Medaka

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
Generalities
  • Name : Oryzias latipes
  • Phylogenetic classification : Chordate, Vertebrate, Actinopterygian, Teleostean, family Adrianichthyidae
  • Sequenced genome complete since 2017
  • Genome size : 800-1000 millions pb.
    Number of genes : about 20.000.
Fields
Infrastructures
Experts
Bibliography
  • Articles

[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

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