Effects of high temperature on sex differentiation and germ cell population in medaka, Oryzias latipes

Medaka, Oryzias latipes, has an XX/XY sex determination system with DMY as the sex-determining gene. Previous studies have confirmed that the process of sex determination in medaka is sensitive to the effects of high temperature. In this study, the fertilized eggs from two inbred strains and a transgenic strain were divided into four groups. Two groups from each strain were exposed to either 27 ± 0.2 °C or 32 ± 0.2 °C. The other two groups were subjected to a temperature shift at stage 25 until hatching, either from 32 to 27 °C or from 27 to 32 °C. All groups of hatchlings were examined for sex ratio as well as gonadal development. The XX sex reversal percentages in Hd-rR and HNI strains were 18.6% and 55.2%, respectively, at 32 °C and 3.8% and 58.7%, respectively, in the group shifted from 27 to 32 °C. Although incubation at a constant 27 °C or a temperature shift from 32 to 27 °C resulted in no sex reversals in Hd-rR, these same treatments resulted in 9.0% and 28.0% sex reversal, respectively, in HNI. These results showed that the sensitive period for sex reversal was mainly after stage 25, although the effects of the exposure during earlier stages could not be neglected. The sex reversal ratio in GFP transgenic fish was similar to that in Hd-rR fish. In this strain, gonadal sex reversal was detected at the time of hatching as judged by the absence of both leucophores on the body surface and GFP signals in the gonad. Fish with GFP signals at 0 days after hatching (dah) developed ovaries in later stages. High temperature had a clear effect on XX fish and minimal effect on XY fish. In groups of Hd-rR fish treated at 32 °C, some XX fish at 0, 5, and 10 dah had very low numbers of gonia cells, similar to normal XY fish; others showed lower numbers of germ cells with little effect on germ cell population; and some fish were not affected. In HNI fish, in which germ cells were more numerous and more variable in type at hatching, developing oocytes appeared earlier and the effect of high temperature was more evident than in Hd-rR fish. These data suggest that high temperature leads first to an inhibition of both the proliferation of germ cells and the development of oocytes and then to induced gonadal and phenotypic sex reversal.