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- The Japanese Medaka fish is a small (2-4 cm) fish commonly found in flooded rice paddies in Japan, Taiwan, and other areas of southeast Asia.
- The name Oryzias latipes reflects the preferred habitat of medaka - the rice (Oryza sativa) fields. This habitat also gave rise to the common English name of medaka - ricefish.
- Medakas are remarkable for the number of eggs produced by females - around 3,000 in a single breeding season.
- Medaka has many attributes that make it a model laboratory organism, among them the clarity of its eggs, hardiness, and lack of aggression.
The potential of medaka as a genetic model
- Medaka has relatively small genome (~800 Mb, half the size of the zebrafish genome).
- Medaka is more closely related to the fugu (Takifugu and Tetraodon) than to zebrafish.
- In contrast with zebrafish, medaka has clearly defined sex chromosomes, and sex determination is intensively studied.
- Medaka is hardier than zebrafish and less susceptible to disease. Embryonic development can proceed at a wide range of temperatures (6-40 degrees Celsius) - a trait that increases a chance of isolating temperature-sensitive mutants.
- For decades, medaka was an important test system for environmental research. It is widely used for carcinogenesis studies and for testing endocrine disruptors in ecotoxicology.
- Unlike in other lower vertebrate genetic systems, inbred and highly fertile strains of medaka are available. Many very important mutant phenotypes of medaka do not have analogs in zebrafish.
Under laboratory conditions, generation time is between 6 and 8 weeks for medaka, compared with 8 and 10 weeks for zebrafish.
egg development stages are given as observed at 26°C
- unfertilized egg
Iwamatsu stage 0
- activated egg egg is stimulated by spermatozoon; fertilization; Iwamatsu stage 1
- fertilized egg
- germinal disc Iwamatsu stage 2; blastodisc stage; zygote
Iwamatsu stages 3-7; 1 h 5 min - 3 h 30 min
post-fertilization; 2-32 cell embryo
Iwamatsu stages 8-9
- early morula
Iwamatsu stage 8; 4 h 5 min
post-fertilization; the cells are
arranged in 3-4 layers but are
still easily dissociated from
Iwamatsu stage 9; 5 h 15 min
post-fertilization; the blastodermal
cells now form 4-5 layers
- early morula Iwamatsu stage 8; 4 h 5 min
Iwamatsu stages 10-11; 6 h 30 min -
10 h 20 min post-fertilization
- gastrula Iwamatsu stages 12-16
Iwamatsu stages 17-18; 1 day 1 h
- 1 day 2 h post-fertilization; head
formation and optic bud formation
Iwamatsu stages 19-32;
1 day 3 h 30 min - 4 days 5 h post-fertilization;
somites stages (2-30 somites); brain
regionalization and optic vesicle formation;
tubular heart development
Iwamatsu stages 32-38; 4 days 5 h - 9 days
post-fertilization; development of heart;
formation pectoral fin; blood vessel
development; spleen and other organs
- cleavage Iwamatsu stages 3-7; 1 h 5 min - 3 h 30 min
- unfertilized egg Iwamatsu stage 0
- hatching Iwamatsu stage 39; 9-10 days post-fertilization; the embryos dissolve the inner layers of the chorion, tear the single outer layer by moving the body and escape from the chorion tail-first
- larva fry; Iwamatsu stage 40; period of morphogenesis; this period extends from hatching until fin rays appear in the caudal and pectoral fins (0-6 days after hatching)
- juvenile young immature fish, until 7-50 days after hatching
- adult medaka that reached sexual maturity (at about 2 months under laboratory conditions); maximal longevity of medaka is about 5 years
Taniguchi Y, Takeda S, Furutani-Seiki M, Kamei Y, Todo T, Sasado T, Deguchi T, Kondoh H, Mudde J, Yamazoe M, Hidaka M, Mitani H, Toyoda A, Sakaki Y, Plasterk RH, Cuppen E. Generation of medaka gene knockout models by target-selected mutagenesis. Genome Biol. 2006;7(12):R116. PMID: 17156454
Various tumors that developed spontaneously in p53 medaka knockouts. (a,b) The tumor that arose in the left gill of a p53E241X/+ fish with the lymphomatous infiltrate, consistent with the diagnosis of thymic lymphoma. (c,d) Adenocarcinoma found in the right gill of a p53E241X/E241X homozygous fish. (e,f) Retinoblastoma in the right eye of a p53E241X/E241X homozygous fish. Note the rosette-like structures throughout the tumor. (g,h) A germ cell tumor found in the anterior upper part of the peritoneal cavity of a p53E241X/E241X homozygous fish. All fish presented here died or were sacrificed at around 8 months of age. Arrowheads indicate tumors. Hematoxylin-eosin staining, original magnification: (b,d) 100x; (f,h) 10x.
- Kobayashi D, Takeda H. Medaka genome project. Brief Funct Genomic Proteomic. 2008 Nov.
- Iwamatsu T. Stages of normal development in the medaka Oryzias latipes. Mech Dev. 2004 Jul;121(7-8):605-18.
- Wittbrodt J, Shima A, Schartl M. Medaka--a model organism from the far East. Nat Rev Genet. 2002 Jan;3(1):53-64.
- Free full-text articles in PubMed: major topic "Oryzias"