Phenotypic analysis of a novel chordin mutant in medaka

We have isolated and characterized a ventralized mutant in medaka (the Japanese killifish; Oryzias latipes), which turned out to have a mutation in the chordin gene. The mutant exhibits ventralization of the body axis, malformation of axial bones, over-bifurcation of yolk sac blood vessels, and laterality defects in internal organs. The mutant exhibits variability of phenotypes, depending on the culture temperature, from embryos with a slightly ventralized phenotype to those without any head and trunk structures. Taking advantages of these variable and severe phenotypes, we analyzed the role of Chordin-dependent tissues such as the notochord and Kupffer's vesicle (KV) in the establishment of left-right axis in fish. The results demonstrate that, in the absence of the notochord and KV, the medaka lateral plate mesoderm autonomously and bilaterally expresses spaw gene in a default state.