Activation of stress response axis as a key process in environment-induced sex plasticity in fish.

The determination of sex is an important hallmark in the life cycle of organisms, in which the fate of gonads and then the individual sex are defined. In gonochoristic teleost fish, this process is characterized by a high plasticity, considering that in spite of genotypic sex many environmental factors can cause shifts from one to another molecular pathway, resulting in organisms with mismatching genotypic and phenotypic sexes. Interestingly, in most instances, both female-to-male or male-to-female sex-reversed individuals develop functional gonads with normal gametogenesis and respective progenies with full viability. The study of these mechanisms is being spread to other non-model species or to those inhabiting more extreme environmental conditions. Although water temperature is an important mechanism involved in sex determination, there are other environmental stressors affected by the climate change which are also implicated in stress response-induced masculinization in fish. In this regard, the brain has emerged as the transducer of the environment input that can influence the gonadal fate. Furthermore, the evaluation of other environmental stressors or their synergic effect on sex determination at conditions that simulate the natural environments is growing gradually. Within such scope, the concerns related to climate change impacts rely on the fact that many of biotic and abiotic parameters reported to affect sex ratios are expected to increase concomitantly as a result of increased greenhouse gas emissions and, particularly worrying, many of them are related to male bias in the populations, such as high temperature, hypoxia, and acidity. These environmental changes can also generate epigenetic changes in sex-related genes affecting their expression, with implications on sex differentiation not only of exposed individuals but also in following generations. The co-analysis of multi-stressors with potential inter- and transgenerational effects is essential to allow researchers to perform long-term predictions on climate change impacts in wild populations and for establishing highly accurate monitoring tools and suitable mitigation strategies.

Dimorphic expression of dmrt1 and cyp19a1 (ovarian aromatase) during early gonadal development in pejerrey, Odontesthes bonariensis.

The pejerrey (Odontesthes bonariensis) is a teleost fish with strong temperature-dependent sex determination (TSD). Several studies have shown that dmrt1 and gonadal aromatase (cyp19a1) are implicated in the sex differentiation process in teleosts but little is known on the expression balance and endocrine regulation of these two genes during TSD. This study was designed to clarify the expression patterns of both genes during gonadal sex differentiation of pejerrey reared at female-, male- and mixed-sex-producing temperatures (FPT, MPT, and MixPT, respectively). The expression of dmrt1 was found to be significantly higher during gonadal sex differentiation at MPT compared to FPT. Conversely, cyp19a1 expression clearly increased during differentiation at FPT but not at MPT. The expression of both genes at MixPT showed a dimorphic profile with individual values resembling either those at the MPT or FPT. Administration of exogenous 17beta-estradiol down- and up-regulated the expression of dmrt1 and cyp19a1, respectively, regardless of temperature, and rescued the female phenotype at the MPT. However, treatment with the aromatase inhibitor Fadrozole caused masculinization without changing the pattern of gene expression. These results are strong evidence of the involvement of both genes in the gonadal differentiation process of pejerrey. The involvement of estradiol is discussed.