Understanding of the mechanisms underlying the evolution and maintenance of biodiversity is an increasingly important topic. Even 200 years after the Darwin’s birthday, little is known yet about the genetic mechanisms underlying species formation. A research group led by Jun Kitano, Assistant Professor at Graduate School of Life Sciences, Tohoku University, has presented new evidence that sex chromosome changes contributed to the origin of a new fish species in the Sea of Japan. Although it was hypothesized that the sex chromosome change might contribute to species formation, the research group has presented clear and empirical evidence supporting this hypothesis in a natural vertebrate system. The results have been published in Nature as Advance Online Publication (AOP) on September 27, 2009.
(http://www.nature.com/nature/index.html)

The joint U.S.-Japan research team has focused on a unique threespine stickleback (Gasterosteus aculeatus) population in the Sea of Japan (Japan Sea stickleback), which diverged from the ancestral Pacific Ocean sticklebacks when the Sea of Japan was geographically isolated from the Pacific Ocean about 2 million years ago during the last ice age. Over 7 years, the research group has been investigating the evolutionary origin of this unique stickleback species.

In the present study, they found that the Japan Sea stickleback has a unique sex chromosome (neo-sex chromosome): in the Japan Sea stickleback, one Y-chromosome is fused to autosome no. 9. They found that the neo-sex chromosome contains a gene that makes their courtship behavior aggressive (the male pricks the female with the dorsal spines during courtship). Japan Sea females can tolerate the aggressive Japan Sea males, whereas the Pacific Ocean females stop mating when faced with such an aggressive pricking, thus preventing these two species from mating freely. The sex chromosome also contains a gene that makes the Japan Sea male smaller, which may also make the male less attractive to the Pacific Ocean female. They also found that the sex chromosome of the Japan Sea stickleback makes the hybrid males sterile.

 Assistant Professor Kitano, who led the research group, said:
“It has been long known that sex chromosome evolves rapidly in certain taxonomic groups, such as fish and reptiles. But, it was a mystery why the sex chromosome changes so rapidly in these taxa. Our results suggest that the sex chromosome changes may contribute to species formation in such taxonomic groups other than sticklebacks.”
“We do not know yet why the Japan Sea females follow such an aggressive male. By investigating the neuronal mechanisms underlying the female promiscuity in the Japan Sea sticklebacks, we may be able to understand the reason why some women follow aggressive men in our society.”

[Contact]
Assistant Professor Jun Kitano
Division of Ecology and Evolutionary Biology at Graduate School of Life Sciences, Tohoku University
Tel: +81-22-795-6689
E-mail: jkitano@m.tains.tohoku.ac.jp
URL: http://www.lifesci.tohoku.ac.jp/teacher/environmental/a_kitano.html