Epigenetics and fish sex ratios: the case of the sea bass

Abstract

Fish and reptiles exhibit sex ratios that are dependent on the temperature experienced during critical windows of early development. However, despite several attempts, a mechanism linking temperature and final sex ratios has not been found. Recently, we showed in the European sea bass that males have double DNA methylation levels than females in the promoter of cyp19a. This gene codes for aromatase, the enzyme that converts androgens into estrogens. In fish exposed to high water temperature, cyp19a promoter methylation levels significantly increased in females, indicating that induced-masculinization involves DNA methylation-mediated control of aromatase gene expression, with an observed inverse relationship between methylation levels and cyp19a expression. Furthermore, induced methylation of the cyp19a promoter in vitro suppressed the ability of SF-1 and Foxl2 to stimulate transcription. Now we show in additional experiments that different CpGs within the cyp19a promoter exhibit different sensitivity to temperature, and that the increased methylation of the cyp19a promoter occurs in the gonads but not in the brain, meaning that is not a generalized effect of temperature. Importantly, these effects were observed also in sexually undifferentiated fish, and were not altered by estrogen treatment. Thus, methylation of the cyp19a promoter is the cause of the lower expression of cyp19a in temperature-masculinized fish. Finally, a CpG differentially methylated by temperature and adjacent to a Sox transcription factor binding site is conserved across species. Thus, an essential component of the long-sought after mechanism connecting environmental temperature and sex ratios in species with temperature-dependent sex determination most likely is the DNA methylation of the aromatase promoter