Abstract
Medaka ric1 homozygous mutant was isolated by large scale ENU mutagenesis screening. It showed high incidence of malformation and lower hatchability after ionizing irradiation at early embryonic stages and has a defect in the repair of DNA double-strand breaks in embryonic cells though it is highly fertile and easy to breed like wild-type fish under non-irradiated condition (Aizawa et al. 2004 Mech. Dev. 121: 895-902). Spermatogonial stem cells in ric1 fish are much more sensitive to ionizing radiation than wild-type fish and show delayed apoptosis, which suggests that the ric1-dependent DNA repair pathway plays a critical role in the SSCs.
Using transgenic medaka Olvas-GFP strain contains the green fluorescent protein (GFP) gene fused to the regulatory region of the vasa gene (Tanaka et al. 2001 PNAS. 98: 2544-2549), we established Ric1Olvas-GFP strain with germ-cell-specific expression of GFP which can be visualized germ cells in living ric1 embryos. Embryonic germ cells in ric1 are sensitive to non-lethal dose of irradiation at stage 7 (32 cells ) and stage 33 (notochord vacuolation). The proliferating female germ cells are more sensitive than male germ cells at stage 33. It is also suggested that ric1 dependent maternal DNA repair pathway plays a major role in radiosensitivity of the PGC at stage 7 and at stage 24.
These data suggests that ric1 mutant strain is a valuable model for germ-line mutagenesis, radiation biology, teratogenesis, carcinogenesis, aging and environmental monitoring
