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Bolder individuals have greater access to food sources and reproductive partners but are also at increased risk of predation. Boldness is believed to be consistent across time and contexts, but few studies have investigated the stability of this trait across variable environments, such as varying stress loads or long periods of time. Moreover, the underlying molecular components of boldness are poorly studied. Here, we report that boldness of 1154 European sea bass, evaluated using group risk-taking tests, is consistent over seven months and for individuals subjected to multiple environments, including a chronically stressful environment. Differences in risk-taking behaviour were further supported by differences observed in the responses to a novel environment test: shy individuals displayed more group dispersion, more thigmotaxic behaviour and lower activity levels. Transcriptomic analyses performed on extreme phenotypes revealed that bold individuals display greater expression for genes involved in social and exploration behaviours, and memory in the pituitary, and genes involved in immunity and responses to stimuli in the head kidney. This study demonstrates that personality traits come with an underpinning molecular signature, especially in organs involved in the endocrine and immune systems. As such, our results help to depict state–behaviour feedback mechanisms, previously proposed as key in shaping animal personality. [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]

• Small within-day increases in temperature are physiologically stressful. • Whole-fish transcriptomic analysis revealed a signature of warming. • Fish kept at high temperature accumulate more cortisol in scales than those kept at low temperature. • We propose fish scales as a relevant indicator of global warming in wild populations. Changes in ocean water temperature associated with global climate change are bound to enormously affect fish populations, with potential major economic consequences in the aquaculture and fisheries industries. A link between temperature fluctuations and changes in fish stress response is well established. In this study, we aimed to assess the effects of a short- (4 days) or a long-term (4 months) exposure to warm temperature in the stress physiology of European sea bass (Dicentrarchus labrax) larvae and juveniles. First, cortisol (i.e. the main stress hormone in fishes) analysis was used to confirm that a steady and short-term elevation of temperature acts as a physiological stressful event in these fish, and cortisol release is indeed above a metabolic increase linked to temperature. Moreover, our results verified that measurement of cortisol released into the water can be reliably employed as a non-invasive indicator of acute thermal stress in experimental conditions. Secondly, the different effects on the genetic cascade underlying the stress response between long-term low or high thermal treatments were evaluated at two larval development stages via candidate-gene and whole-transcriptome approaches. Interestingly, opposite expression for some key stress genes (nr3c1, nr3c2 and hsd11b2) were observed between developmental stages, highlighting the distinct adaptive mechanisms controlling the primary and secondary responses to a stressor. Surprising expression patterns for some understudied genes involved in the stress axis were also revealed, including crhr1, mc2r, mc5r, trh or trhr, which should be further explored. Finally, evaluation of cortisol content in scales was successfully used as a biomarker of chronic thermal stress, with 10x more cortisol in fish kept at 21 °C vs 16 °C after 4 months, supporting the gene expression results observed. The use of such a method as a proxy of long-term stress, unprecedented in the literature, holds a vast array of applications in further research, in particular, in the investigation of the impact of global warming on wild fish populations. [ABSTRACT FROM AUTHOR]