Biohistory Jyournal, Autumn, 2004

Research: Index > Medaka-A model animal raised by Japanese

Narrative science Medaka A model animal raised by Japanese

Hiroyuki Takeda, School of Science, The University of Tokyo

The medaka (oryzias latipes) is a fish well known to the Japanese. Japanese researchers have established pure strains of the medaka in the wild, broadly divided into the northern Japan group and the southern Japan group. These two groups have differences in facial characteristics and physical form.     Although developmental biology research of fish is primarily conducted using the zebrafish, a small tropical fish or blowfish because of their short generation time, the medaka is also interesting and unique for its smaller genome size and its transparency, making their organs visible.     The medaka genome project has gotten underway, and now, we can deal with genetic variants in the pure strain. Therefore, investigations can be made into the relation-ship among DNA, shape, and ecology.

The northern medaka and the southern medaka

To understanding of the mechanisms of development, biologists look for the cause of the aberration in the shape of medaka mutants. The mutant toguro (toguro means coiled snake in Japanese) has a different shape with no bones in the body. Interestingly, this phenotype was not observed in individuals hatched at low temperatures. This phenomenon is observed only for the medaka, which has a wide temperature range (4-40C°) for its growth.     Another mutant is Da, commonly known as the hikari (light in Japanese) medaka. (The silver color of its underbody extends to its back.) Its flat back, which is suited for swimming near the surface, has changed to resemble the tuna (the teardrop shape for swimming at medium depth in the ocean). It can be speculated that the medaka's ancestors were of the tuna type.     There is a great variation in the body types of fish depending on their living environment, even if they are a closely related species. This is likely the key to understanding the evolution of their body type.

Coil-shaped medaka The upper photo shows a normal medaka fingerling, and the lower photo shows the toguro mutant. The toguro has no bones at all in the back. This mutant indicates a susceptibility to temperature. Changes in temperature during its growth control the period in which the aberrations occur. Teardrop-shaped medaka The trunk of a normal medaka has a flat, orange back with a dorsal fin and a green underbody with an anal fin (above). With the change of a single gene, the back of the medaka assumes the structure of the underbelly, and it becomes shaped like a tuna (below).

Medaka has been kept as pets, and they flourished in wet rice paddies until the days of my childhood. The medaka swimming in water tanks in the laboratory and those in wet rice paddies close to extinction are superimposed within me. I want to apply my thoughts to history and the environment as a Japanese to compile stories about the medaka and conduct research.

Hiroyuki Takeda Ph.D. Born in 1958, Takeda is currently a professor at the University of Tokyo. He specializes in developmental genetics and experimental embryology.

		Why do we pay attention to the spider now? :Yasuko Akiyama-Oda,Hiroki Oda
		Searching for the living creatures depicted :Naoko Kitaji

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