31. A Medaka Mutant, Oot(ONE-sided optic tectum), That Breaks Bilateral Symmetry of the Developing Brain
Yuji Ishikawa
Keywords: mutation, brain, mutant, morphology anomaly, development, teleost fish, medaka
A spontaneous viable maternal-effect mutation, Oot (One-sided optic tectum), was newly found in a population of the HO4C inbred strain of the medaka (Oryzias latipes). The mutant phenotype was observed at only restricted stages of the morphogenesis of the brain. The Oot phenotype is as follows: In a normal embryo, the hemilobes of the optic tectum (dorsal part of the midbrain) bulge out laterally, or evaginate, on both sides at early embryonic stages (at stages 23-25) , forming a mirror symmetry along the midline axis; namely, the left hemilobe is a mir ror image of the right hemilobe. However, in the mutant embryo at the same stages, only one of the hemilobes of the optic tectum evaginates, while the hemilobe on the other side does not evaginate but instead invaginates or bulges medially. The frequency ratio of the right/left directions of the expansion is 1/1 among sibling mutant embryos. Other body parts of the mutant embryo are completely normal without any associated situs inversus or unusual car diac looping. Thus, in the Oot phenotype the mirror symmetry or bilateral symmetry is broken only in the developing optic tectum, and both hemilobes exhibit the same morphology on both sides. At stage 26, the invaginated optic tectum hemilobe starts to evaginate in the mutant embryo, and the optic lobes become normally symmetrical at later stages. The mutant fry hatch and grow almost normally. This kind of brain mutant has never been reported so far in other vertebrates including zebrafish. Thus, this is the first report to show that there exists a maternal gene Lhat affects the bilaterally symmetrical evagination of the developing optic tectum.
Publications:
1)Ishikawa Y.: Bioessays, 22, 487-495, 2000.
2)Ishikawa, Y., Hyodo-Taguchi, Y., Aoki, K. Yasuda, T., Matsumoto, A. and Sasanuma, M. Fish Biol. J. Medaka, 10, 27-29, 1999.
3)Ishikawa, Y., Yoshimoto, M. and Ito, II.: Fish Biol. J. Medaka, 10, 1-26, 1999.