It came from Earth: green-blooded fish.

Green-blooded visitors from outer space have long been a staple of science fiction films, TV shows, and books. With the aid of genetic engineering and blue light, scientists have now bestowed that alien hue upon the blood of a more down-to-earth creature, the zebrafish.

The researchers took the gene for green fluorescent protein (GFP), a glowing molecule used by jellyfish, and inserted it into single-celled zebrafish embryos. Moreover, by adding to the gene a short stretch of regulatory DNA, they engineered the gene so that it turns on only in red blood cells.

"This is the first vertebrate animal that has tissue-specific expression of GFP," says Shuo Lin of the Medical College of Georgia in Augusta. In the Oct. 15 Development, Lin and his colleagues describe the GFP-endowed zebrafish, whose blood cells glow green when exposed to blue fight.

One payoff of this trick is that it lets the scientists see readily when and where red blood cells arise in the transparent zebrafish embryos. Confirming work by other researchers, they found that blood cell production occurs in the heart during early development but shifts to the zebrafish kidney towards adulthood.

"As early as 12 hours postfertilization, we're able to see where the blood progenitors come from," says Lin. "We can purify those cells as soon as they show up."

By studying the young cells, the investigators hope to identify the genes used in making a mature red blood cell.

The researchers' plans aren't limited to blood: "We've made fish that have green neurons," notes Lin.

These glowing cells may help scientists understand how nerve cells form connections within the developing brain, he says.

Zebrafish have recently become the catch of the day for scientists studying the development of vertebrates. Last year, scientists published descriptions of hundreds of mutant zebrafish (SN: 12/7/96, p. 360); they are now searching for the genes responsible. By highlighting specific cell populations with GFP, investigators may spot even more subtle mutations, says Lin.

Beyond its effective use of GFP, Lin's work marks the first demonstration that scientists can permanently add functioning genes to the zebrafish, notes Mark C. Fishman of Massachusetts General Hospital in Boston.

While scientists had previously inserted working genes into zebrafish, the genes never remained active in future generations, he explains. In contrast, Lin's original green-blooded fish are already grandparents of fish whose blood also glows.