Underwater Freak?

Here's a mystery for you nature lovers: Why do some Mexican salamanders metamorphose, or change their physical appearance as they grow, while others don't? Salamanders that undergo metamorphosis grow from water-loving, tadpole, like larvae into land-dwelling adults.

Those that don't change, like the Mexican axolotl (AK-suh-luh-tl) at left, just keep growing until they look like big larvae about 38 cm (15 in.) long. And they never emerge from water. In fact, axolotls die if their ponds dry up.

So, why don't axolotls morph like their amphibious cousins? Researchers at the University of California at Davis think they have the answer--a single gene, the part of DNA that influences the development of a trait. This particular gene appears to block metamorphosis in axolotls and sentences the critters to life underwater. The fact that a single gene can dictate an entire species' lifestyle is mind-boggling, says researcher Bradley Shaffer. "It's usually thought that characteristics, like metamorphosis, are controlled by many genes, not just one" he adds.

Talk about "designer" genes.