Cardiovascular oxygen transport and peripheral oxygen extraction capacity contribute to acute heat tolerance in European seabass.

This study evaluated whether different parameters describing cardiovascular function, energy metabolism, oxygen transport and oxidative stress were related to the critical thermal maximum (CTMAX) of European seabass (Dicentrarchus labrax) and if there were differential changes in these parameters during and after heat shock in animals with different CTMAX in order to characterize which physiological features make seabass vulnerable to heat waves. Seabass (n = 621) were tested for CTMAX and the physiological parameters were measured in individuals with good or poor temperature tolerance before and after a heat shock (change in temperature from 15 °C to 28 °C in 1.5 h). Fish with good thermal tolerance had larger ventricles with higher maximal heart rate during the heat shock than individuals with poor tolerance. Furthermore, they initially had a high ventricular Ca2+-ATPase activity, which was reduced to a similar level as in fish with poor tolerance following heat shock. The activity of heart lactate dehydrogenase increased in fish with high tolerance, when they were exposed to heat shock, while the aerobic enzyme activity did not differ between groups. The tolerant individuals had smaller red muscle fibers with higher myoglobin content than the poorly tolerant ones. The poorly tolerant individuals had higher hematocrit, which increased with heat shock in both groups. The poorly tolerant individuals had also higher activity of enzymes related to oxidative stress especially after heat shock. In general, CTMAX was not depending on merely one physiological factor but several organ and cellular parameters were related to the CTMAX of seabass and when working in combination they might protect the highly tolerant seabass from future heat waves.

Hsp70s transcription-translation relationship depends on the heat shock temperature in zebrafish.

Virtually all organisms respond to heat shock by transcription of genes encoding for heat shock proteins (HSPs), but the mechanisms behind post-transcriptional regulation are not known in detail. When we exposed zebrafish to 5 and 7 °C above normal rearing temperature for 30 min, hsp70 mRNA expression was 28 and 150 -fold higher than in control, respectively. Protein expression, on the other hand, showed no significant change at the +5 °C and a 2-fold increase at the +7 °C exposure. This suggests that the transcription of hsp70 gene does not immediately correspond to translation to related proteins under certain stress temperatures, but, when the temperature is higher, and potentially detrimental, transcription and translation are intimately coupled. Those results confirm that temperature is an important abiotic factor involved in heat shock post-transcriptional regulation mechanisms in fish. However, further studies are needed to determine the relationship between this environmental factor and post-transcriptional regulation mechanisms. Earlier, the coupling/uncoupling of hsp transcription and translation has only been studied using cold-water fish, or zebrafish embryos. With current findings, we suggest this mechanism might be present even in adult warm water fish like the zebrafish.

Circadian rhythms and environmental disturbances - underexplored interactions.

Biological rhythms control the life of virtually all organisms, impacting numerous aspects ranging from subcellular processes to behaviour. Many studies have shown that changes in abiotic environmental conditions can disturb or entrain circadian (∼24 h) rhythms. These expected changes are so large that they could impose risks to the long-term viability of populations. Climate change is a major global stressor affecting the fitness of animals, partially because it challenges the adaptive associations between endogenous clocks and temperature - consequently, one can posit that a large-scale natural experiment on the plasticity of rhythm-temperature interactions is underway. Further risks are posed by chemical pollution and the depletion of oxygen levels in aquatic environments. Here, we focused our attention on fish, which are at heightened risk of being affected by human influence and are adapted to diverse environments showing predictable changes in light conditions, oxygen saturation and temperature. The examined literature to date suggests an abundance of mechanisms that can lead to interactions between responses to hypoxia, pollutants or pathogens and regulation of endogenous rhythms, but also reveals gaps in our understanding of the plasticity of endogenous rhythms in fish and in how these interactions may be disturbed by human influence and affect natural populations. Here, we summarize research on the molecular mechanisms behind environment-clock interactions as they relate to oxygen variability, temperature and responses to pollutants, and propose ways to address these interactions more conclusively in future studies.

Cold temperature represses daily rhythms in the liver transcriptome of a stenothermal teleost under decreasing day length.

The climate-change-driven increase in temperature is occurring rapidly and decreasing the predictability of seasonal rhythms at high latitudes. It is therefore urgent to understand how a change in the relationship between photoperiod and temperature can affect ectotherms in these environments. We tested whether temperature affects daily rhythms of transcription in a cold-adapted salmonid using high-throughput RNA sequencing. Arctic char (Salvelinus alpinus) from a subarctic population were reared at a high and a low temperature (15 and 8°C) for 1 month under natural, decreasing day length during late summer. Liver transcriptomes were compared between samples collected in the middle and towards the end of the light period and in the middle of the dark period. Daily variation in transcription was lower in fish from the low temperature compared with strong daily variation in warm-acclimated fish, suggesting that cold temperatures dampen the cycling of transcriptional rhythms under a simultaneously decreasing day length. Different circadian clock genes had divergent expression patterns, responding either by decreased expression or by increased rhythmicity at 15°C compared with 8°C. The results point out mechanisms that can affect the ability of fish to adapt to increasing temperatures caused by climate change.

The evolution and adaptive potential of transcriptional variation in sticklebacks--signatures of selection and widespread heritability.

Evidence implicating differential gene expression as a significant driver of evolutionary novelty continues to accumulate, but our understanding of the underlying sources of variation in expression, both environmental and genetic, is wanting. Heritability in particular may be underestimated when inferred from genetic mapping studies, the predominant "genetical genomics" approach to the study of expression variation. Such uncertainty represents a fundamental limitation to testing for adaptive evolution at the transcriptomic level. By studying the inheritance of expression levels in 10,495 genes (10,527 splice variants) in a threespine stickleback pedigree consisting of 563 individuals, half of which were subjected to a thermal treatment, we show that 74-98% of transcripts exhibit significant additive genetic variance. Dominance variance is also prevalent (41-99% of transcripts), and genetic sources of variation seem to play a more significant role in expression variance in the liver than a key environmental variable, temperature. Among-population comparisons suggest that the majority of differential expression in the liver is likely due to neutral divergence; however, we also show that signatures of directional selection may be more prevalent than those of stabilizing selection. This predominantly aligns with the neutral model of evolution for gene expression but also suggests that natural selection may still act on transcriptional variation in the wild. As genetic variation both within- and among-populations ultimately defines adaptive potential, these results indicate that broad adaptive potential may be found within the transcriptome.

Microarray analysis of di-n-butyl phthalate and 17α ethinyl-oestradiol responses in three-spined stickleback testes reveals novel candidate genes for endocrine disruption.

Phthalate esters are plasticizers frequently found in wastewater effluents. Previous studies on phthalates have reported anti-androgenic activity in mammals, causing concerns of their potential effects on the reproduction of aquatic organisms. Another group of environmental endocrine disrupters, steroidal estrogens, are known to inhibit steroid biosynthesis in the gonads, but the effects related to spermatogenesis are not well understood in fish. In this study, three-spined sticklebacks were exposed to di-n-butyl phthalate (DBP) and 17α ethinyl-oestradiol (EE2) at nominal concentrations 35µg/L and 40ng/L, respectively, for four days. The aim of the study was to obtain insight into the acute transcriptional responses putatively associated with endocrine disruption. RNA samples from eight individual male fish per treatment (including controls) were used in microarray analysis, covering the expression of approximately 21,000 genes. In the EE2 treatment the results show transcriptional downregulation of genes associated with steroid biosynthesis pathway and up-regulation of genes involved in pathways related to epidermal growth factor signaling and xenobiotic metabolism. The transcriptional response to DBP was in general weaker than to EE2, but based on enrichment analysis, we suggest adverse effects on retinoid metabolism, creatine kinase activity and cell adhesion. Among the genes showing highest fold changes after DBP treatment compared to control was the teleost fish -specific cytochrome P450 17A2. Overall, this study promotes our understanding on molecular responses to anti-androgens and estrogens in fish testes.

Warm acclimation and oxygen depletion induce species-specific responses in salmonids.

Anthropogenic activities are greatly altering the habitats of animals, whereby fish are already encountering several stressors simultaneously. The purpose of the current study was to investigate the capacity of fish to respond to two different environmental stressors (high temperature and overnight hypoxia) separately and together. We found that acclimation to increased temperature (from 7.7±0.02°C to 14.9±0.05°C) and overnight hypoxia (daily changes from normoxia to 63-67% oxygen saturation), simulating climate change and eutrophication, had both antagonistic and synergistic effects on the capacity of fish to tolerate these stressors. The thermal tolerance of Arctic char (Salvelinus alpinus) and landlocked salmon (Salmo salar m. sebago) increased with warm acclimation by 1.3 and 2.2°C, respectively, but decreased when warm temperature was combined with overnight hypoxia (by 0.2 and 0.4°C, respectively). In contrast, the combination of the stressors more than doubled hypoxia tolerance in salmon and also increased hypoxia tolerance in char by 22%. Salmon had 1.2°C higher thermal tolerance than char, but char tolerated much lower oxygen levels than salmon at a given temperature. The changes in hypoxia tolerance were connected to the responses of the oxygen supply and delivery system. The relative ventricle mass was higher in cold- than in warm-acclimated salmon but the thickness of the compact layer of the ventricle increased with the combination of warm and hypoxia acclimation in both species. Char had also significantly larger hearts and thicker compact layers than salmon. The results illustrate that while fish can have protective responses when encountering a single environmental stressor, the combination of stressors can have unexpected species-specific effects that will influence their survival capacity.

Circulating N-terminal brain natriuretic peptide and cardiac function in response to acute systemic hypoxia in healthy humans.

BACKGROUND: As it remains unclear whether hypoxia of cardiomyocytes could trigger the release of brain natriuretic peptide (BNP) in humans, we investigated whether breathing normobaric hypoxic gas mixture increases the circulating NT-proBNP in healthy male subjects. METHODS: Ten healthy young men (age 29 ± 5 yrs, BMI 24.7 ± 2.8 kg/m2) breathed normobaric hypoxic gas mixture (11% O2/89% N2) for one hour. Venous blood samples were obtained immediately before, during, and 2 and 24 hours after hypoxic exposure. Cardiac function and flow velocity profile in the middle left anterior descending coronary artery (LAD) were measured by Doppler echocardiography. RESULTS: Arterial oxygen saturation decreased steadily from baseline value of 99 ± 1% after the initiation hypoxia challenge and reached steady-state level of 73 ± 6% within 20-30 minutes. Cardiac output increased from 6.0 ± 1.2 to 8.1 ± 1.6 L/min and ejection fraction from 67 ± 4% to 75 ± 6% (both p < 0.001). Peak diastolic flow velocity in the LAD increased from 0.16 ± 0.04 to 0.28 ± 0.07 m/s, while its diameter remained unchanged. In the whole study group, NT-proBNP was similar to baseline (60 ± 32 pmol/ml) at all time points. However, at 24 h, concentration of NT-proBNP was higher (34 ± 18%) in five subjects and lower (17 ± 17%), p = 0.002 between the groups) in five subjects than at baseline. CONCLUSION: In conclusion, there is no consistent increase in circulating NT-proBNP in response to breathing severely hypoxic normobaric gas mixture in healthy humans, a possible reason being that the oxygen flux to cardiac myocytes does not decrease because of increased coronary blood flow. However, the divergent individual responses as well as responses in different cardiac diseases warrant further investigations.

Spatial variation in transcript and protein abundance of Atlantic salmon during feeding migration in the Baltic Sea.

The fitness and reproductive output of fishes can be affected by environmental disturbances. In this study, transcriptomics and label-free proteomics were combined to investigate Atlantic salmon (Salmo salar) sampled from three different field locations within the Baltic Sea (Baltic Main Basin (BMB), Gulf of Finland (GoF), and Bothnian Sea (BS)) during marine migration. The expression of several stress related mRNAs and proteins of xenobiotic metabolism, oxidative stress, DNA damage, and cell death were increased in salmon from GoF compared to salmon from BMB or BS. Respiratory electron chain and ATP synthesis related gene ontology-categories were upregulated in GoF salmon, whereas those associated with RNA processing and synthesis, translation, and protein folding decreased. Differences were seen also in metabolism and immune function related gene expression. Comparisons of the transcriptomic and proteomic profiles between salmon from GoF and salmon from BMB or BS suggest environmental stressors, especially exposure to contaminants, as a main explanation for differences. Salmon feeding in GoF are thus "disturbed by hazardous substances". The results may also be applied in evaluating the conditions of pelagic ecosystems in the different parts of Baltic Sea.

Hypoxia increases the release of salmon cardiac peptide (sCP) from the heart of rainbow trout (Oncorhynchus mykiss) under constant mechanical load in vitro.

Our aim was to study the effects of hypoxia on the release of salmon cardiac peptide (sCP) from an isolated heart ventricle of trout during a constant mechanical load. Trout heart ventricles were studied in vitro. The ventricle was placed in an organ bath at 12 °C in which a constant mechanical load could be imposed on the ventricle while buffer solution was circulating. Ventricles were field-stimulated with a supramaximal voltage pulse at a rate of about 0.3 s⁻¹. Samples of 1 ml were collected at an interval of 10 min for 200 min from the organ bath and assessed with a radioimmunoassay for sCP. After a control period of 20 min, ventricles were exposed to hypoxia produced with N2 gassing (n = 9) or to hypoxia with 20 mM BDM, a nonselective myosin ATPase inhibitor locking cross-bridges in a pre-power-stroke state inhibiting force production with normal electrical activity (n = 10). In this model and setup, hypoxia stimulated the release of sCP, but the interindividual variation in the response was large. At the end of hypoxia exposure, the concentration of sCP in the organ bath was about sixfold higher than at the start of the exposure (P < 0.05, one-way ANOVA for repeated measurements, followed by Dunnett's multiple comparison test). When BDM was introduced into the bath, the ventricle still secreted sCP but the hypoxic response was smaller than in the experiments without BDM. In the trout heart ventricle, there is a hypoxia-sensitive component in the release mechanism of sCP which is independent of contraction.

Revisiting redox-active antioxidant defenses in response to hypoxic challenge in both hypoxia-tolerant and hypoxia-sensitive fish species.

It is not known whether changes in antioxidant levels always occur in fish in response to the oxidative stress that usually accompanies a hypoxic challenge. The studies of antioxidant responses to hypoxia in fish have mostly focused on very anoxia-tolerant species and indicate that there is an enhancement of antioxidant defenses. Here we present new data on redox-active antioxidants from three species, which range in their tolerance to hypoxia: the epaulette shark, threespine stickleback, and rainbow trout, together with a compilation of results from other studies that have measured oxidative stress parameters in hypoxia-exposed fish. The results suggest that in general, fish do not show an increase in redox-active antioxidant defense in response to oxidative stress associated with hypoxia. Rather, the changes in antioxidant defenses during hypoxia are very much species- and tissue-specific and are not linked to the level of hypoxia tolerance of the fish species.

Tissue oxygen demand in regulation of the behavior of the cells in the vasculature.

The control of arteriolar diameters in microvasculature has been in the focus of studies on mechanisms matching oxygen demand and supply at the tissue level. Functionally, important vascular elements include EC, VSMC, and RBC. Integration of these different cell types into functional units aimed at matching tissue oxygen supply with tissue oxygen demand is only achieved when all these cells can respond to the signals of tissue oxygen demand. Many vasoactive agents that serve as signals of tissue oxygen demand have their receptors on all these types of cells (VSMC, EC, and RBC) implying that there can be a coordinated regulation of their behavior by the tissue oxygen demand. Such functions of RBC as oxygen carrying by Hb, rheology, and release of vasoactive agents are considered. Several common extra- and intracellular signaling pathways that link tissue oxygen demand with control of VSMC contractility, EC permeability, and RBC functioning are discussed.

Corticosterone secretion patterns prior to spring and autumn migration differ in free-living barn swallows (Hirundo rustica L.).

Recent studies of long-distance migratory birds show that behavioural and physiological changes associated with predictable or unpredictable challenges during the annual cycle are distinctively regulated by hormones. Corticosterone is the primary energy regulating hormone in birds. Corticosterone levels are elevated during stresses but they are also modulated seasonally according to environmental conditions and life-history demands. We measured the baseline and stress-induced levels of corticosterone in the barn swallow (Hirundo rustica L.) just before spring and autumn migrations in South Africa and Finland, respectively. Barn swallows completing their pre-breeding moult had low body condition (residual body mass) and high baseline corticosterone levels in the wintering grounds. In contrast, baseline corticosterone levels in Finland were low and not related to residual mass. These data contradict the first prediction of the migration modulation hypothesis (MMH) by showing no association with baseline corticosterone levels and pre-migratory fuelling. Yet, the adrenocortical response to the capture and handling stress was notably blunted in South Africa compared to a strong response in Finland. Further, individuals that had started fuelling in Finland showed a reduced response to the handling stress. Taken together, elevated baseline corticosterone levels and high residual mass may blunt the adrenocortical response in long-distance migrants and aerial feeders such as the barn swallow. This observation lends support to the second prediction of the MMH.

Developmental transcription of genes putatively associated with growth in two sturgeon species of different growth rate.

In the present study, we surveyed developmental changes in the transcription of growth hormone (gh), insulin-like growth factor-I (igf-I), ghrelin (ghrl) and vascular endothelial growth factor (vegf) genes in the largest freshwater fish, European sturgeon (Beluga, Huso huso) and compared the same parameters to that of its phylogenically close moderate-sized species, Persian sturgeon (Acipenser persicus). The transcripts of gh, igf-I, ghrl and vegf were detected at all developmental time-points of Persian sturgeon and Beluga from embryos to juvenile fish. Changes in normalized gh, igf-I, ghrl and vegf transcription by using the geometric average of genes encoding ribosomal protein L6 (RPL6) and elongation factor (EF1A) over the time of development of Persian sturgeon and Beluga were statistically significant (P<0.05). Our results showed that the mRNA expression levels of both igf-I and ghrl were low during early larval development and then increased significantly to the late larval time-points when larvae started exogenous feeding. In both Beluga and Persian sturgeon, after a low mRNA expression during the embryonic stage, the transcript levels of vegf displayed an increasing trend during yolk-sac fry, consistent with organogenesis. The vegf level remained constantly high in the time of exogenous feeding. The highest detection of gh transcripts coincided with the end of the embryonic stage (hatching time) in Persian sturgeon and 3 days-post-hatching (dph) in Beluga. In Persian sturgeon, the gh transcript started to decrease to the rest of the developmental time-points, whereas in Beluga gh transcript had a marked second increase from the time of exogenous feeding (20-dph). This Beluga specific increase in gh transcription may be associated with the marked growth rate and extraordinary size of this fish species.

Transcriptional responses to hypoxia are enhanced by recurrent hypoxia (hypoxic preconditioning) in the epaulette shark.

All animals require molecular oxygen for aerobic energy production, and oxygen availability has played a particularly important role in the evolution of aquatic animals. This study investigates how previous exposure to hypoxia (preconditioning) primes protective transcriptional responses in a hypoxia-tolerant vertebrate species, the epaulette shark (Hemiscyllium ocellatum). The epaulette shark is a basal cartilaginous fish that in its natural environment experiences cyclic hypoxic periods. We evaluated whether the transcription of a set of crucial prosurvival genes is affected differently by a single short-term (2 h) exposure to sublethal hypoxia compared with eight such successive hypoxia exposures (hypoxia preconditioning). We discovered that hypoxia preconditioning amplifies transcriptional responses compared with animals that experienced a single hypoxic bout. In the heart we observed that hypoxic preconditioning, but not a single hypoxic exposure, resulted in higher transcript levels of genes that regulate oxygen and energy homeostasis, including those of hypoxia-inducible factor-1 alpha, adenosine signaling pathway components, and genes affecting circulation [prostaglandin synthetase 2 (cox-2) and natriuretic peptide C]. This suggests that in a single short-term hypoxic bout, the responses to low oxygen are regulated at the level of pre-existing proteins or translational and posttranslational machinery, whereas transcriptional responses are induced in experiments that parallel the natural environmental cycles of oxygen availability. These findings have general implications for understanding how vertebrates regulate protective gene expression upon physiological stress.

Molecular evolution of the metazoan PHD-HIF oxygen-sensing system.

Metazoans rely on aerobic energy production, which requires an adequate oxygen supply. During reduced oxygen supply (hypoxia), the most profound changes in gene expression are mediated by transcription factors known as hypoxia-inducible factors (HIFs). HIF alpha proteins are commonly posttranslationally regulated by prolyl-4-hydroxylase (PHD) enzymes, which are direct "sensors" of cellular oxygen levels. We examined the molecular evolution of the metazoan PHD-HIF oxygen-sensing system by constructing complete phylogenies for PHD and HIF alpha genes and used computational tools to characterize the molecular changes underlying the functional divergence of PHD and HIF alpha duplicates. The presence of PHDs in metazoan genomes predates the emergence of HIF alphas. Our analysis revealed an unexpected diversity of PHD genes and HIF alpha sequence characteristics in invertebrates, suggesting that the simple oxygen-sensing systems of Caenorhabditis and Drosophila may not be typical of other invertebrate bilaterians. We studied the early vertebrate evolution of the system by sequencing these genes in early-diverging cartilaginous fishes, elasmobranchs. Cartilaginous fishes appear to have three paralogs of both PHD and HIF alpha. The novel sequences were used as outgroups for a detailed molecular analysis of PHD and HIF alpha duplicates in a major air-breathing vertebrate lineage, the mammals, and a major water-breathing vertebrate lineage, the teleosts. In PHDs, functionally divergent amino acid sites were detected near the HIF alpha-binding channel and beta2beta3 loop that defines its substrate specificity. In HIF alphas, more functional divergence was found in teleosts than in mammals, especially in the HIF-1 alpha PAS domain and HIF-2 alpha oxygen-dependent degradation (ODD) domains, which interact with PHDs. Overall, in the vertebrates, elevated substitution rates in the HIF-2 alpha N-terminal ODD domain, together with a functional divergence associated with the known differences in PHD2 versus PHD1/3 substrate specificity, have contributed to the tighter oxygen-sensitive regulation of HIF-1 alpha than that of HIF-2 alpha.

Impact of tributyltin on immune response and life history traits of Chironomus riparius: single and multigeneration effects and recovery from pollution.

Chironomids play an important role in the detritus cycle and as a component in brackish- and freshwater benthic and terrestrial food webs. If TBT is present in their environment, then they may accumulate tributyltin (TBT) during their juvenile period, which negatively affects many of their life history characteristics. The aim of this experiment is to test the effects of three TBT sediment concentrations (nominal 30, 90, and 180 µg/kg) on life history traits (development time, survival, fecundity, and weight) and immune response (number of hemocytes and phenoloxidase activity) of the nonbiting midge, Chironomus riparius. These responses were recorded immediately after one generation of TBT exposure, and in the long run during five consecutive generations. We also assessed recovery from pollution after four generations of TBT exposure. In a single generation, TBT affected all measured parameters, except phenoloxidase activity, when compared to the control. Long-term-effects of TBT lead to extinction of all treatments after the fifth generation. Again, all measured variables significantly differ from the control, although TBT had varying effects on the measured variables. Most of the effects of TBT on population viability were not evident during recovery, once TBT was removed from the sediment. The effect of previous TBT contamination was observed only in delayed larval development, suggesting that TBT has only limited maternal/epigenetic effects on individual condition. However, altered schedules in the life-cycle can have unexpected ecological impacts. TBT decreases the viability of Chironomus riparius and the effect will become stronger if exposure to TBT continues for many generations. Yet, the harmful effect of TBT disappears quickly as the TBT is removed from the environment.

Functional Genomics of Fish Erythrocytes.

Fish red blood cells contain nucleus and therefore carry on active transcription throughout their life. As the steady state level of mRNA depends both on the production and breakdown of the mRNA, it is important to pay attention to sampling, cell separation, mRNA extraction, and transcript stability to conduct a comprehensive erythrocyte genomics study. In addition, particular attention should be made to tie the transcriptional changes to corresponding protein activities, as only those would impact cellular functions. Here, we thus describe the different steps to conduct comprehensive functional genomics studies with fish erythrocytes.

Copper exposure improves the upper thermal tolerance in a sex-specific manner, irrespective of fish thermal history.

Ectotherms can respond to climate change via evolutionary adaptation, usually resulting in an increase of their upper thermal tolerance. But whether such adaptation influences the phenotypic plasticity of thermal tolerance when encountering further environmental stressors is not clear yet. This is crucial to understand because organisms experience multiple stressors, besides warming climate, in their natural environment and pollution is one of those. Here, we studied the phenotypic plasticity of thermal tolerance in three-spined stickleback populations inhabiting spatially replicated thermally polluted and pristine areas before and after exposing them to a sublethal concentration of copper for one week. We found that the upper thermal tolerance and its phenotypic plasticity after copper exposure did not depend on the thermal history of fish, suggesting that five decades of thermal pollution did not result in evolutionary adaptation to thermal tolerance. The upper thermal tolerance of fish was, on the other hand, increased by ∼ 1.5 °C after 1-week copper exposure in a sex-specific manner, with males having higher plasticity. To our knowledge this is the first study that shows an improvement of the upper thermal tolerance as a result of metal exposure. The results suggest that three-spined sticklebacks are having high plasticity and they are capable of surviving in a multiple-stressor scenario in the wild and that male sticklebacks seem more resilient to fluctuating environmental conditions than female.

Female-biased expression on the X chromosome as a key step in sex chromosome evolution in threespine sticklebacks.

Given that the genome of males and females are almost identical with the exception of genes on the Y (or W) chromosome or sex-determining alleles (in organisms without sex chromosomes), it is likely that many downstream processes resulting in sexual dimorphism are produced by changes in regulation. In early stages of sex chromosome evolution, as the Y-chromosome degenerates, gene expression should be significantly impacted for genes residing on the sex chromosome pair as regulatory mutations accumulate. However, this has rarely been examined because most model organisms have clearly diverged sex chromosomes. Fish provide a unique opportunity to examine the evolution of sex chromosomes because genetic sex determination has evolved quite recently in some groups of fish. We compared sex-specific transcription in threespine stickleback (Gasterosteus aculeatus) liver tissue using a long-oligo microarray. Of the 1,268 genes that were differentially expressed between sexes, a highly significant proportion (23%) was concentrated on chromosome 19, corresponding to the recently described nascent sex chromosomes. The sex-biased genes are enriched for different functional categories in males and females, although there is no specific functional enrichment on the sex chromosomes. Female-biased genes are concentrated at one end of the sex chromosome, corresponding to a deletion in the Y, suggesting a lack of global dosage compensation. Prior research on threespine sticklebacks has demonstrated various degrees of dissimilarity in upstream regions of genes on the Y providing a potential mechanism for the observed patterns of female-biased expression. We hypothesize that degeneration of the Y chromosome results in regulatory mutations that create a sex-specific expression pattern and that this physical concentration of sex-biased expression on the nascent sex chromosome may be a key feature characterizing intermediate phases of sex chromosome evolution.