Environmental effects during gonadal development in fish: role of epigenetics

Abstract

In many organisms, sex is determined by a combination of genetic and environmental influences. Epigenetic regulatory mechanisms such as DNA methylation integrate both types of influences. However, understanding how genetic and environmental changes shape the sexual phenotype and the role of epigenetics in this process is far from clear.

To address these questions that are of major interest in reproductive and evolutionary biology, in this study we used two zebrafish (Danio rerio) wild strains, with an intact sex­ determinirig loci (sar4), where sex determination is thought to follow a monofactorial system with female heterogamety (ZZ/ZW). We also used the laboratory AB strain, a consolidated model for many research areas, with loss of sar4 during the process of domestication, and with polygenic sex determination. Genetic variation was accounted for by using different families of each strain, which were exposed to biotic (rearing density) and abiotic (temperature) environmen4Il perturbations during critical stages of / sex differentiation.

Elevated rearing density resulted in lower survival and growth, increased masculinization and delayed gonad maturation. Transcriptomic analysis of the adult gonads showed that masculinization was achieved by upregulation of male-related genes and downregulation of female-related genes and, importantly, the participation of the cortisol-mediated stress response. By comparing the gonadal transcriptomes of females resistant to heat- and crowding-induced stress, a common set of differentially expressed genes were identified, constituting novel biomarkers to aid in the identification of hidden effects environmental perturbations. Temperature was able to alter DNA methylation levels of the regulatory regions of sex- and stress-related genes in a clear sex-related fashion. By using machine­ learning procedures, we identified specific methylation profiles of some CpG sites in the promoter regions of key genes (cyp19ala, amh and foxl2a) involved in sex differentiation and in the response to the environment. In wild strains, we discovered elevated rates of spontaneous sex reversal at control temperature, identified novel sexual genotypes and showed genotype-dependent rates of sex reversal under elevated temperature, with possible consequences in sperm production. Contrary to expectations, the presence of sar4 in wild strains did not confer higher resistance to temperature when compared to the situation in the AB strain. Finally, effects of elevated temperature on sex ratio and/or DNA methylation in the gonads were inherited, at least in males, in the F1 but only in a family-dependent manner while effects were never detected in the F2.

In summary, we developed novel DNA methylation-based biomarkers capable of predicting phenotypic sex and whether fish had been previously exposed to abnormal environmental conditions, paving the way for similar developments in other' species. Taken together, these results contribute to our understanding of the role of DNA methylation in shaping the sexual phenotype and can aid towards obtaining a better picture of how environmental changes may affect natural populations in a global warning scenario.