brain. "That initiates a cascade in which the body is reengineered," says Grober, who works through the Center for Behavioral Neuroscience.

For his most recent work, he studied the bluebanded goby, a guppy-size fish that lives along the Southern California coast. These gobies cluster in groups of up to 10, each with just one male and a harem of females. The male keeps them in line by making threatening approaches like a school bully, Grober says. Males also do a courtship dance called the "jerk swim."

If the male dies or is removed, Grober says, one dominant female "goes nuts," carrying on a ritualistic bullying of the others. Over the next few days, her body becomes male. The ovaries stop making eggs and turn into testicles. She/he grows a prostate gland and develops a penislike external sex organ from a different organ that females use for sticking eggs to their nests.

How could such a transformation occur all by itself? Normally in females, Grober says, male hormones are constantly being changed into female estrogens in a reaction that depends on a catalyst called aromatase. By measuring brain chemistry, he found aromatase levels plummeted just as a female started her sex change. That would allow male hormones to build up.

Grober says it's possible the female, sensing her own dominance, acts aggressively, and this new behavior is what triggers the physical sex change.

Researchers also found the process can reverse. When they put multiple males together, one established himself as boss; the others turned female.

Will any of this mean anything for transsexualism in humans? Certainly it will give pause to anyone making smug claims about the workings and intentions of Mother — or would it be Father?