Phenotypic Analyses and Identification of Causal Mutations In Medaka Mutants with Gonadal Dysgenesis.

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Before analysis of the mutants displaying defects in gonadogenesis, the cell lineages that constitute the gonad have been characterized in terms of the motility, origin and translational regulation. Then, phenotypic analyses and identification of the mutations have been performed for the mutant medaka.Mutant medaka, totoro (tot), was identified as medaka showing a hypertrophic gonad. Phenotypic analyses showed that tot possessed an excessive number of germ cells and a half of genetic male exhibited male-to-female sex reversal. A mutation was found in the kinase domain of anti-Mullerian hormone receptor type II, resulting in tyrosine to cysteine substitution. This gene has been known to be involved in sex-specific development of accessory organs found in only mammals and function in other vertebrates remained to be elucidated. Our result indicated that this gene is involved in proliferation of germ cells in a sex-dependent manner. Since sex-reversal was speculated to be due to a result of a burst number of germ cells, medaka with the complete loss of germ cells were generated. The germ-cell deficient medaka displayed a female-male sex reversal, suggesting that germ cells are deeply committed to sexual dimorphic gonadogenesis.A mutation of zenzai mutant has also been identified in one of the surface antigen genes. In vivo germ cell-tracing analysis indicated that two types of proliferation are present in the gonads and the process of proliferation involved in germ cell maintenance was affected in this mutant, which brings about sterility in adult stage.