The tumor suppressor protein p53 is considered a guardian of genome integrity, regulating the induction of apoptosis and cell cycle arrest in response to irradiation to block the transmission of teratogenic mutations to progeny cells. We examined the function of p53 in highly radiosensitive tissues, the developing brain and mature testis, using a small fish model, medaka (Oryzias latipes). Medaka offer advantages as a vertebrate model system, as the transparency and small size of the embryos enables clear detection of apoptotic cells in the developing brain. In addition, the simple architecture of medaka testes enables more precise identification of the differentiating spermatogenic stages compared with mammals. We found that in irradiated p53-deficient embryonic brain, diminished induction of apoptosis facilitated tissue regeneration earlier compared to irradiated wild-type embryos, which remained structural abnormalities in the retina at hatching. Moreover, the prominent delay in apoptotic induction in irradiated p53-deficient testes could induce transient mis-differentiation during spermatogenesis, such as the formation of ovum-like cells (testis-ova). However, all testis-ova cells were eliminated via p53-independent apoptosis, and spermatogenesis was completely restored within 1 month after irradiation. Collectively, these data indicate that p53 is not indispensable for the restoration of irradiation-induced damaged tissues.

Recently, medaka has been used as a model organism in various research fields. However, even though it possesses several advantages over zebrafish, fewer studies were done in medaka than zebrafish, especially its behaviour. Thus, to provide more information regarding its behaviour and to demonstrate the be-havioural differences between several species of medaka, we compared the behavioural performance and biomarker expression in the brain between four medaka fishes, which were Oryzias latipes, O. dancena, O. woworae, and O. sinensis. From the results, we found that each medaka species explicitly exhib-ited different behaviours to each other, which might be related to the different basal levels of several biomarkers. Furthermore, by phenomics and ge-nomic-based clustering, the differences between these medaka fishes were further investigated. Interestingly, even though both phenomics and ge-nomic-based clustering showed some resemblances to each other in terms of the interspecies relationship between medaka and zebrafish, however, in the medaka interspecies comparisons, this similarity was not displayed. Therefore, these results suggest a re-interpretation of several prior studies in comparative biology. We hope that these results contribute to the growing database of medaka fish phenotypes and provide one of the foundations for future phe-nomics studies of medaka fish.