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In recent years, observations of distinct organisms have linked the quality of the environment experienced by a given individual and the sex it will develop. In most described cases, facing relatively harsh conditions resulted in masculinization, while thriving in favorable conditions promoted the development of an ovary. This was shown indistinctively in some species presenting a genetic sex determination (GSD), which were able to sex-reverse, and in species with an environmental sex determination (ESD) system. However, this pattern strongly depends on evolutionary constrains and is detected only when females need more energy for reproduction. Here, I describe the mechanisms involved in this environmentally driven sex allocation (EDSA), which involves two main energy pathways, lipid and carbohydrate metabolism. These pathways act through various enzymes and are not necessarily independent of the previously known transducers of environmental signals in species with ESD: calcium–redox, epigenetic, and stress regulation pathways. Overall, there is evidence of a link between energy level and the sexual fate of individuals of various species, including reptiles, fish, amphibians, insects, and nematodes. As energy pathways are evolutionarily conserved, this knowledge opens new avenues to advance our understanding of the mechanisms that allow animals to adapt their sex according to the local environment. The quality of the environment directly or indirectly influences the sex of a variety of species. The phenotypic sex of some animals is linked to energy level at a given, key moment. Regulation of lipid and carbohydrate metabolism is involved in the process. This change in metabolism is accompanied by changes in stress hormone levels and epigenetic modifications. Findings in invertebrates, fish, amphibians, and reptiles open new avenues in our understanding of sex allocation in mammals. [ABSTRACT FROM AUTHOR]

In most animals, sex determination occurs at conception, when sex chromosomes are segregated following Mendelian laws. However, in multiple reptiles and fishes, this genetic sex can be overridden by external factors after fertilization or birth. In some species, the genetic sex may also be governed by multiple genes, further limiting our understanding of sex determination in such species. We used the European sea bass (Dicentrarchus labrax) as a model and combined genomic (using a single nucleotide polymorphism chip) and transcriptomic (RNA-Sequencing) approaches to thoroughly depict this polygenic sex determination system and its interaction with temperature. We estimated genetic sex tendency (eGST), defined as the estimated genetic liability to become a given sex under a liability threshold model for sex determination, which accurately predicts the future phenotypic sex. We found evidence that energetic pathways, concerning the regulation of lipids and glucose, are involved in sex determination and could explain why females tend to exhibit higher energy levels and improved growth compared to males. Besides, early exposure to high temperature up-regulated sox3, followed by sox9a in individuals with intermediate eGST, but not in individuals showing highly female-biased eGST, providing the most parsimonious explanation for temperature-induced masculinization. This gonadal state was maintained likely by DNA methylation and the up-regulation of several genes involved in histone modifications, including jmjd1c. Overall, we describe a sex determination system resulting from continuous genetic and environmental influences in an animal. Our results provide significant progress in our understanding of the mechanisms underlying temperature-induced masculinization in fish. [ABSTRACT FROM AUTHOR]

Sex ratio variation is a key topic in ecology, because of its direct effects on population dynamics and thus, on animal conservation strategies. Among factors affecting sex ratio, types of sex determination systems have a central role, since some species could have a sex determined by genetic factors, environmental factors or a mix of those two. Yet, most studies on the factors affecting sex determination have focused on temperature or endocrine-disrupting chemicals (EDCs), and much less is known regarding other factors. Exposure to gamma irradiation was found to trigger offspring masculinization in zebrafish. Here we aimed at deciphering the potential mechanisms involved, by focusing on stress (i.e. cortisol) and epigenetic regulation of key genes involved in sex differentiation in fish. Cortisol levels in exposed and control (F0) zebrafish females' gonads were similar. However, irradiation increased the DNA methylation level of foxl2a and cyp19a1a in females of the F0 and F1 generation, respectively, while no effects were detected in testis. Overall, our results suggest that parental exposure could alter offspring sex ratio, at least in part by inducing methylation changes in ovaries. [Display omitted] • Irradiation-induced masculinization was associated to parental exposure. • Irradiation impacted methylation level of genes involved in sex determination. • foxl2a methylation level in mother gonads was associated with offspring sex ratio. • Increased methylation of cyp19a1a in female gonads was associated to biased sex ratio. • DNA methylation could be a key transducer between environment and sex. [ABSTRACT FROM AUTHOR]

In fishes, sex is determined by genetics, the environment or an interaction of both. Temperature is among the most important environmental factors that can affect sex determination. As a consequence, changes in temperature at critical developmental stages can induce biases in primary sex ratios in some species. However, early sex ratios can also be biased by sex‐specific tolerances to environmental stresses that may, in some cases, be amplified by changes in water temperature. Sex‐specific reactions to environmental stress have been observed at early larval stages before gonad formation starts. It is therefore necessary to distinguish between temperature effects on sex determination, generally acting through the stress axis or epigenetic mechanisms, and temperature effects on sex‐specific mortality. Both are likely to affect sex ratios and hence population dynamics. Moreover, in cases where temperature effects on sex determination lead to genotype–phenotype mismatches, long‐term effects on population dynamics are possible, for example temperature‐induced masculinization potentially leading to the loss of Y chromosomes or feminization to male‐biased operational sex ratios in future generations. To date, most studies under controlled conditions conclude that if temperature affects sex ratios, elevated temperatures mostly lead to a male bias. The few studies that have been performed on wild populations seem to confirm this general trend. Recent findings suggest that transgenerational plasticity could mitigate the effects of warming on sex ratios in some populations. [ABSTRACT FROM AUTHOR]

The impact of early stress on juvenile development has intrigued scientists for decades, but the adaptive significance of such effects remains an ongoing debate. This debate has largely ignored some characteristics of the offspring, such as their sex, despite strong evolutionary and demographic implications of sex-ratio variation. We review recent studies that examine associations between glucocorticoids (GCs), the main class of stress hormones, and offspring sex. Whereas exposure to GCs at around the time of sex determination in fish consistently produces males, the extent and direction of sex-ratio bias in response to stress vary in reptiles, birds, and mammals. We propose proximate and ultimate explanations for most of these trends. Offspring sex ratios have been shown to correlate with environmental stressors and maternal stress in many vertebrate species. There is an adaptive advantage for parents to produce the sex that is more likely to survive and reproduce in a future hazardous environment. In the fastest life histories, there is more likely to be a close good match between the environment around sex determination and the future environment of the offspring. GCs are key messengers of environmental contexts that likely influence the sex-determination processes of various species. A combination of field and laboratory studies will be necessary to understand the extent to which stress influences offspring sex from conception to birth. [ABSTRACT FROM AUTHOR]

The aim of this review is to present an overview of the sex differentiation and sex determination processes in eels in relation to the urgent need to provide scientific knowledge to better protect and manage the Anguilla genus. Indeed, the global decline of the three main temperate eel stocks, Anguilla anguilla, Anguillidae (Fisheries Management and Ecology, 2003, 10, 365); Anguilla japonica, Anguillidae (Casselman, Eel Biology, Springer Japan, 2003, 293) and Anguilla rostrata, Anguillidae (Tatsukawa, Eel Biology, Springer, Japan, 2003, 255), raises concerns about the necessity to better understand all stages of the life cycle of eels (Righton and Walker, Journal of Fish Biology, 2013, 83, 754). Little is known about the mechanisms involved in the production of males and females in this species with environmental sex determination. Previous reviews identifying the density of individuals as the major factor influencing sex determination were undertaken (Krueger and Oliveira, Environmental Biology of Fishes, 1999, 55, 381; Reviews in Fish Biology and Fisheries, 2005, 15, 37). Here, we review the current advances on the subject, focusing on the roles of early growth rate and interindividual relationships, which are mechanisms underpinned by density, as well as the sex differentiation process, and we question how this knowledge might influence global conservation measures. [ABSTRACT FROM AUTHOR]

• Energy allocation genes are involved in determining phenotypic sex of some animals. • Temperature differentially modulates ampk1, ghrelin, mtor, and nucb1. • Temperature and sex interaction occurs for the expression of spry1 and pank1a. • Ampk1 , mtor , pank1 , and spry1 are involved in sex differentiation. • Spry1 and sox3 are early markers of masculinization in the European sea bass. Water temperature governs physiological functions such as growth, energy allocation, and sex determination in ectothermic species. The European sea bass (Dicentrarchus labrax) is a major species in European aquaculture, exhibiting early dimorphic growth favoring females. The species has a polygenic sex determination system that interacts with water temperature to determine an individual's sex, with two periods during development that are sensitive to temperature. The current study investigated the influence of water temperature on energy allocation and sex-biased genes during sex determination and differentiation periods. RNA-Sequencing and qPCR analyses were conducted in two separate experiments, of either constant water temperatures typical of aquaculture conditions or natural seasonal thermal regimes, respectively. We focused on eight key genes associated with energy allocation, growth regulation, and sex determination and differentiation. In Experiment 1, cold and warm temperature treatments favored female and male proportions, respectively. The RNA-seq analysis highlighted sex-dependent energy allocation transcripts, with higher levels of nucb1 and pomc1 in future females, and increased levels of egfra and spry1 in future males. In Experiment 2, a warm thermal regime favored females, while a cold regime favored males. qPCR analysis in Experiment 2 revealed that ghrelin and nucb1 were down-regulated by warm temperatures. A significant sex-temperature interaction was observed for pank1a with higher and lower expression for males in the cold and warm regimes respectively, compared to females. Notably, spry1 displayed increased expression in future males at the all-fins stage and in males undergoing molecular sex differentiation in both experimental conditions, indicating that it provides a novel, robust, and consistent marker for masculinization. Overall, our findings emphasize the complex interplay of genes involved in feeding, energy allocation, growth, and sex determination in response to temperature variations in the European sea bass. [ABSTRACT FROM AUTHOR]

One of the primary challenges in European seabass farming is optimizing the number of females, which grow faster and mature later than males. However, typical rearing practices often result in populations with a high proportion of males. To test whether photoperiod, temperature and fasting impact the sex ratio, two distinct experiments were conducted. In the first one, we tested the effects of two photoperiods (12L:12D vs 10L:14D), applied until 90 dph at 16 °C. Then, on the same fish, photoperiod was set to 12D:12L and four different temperature treatments (19 °C, 21 °C, 23 °C, and 25 °C) were applied from 90 to 160–192 dph. In a second experiment, we tested the impact of fasting at four different times during the sex determination window: from 60 to 65 dph; 81 to 87 dph; 102 to 110 dph and 123 to 133 dph, at a temperature of 21 °C. Neither photoperiod (in the first experiment) nor starvation during larval development (in the second experiment) altered sex ratio. Conversely, we revealed (in the first experiment) that the proportion of females increased with post-larval rearing temperature: 30.4% at 19 °C, 36.3% at 21 °C, 44.1% at 23 °C, and 48.7% at 25 °C. Thus, late warm thermal treatments, following an initial cold treatment at 16 °C until 90 dph, can be useful to compensate the deficit of females in farmed sea bass production. • Late high temperature can yield females in seabass. • This can be applied to improve seabass production • We found no effect of photoperiod or starvation on sex ratio. [ABSTRACT FROM AUTHOR]