Epigenetics of Sex Determination and Differentiation in Fish

The current knowledge on epigenetics and gene expression regulation during sexual development in fish is reviewed. Gonochoristic species with different types of sex determining mechanisms and different types of hermaphrodites are considered. Most research has targeted only a few genes involved in ovarian differentiation – mainly the gonadal aromatase, cyp19a1a, and foxl2, and genes related to testicular differentiation, mainly dmrt1. However, two whole genome studies have shown species-specific variation in global methylation differences between sexes. Based on this information, a testable model is proposed for the epigenetic regulation of sexual development in fish – the conserved model of epigenetic regulation of sex, which deals with the relationship between gene silencing and gene expression levels. This model is based on the assumption that there are "pro-male" and "pro-female" genes, and on the canonical inverse relationship between gene silencing and gene expression levels emerging from the available data. Importantly, it also postulates that a given set of epigenetic and gene expression patterns are more associated with a particular gonadal phenotype (e.g., males) than the means by which this phenotype is obtained (e.g., natural vs. induced masculinization). Finally, the application of epigenetics in breeding programs to favor the appearance of a desired phenotype and, in particular, to control sex, is briefly explored. To that aim, it will be necessary first to identify what are defined as the essential epigenetic marks (EEMs) – the number and identity of informative epigenetic marks that are strictly necessary, although perhaps not sufficient, to bring about a specific and measurable phenotype of interest.