Rapid Induction of Female-to-Male Sex Change in Adult Zebrafish by Injection of an Aromatase Inhibitor.

Previously, we examined whether aromatase inhibitor (AI) treatment induces a sex change in adult female zebrafish. A 5-month AI treatment regime resulted in the retraction of the ovaries and testis formation. Eight weeks after changing the diet to AI-free food, a large number of normal sperm were obtained. Artificial fertilization using sperm from the sex-changed females was successful. These results demonstrated that sex plasticity remains in the mature ovaries of zebrafish. However, >7 months of treatment was necessary; thus, pairing was unsuccessful. In this study, we tried to induce sex change through the injection of an AI to shorten the time course of sex change. When the AI solution was directly injected into the abdomen of zebrafish, retraction of the ovary was induced within 2 months. The natural mating of sex-changed females with normal females was successful at 3 months. Although the fertilization rate was low, juveniles resulting from these matings developed normally. We succeeded in establishing a method for inducing sex changes in adult zebrafish within 3 months. The procedure will support the study of how sexual plasticity persists in adult zebrafish following sex differentiation and the identification of undifferentiated stem cells.

Generation of Transparent Zebrafish with Fluorescent Ovaries: A Living Visible Model for Reproductive Biology.

The transparent zebrafish enables researchers to study the morphology and distribution of cells and tissues in vivo. To capture the dynamic processes of germ cell proliferation and juvenile ovarian development in zebrafish in vivo, we established transgenic (TG) lines to allow us to monitor the changes in the ovaries of living fish. The original transgenic line with ovarian fluorescence was occasionally established. Although the cDNA integrated in the strain was constructed for the expression of enhanced green fluorescent protein (EGFP) driven by the medaka ß-actin promoter, expression of EGFP is restricted to the oocytes and gills in adult fish. Mutant strains with transparent bodies, roy and ruby, were isolated in zebrafish. In this study, we crossed the TG strain with fluorescent ovary with transparent strains and established the TG (ß-actin:EGFP);ruby strain. The strain is highly transparent, and the oocytes are easily observed in living fish. We identified a fluorescent tissue that might contain the undifferentiated germ cells close to the cloaca in the strain. This strain can be used for analysis of ovarian development in vivo.