When Catherine Peichel and her colleagues went fishing for genes underlying behavioural evolution in a population of sticklebacks, they were prepared for a long haul. It was worth the wait; the five-plus years of work netted a great catch. The group found genetic evidence that a stickleback population living in the Japan Sea has a new sex chromosome that accounts for its males' aggressive mating behaviour.

Peichel became interested in stickleback behaviour shortly after starting her current lab at the Fred Hutchinson Cancer Research Center in Seattle, Washington, in 2003. Jun Kitano, a postdoc who had joined her lab from Japan, told her about two groups of fish in Japanese waters that look similar but exhibit strikingly different mating behaviours. Three-spine sticklebacks from the Pacific Ocean swim in a zigzag pattern, whereas those from the Japan Sea roll over.

Despite these differences, both fish share a mating ground, breeding in Lake Akkeshi and the Bekanbeushi Marsh in Japan's northern island. In the spring of 2003, Peichel's group visited the area. When Japanese fishermen brought their daily haul ashore, Peichel and her group looked for spiny creatures that might have been caught along with the more commercial species. They spent many cold, wet mornings plucking fish from nets and sorting them into two groups by examining the bony plates, or keels, on the fishes' tails. The Pacific Ocean stickleback sports a broader keel than its Japan Sea counterpart.

The researchers sent their live catch to Seattle. In the lab, Kitano spent hundreds of hours recording the fishes' behaviour. He noticed that, in addition to performing different moves from Pacific Ocean fish, the Japan Sea males were more aggressive with their females — vigorously bumping them from underneath and pricking them with their spiky dorsal fins. “It's pretty dramatic,” Peichel says of the behaviour.

To delve into the genetics of these behavioural differences, the group crossbred the two fish populations over two generations. Having analysed the mating behaviours of the adult male progeny resulting from the crosses, the researchers extracted DNA from the fish to look for genetic markers that were inherited along with either of the two behaviours.

The team found that male fish that performed the aggressive bumping had inherited a piece of chromosome 9 from the Japan Sea sticklebacks. Chromosome 9 is not a sex chromosome in other stickleback populations, but Peichel's group discovered that, in the Japan Sea population, it is always inherited together with — or linked to — a sex chromosome. “The fact that the two chromosomes are linked was weird, because we had done lots of genetic mapping and had never seen that result,” Peichel says.

So the team turned to microscopy to get a close-up look at the fishes' chromosomes. The analysis showed that certain genetic elements, or markers, that the team had earlier mapped to two separate chromosomes — chromosome 9 and a sex chromosome — in other three-spine stickleback populations were fused together during evolution into a new sex chromosome in Japan Sea sticklebacks. This seems to be responsible for the aggressive mating behaviour (see page 1079). Although sex chromosome regions tend to change little in mammals and birds, they evolve quickly in fish and some amphibians. “How and why this occurs is still a really big mystery,” says Peichel.