Hey man, dig those spaced-out fish. Researchers have found a real space
cadet - a mutant zebrafish that swims towards danger, rather than away from
it
Poke a baby zebrafish (Danio rerio) in the head and it normally flips and swims for its life. However, Michael Granato and colleagues at the University of Pennsylvania School of Medicine in Philadelphia searched for mutants whose escape response is up the spout.
The researchers pretended to be predators by prodding the fish. One strain, christened space cadet, turned back towards the prod or even spun on the spot rather than swimming away.
The addled fish have abnormal connections in the back of their brain, the team found. Severing the same connections in normal fish recreates the intrepid activity. "It's a clear example of one cell type involved in a simple behaviour," says Granato - one of the first times this phenomenon has been seen in vertebrates.
Giant nerve cells at the back of the brain called Mauthner cells control the escape response in fish. Neighbouring nerves feed into these cells through extensions that spiral around the body of the Mauthner cell. It is these extensions that are off-course in space cadet fish.
The gene involved has yet to be identified, but it probably guides the brain's growing nerve cells to their destination. "They've taken the first steps towards understanding the circuitry," says Chi-Bin Chien, who studies zebrafish behaviour at the University of Utah Medical Center in Salt Lake City.
Apart from space cadet, Granato has two more flipping fish up his sleeve: spaced out and twitch twice. Together, they should help to unravel other aspects of the escape response.
The fish are part of a huge collection of mutants created by researchers in 1996, each containing a tiny change in its DNA. The aim is to pin down genes that are critical for vertebrate growth and development by observing the defects they cause.
The zebrafish larvae were put through an assault course of trials, such as jogging the side of the dish to test for their startle response. "This also tests for fish that can't swim," notes Chien.