Academic journal article Genetics

The Genomic and Genetic Toolbox of the Teleost Medaka (Oryzias Latipes)

Academic journal article Genetics

The Genomic and Genetic Toolbox of the Teleost Medaka (Oryzias Latipes)

Article excerpt

MEDAKA ( Oryzias latipes ) is a small freshwater fi sh of the family Adrianichthyidae in the order Beloniformes. It is closely related to other members of the superorder Acanthopterygii of ray-finned fish such as pufferfish (tetraodon and fugu), stickleback and killifish, while it is separated from zebrafish by ^150 million years (MY) of divergent evolution (Figure 1). A growing number of sequenced teleost genomes and several established genetic model systems (zebrafish, stickleback, medaka) thus provide a unique resource for comparative studies relating to vertebrate evolution. Medaka is native to Taiwan, Korea, China, and Japan. In Japan it is found in small rivers, creeks, and rice paddies on all main islands with the exception of Hokkaido. Medaka is a euryhaline species and can also live in brackish water (Figure 2, A and B) (Inoue and Takei 2002). As a resident of a temperate zone, medaka can tolerate a wide range of temperatures (4 -40^) both as adult and embryo (Sampetrean et al. 2009). Thus, in the laboratory, temperature can be used to control the developmental speed without adverse effects.

Medaka native to Japan and East Korea have a diploid karyotype of 48 chromosomes, but 46 chromosomes in medakafromWestKoreaandChina(UwaandOjima 1981). The different chromosome number is due to a fusion of chromosome 11 and 13 by a Robertsonian translocation (Myosho et al. 2012b). The haploid genome size is ^800 Mb (Kasahara et al. 2007). Adults can reach a length of up to 4 cm and the wild-type pigmentation is greyish-brown (Figure 2D). However, mutant strains are available where body pigmentation is strongly reduced both at embryonic and adult stages (Figure 2C) (Wakamatsu et al. 2001; Kelsh et al. 2004). Interestingly, sex-specific pigmentation can be used to distinguish male from female embryos as early as 3 days postfertilization (dpf) (organogenesis stages) (Wada et al. 1998), since the leucophore free (lf, slc2a15b) locus that is required for pigmentation by leucophores is located on the sex chromosomes X and Y (Kimura et al. 2014). The XX-XY sex determination system (see below) allows the establishment of mutant strains with females (XX) that are homozygous for a loss-of-function lf allele and males (XY) that are heterozygous and will therefore exhibit wild-type leucophore pigmentation. The strains, Qurt and FLF2, have the wild allele on the Y chromosome and a mutant allele on the X chromosome, thus males have leucophores and female do not.

The life span of medaka under constant mating conditions (14 hr light/10 hr dark at 25-28^) in the laboratory is ^12 months. This can be extended to .2 years under conditions where the fish do not mate in combination with a reduced temperature (10 hr light/14 hr dark at 19 ^ ). Medaka originate from a temperate zone and show a seasonal mating behavior, requiring long light phases and short dark phases for reproduction, whereas temperature has little effect on fecundity (Koger et al. 1999). Medaka males and females can easily be distinguished due to a distinct sexual dimorphism. Most conspicuous is a slit in the male dorsal fin (Figure 2D). Medaka is oviparous with transparent eggs and embryos. Embryos hatch after 7-8 days at 28^ as fully developed juvenile fish. The generation time of medaka is 8-12 weeks depending on strain and husbandry conditions.

Medaka Wild Populations and Laboratory Strains

In Japan, medaka is divided into two geographically separated populations: the northern population of the northern region of Honshu Island and the southern population of southern Honshu, Kyushu, Shikoku, and Okinawa with a divergence time between these two groups of ^4 MYA (Figure 3) (Takehana et al. 2004a,b). Based on morphological criteria, it has recently been suggested that the northern population is an independent species (O. sakaizumii) (Asai et al. 2011). Under laboratory conditions, productive matings between these groups can be obtained and the resulting F1 and F2 male and female hybrids are fully fertile (Sakaizumi et al. …

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