Medaka

Generalities

The medaka Oryzias latipes is a freshwater teleost fish native to Southeast Asia (Japan, China, Korea, Vietnam). Its ability to survive in a variety of environmental conditions and the fast responses to different drugs, both at the organ level and at the cellular and molecular levels, justify its wide use in toxicology and ecotoxicology. The ease of obtaining and manipulating the medaka (adults and embryos of all stages), the possibility of directing cross-breeding, and its the short life cycle make it a highly appreciated model by geneticists and embryologists, particularly those interested in the development of vertebrates.

Reproduction

The medaka is oviparous. Reproduction is highly dependent on photoperiod, and only takes place during long day periods (14/15 hours day – 9-10 hours night). Illumination stimulates mating, egg laying by the female and sperm emission by the male; fertilization is external. After laying, the eggs remain attached in clusters, by filaments, under the female’s belly. The eggs can then be harvested by catching the females one by one and carefully removing the cluster. About 30 eggs are laid per day by each female, more than 3000 eggs are produced in a season. Under natural conditions, egg laying takes place at dawn in the presence of mating males. Egg-laying can be controlled in the laboratory by light cycles that allow the time of harvest to be predicted.

Medaka grows quickly. The first cleavages last about 30 minutes at 28°C, gastrulation begins about 8 hours after fertilization and the neural tube is visible after 15 hours. The eggs hatch in about 9 days (at 27-28°C) to produce an autotrophic fry and then a young heterotrophic fish. In the laboratory, medaka eggs can be incubated in Petri dishes. The embryos can be cooled without damage (up to 4°C) to slow down their development. As adults, medakas are 2 to 4 cm long.

Tools

Stable lines from different populations (called Northern and Southern) characterized by a high degree of polymorphism
Mutant lines, spontaneous or induced (by chemical mutagenesis, irradiation, etc...) have been produced by several laboratories
A line of completely transparent fish, whose most of organs are visible to the naked eye, has been produced by a Japanese laboratory, which makes it possible to study organogenesis from hatching to adulthood, and could allow the detection of internal abnormalities without sacrificing the animal (particularly interesting in oncology)
Transgenesis techniques by micro-injection of DNA constructs or transposons into the egg are well established in the medaka
Foreign DNA fragments are injected into the cytoplasm of the egg at the one-cell stage
Knock-down methods using morpholine-type antisense are routinely used in the medaka

Databases

Genomic databases :

http://genome.ucsc.edu/cgi-bin/hgGateway?db=oryLat2

http://www.shigen.nig.ac.jp/medaka/genome/top.jsp

http://www.ensembl.org/Oryzias_latipes/Info/Index

http://park.itc.u-tokyo.ac.jp/K-medaka/MGI2/MGI.html

In situ hybridization database :

http://ani.embl.de:8080/mepd/

Tokyo Bio-Resource Library (including a library of transgenic medaka lines) :

http://park.itc.u-tokyo.ac.jp/K-medaka/Kashiwa-TGM.html

Infrastructures

Experts

Bibliography

[FR] Anne-Laure Bauchet thesis (available here : http://theses.vet-alfort.fr/telecharger.php?id=1010)

« The medaka draft genome and insights into vertebrate genome evolution », Kasahara et al., Nature, 2007

« Medaka – a model organism from the far East », Wittbrodt, Shima et Schartl, Nature Rev. Genet., 2002

http://www.ensembl.org/Oryzias_latipes/Info/Index

http://www.fishbase.org/Summary/speciesSummary.php?id=4669