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Macho or wimp? With cichlid fish, it's all in the brain
STANFORD -- Male sexuality among African cichlid fish - put bluntly, whether an individual is macho or a wimp - is determined by its perception of its social role, a Stanford neurobiologist has found.
For 15 years, Russell Fernald has studied the fish, first in their wild habitat in the shallows of Lake Tanganyika, and now in recreated conditions in the laboratory. About 10 percent of the Haplochromis burtoni males control the food-rich territories that attract spawning females. The rest of the males remain sexually immature and blandly marked.
When a dominant male is snatched up by a predator, however, a remarkable transformation takes place in minutes. One of the subservient males physically changes from shy, colorless "wimp" to aggressive "macho" with bright, bold markings. It develops a bold black stripe across its cheek and orange spots on its anal fin to attract females. And it takes over the prized territory.
Within days, the fish begins to grow in body size and its sex organs mature. Brain cells that signal sex-hormone production swell to eight times their former size.
Some of the transformation is reversible. Placed in a tank where it is bullied by larger fish, a once-macho male returns to protective blandness, losing its stripe and spots. It stops growing, and its hormone- signaling brain cells shrink.
"We are seeing changes in the brain in response to the fish's perception of its social role," Fernald said.
Fish that change color - or even gender - are not unusual, and changes in brain cells have been linked to male songbirds' seasonal songs. With his cichlid fish, Fernald is investigating how such changes are controlled -- and reversed -- by the brain and ultimately by genes. In the process, he is documenting how these fundamental processes are linked to social controls.
The small, easily bred cichlid fish offer one of the few scientific models where captive animals establish social controls much as they do in the wild. Fernald has documented differences in growth, sexual maturation and development of vision between dominant and non- dominant fish of the same age. Now he is looking for the first step in the transformation from wimp to macho fish: a change in gene expression in response to social signals.
Those changing brain cells are an important clue. The neurons that swell when a fish becomes macho are the cells that produce gonadotropin-releasing hormone (GnRH) -- an ancient substance shared by all vertebrates. GnRH is the brain's signal to the pituitary and gonads to produce the sex-specific hormones responsible for puberty in humans, mating songs in birds and territorial displays in cichlid fish. So far, it is not know whether GnRH-secreting neurons change size in mammals.
Now Fernald has joined with molecular biologist John Adelman and colleagues to clone the gene for the precursor of GnRH in cichlid fish. With the DNA code, Fernald's research team can look at other questions raised by his mutable fish - for example, how quickly the fish's new social status stimulates a change in the gene, what molecular signals translate that information, and how such other factors as stress hormones fit into the picture.
"There is a great deal still to do," Fernald said. "There are a lot of intermediaries we don't know very much about."
Fernald said he was surprised at the interest from commercial fisheries in this work: It may lead to breeding improvements for farm- raised fish.
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