Triploidy in white sturgeon (Acipenser transmontanus): Effects of acute stress and warm acclimation on physiological performance

Hot and toxic: Accumulation dynamics and ecotoxicological responses of mussel Mytilus galloprovincialis exposed to marine biotoxins during a marine heatwave

Climate change is increasing marine heatwaves (MHWs) frequency and severity worldwide. These extreme events often cause bivalves' mass mortality and facilitate the growth, proliferation and dispersion of toxin-producing microalgae blooms associated with threats to seafood safety. Yet, the interactive effects between MHW and uptake of marine biotoxins by biota are a novel topic still lacking thorough research, from both the ecotoxicological and seafood safety standpoints. This study assessed the effects of a MHW event on the accumulation/elimination dynamics of diarrhetic shellfish toxins in Mytilus galloprovincialis exposed to Prorocentrum lima and the ecotoxicological responses of mussels co-exposed to these two stressors. Results showed that acute exposure to +4 °C reduced toxins accumulation (-49 %) and elimination (-77 %) compared to control temperature. Moreover, exposure to MHW and toxins affected mussels' antioxidant activity, lipid and protein damage, and metabolism in a tissue-specific manner. These findings highlight that M. galloprovincialis can face higher vulnerability to toxins when MHW events strike.

Understanding the spleen response of Russian sturgeon (Acipenser gueldenstaedtii) dealing with chronic heat stress and Aeromonas hydrophila challenge

Sturgeon aquaculture has grown in recent years, driven by increasing global demand for its highly valued products. Russian sturgeon (Acipenser gueldenstaedtii), recognised as one of the most valuable species for caviar production, is farmed in several warm-temperate regions. However, the substantial temperature increase due to global warming represents a challenge for developing sturgeon aquaculture. Previously we demonstrated that Russian sturgeon under chronic heat stress (CHS) exhibited a liver metabolic reprogramming to meet energy demands, weakening their innate defences and leading to increased mortality and economic losses. Here, we used RNA-seq technology to analyse regulated genes in the spleen of Russian sturgeons exposed to CHS and challenged with Aeromonas hydrophila. The assembly gave 253,415 unigenes, with 13.7 % having at least one reliable functional annotation. We found that CHS caused mild splenitis and upregulated genes related to protein folding, heat shock response, apoptosis and autophagy while downregulated genes associated with the cell cycle. The cell cycle arrest was maintained upon A. hydrophila challenge in heat-stressed fish, potentially inducing cell senescence. Surprisingly, immunoglobulin heavy and light chains were upregulated in the spleen of stressed sturgeons but not in those maintained at tolerable temperatures; however, no changes in IgM serum levels were observed in any condition. Our findings indicate that long-term exposure to non-tolerable temperatures induced a heat shock response and activated apoptosis and autophagy processes in the spleen. These mechanisms may enable the control of tissue damage and facilitate the recycling of cell components in a condition where the nutrient supply by the liver might be insufficient. Stressed sturgeons challenged with A. hydrophila maintain these mechanisms, which could culminate in cellular senescence.

Diploid and triploid Chinook salmon (Oncorhynchus tshawytscha) exhibit differential immunological responses to acute thermal stress

Exposure to temperatures outside of a fish's optimal range results in suppression of the immune system, ultimately leaving aquaculture stocks susceptible to disease outbreaks. This effect is exacerbated in triploid fishes, which demonstrate greater susceptibility to stress than their diploid counterparts. This study investigates the impacts of acute heat stress on the abundance of immune transcripts and proteins in diploid and triploid Chinook salmon (Oncorhynchus tshawytscha), an important finfish crop. This study also demonstrates that acute heat stress induces significant increases in the abundance hsp70, hsp90 and il1b transcripts in the head kidneys, gills and heart ventricles of both diploid and triploid Chinook salmon. Widespread dysregulation of antigen-presentation transcripts was also observed in fish of both ploidies. These results suggest that acute heat stress activates acute-phase responses in Chinook salmon and dysregulates antigen presentation, potentially leaving fish more susceptible to infection. At the protein level, IL-1β was differentially expressed in the head kidney and ventricles of diploid and triploid salmon following heat shock. Differential expression of two tapasin-like proteins in diploid and triploid salmon subjected to heat shock was also observed. Altogether, these data indicate that diploid and triploid Chinook salmon respond differently to acute thermal stressors.

Triploid Pacific oysters exhibit stress response dysregulation and elevated mortality following heatwaves

Polyploidy has been suggested to negatively impact environmental stress tolerance, resulting in increased susceptibility to extreme climate events. In this study, we compared the genomic and physiological response of diploid (2n) and triploid (3n) Pacific oysters (Crassostrea gigas) to conditions present during an atmospheric heatwave that impacted the Pacific Northwestern region of the United States in the summer of 2021. Climate stressors were applied either singly (single stressor; elevated seawater temperature, 30°C) or in succession (multiple stressor; elevated seawater temperature followed by aerial emersion at 44°C), replicating conditions present within the intertidal over a tidal cycle during the event. Oyster mortality rate was elevated within stress treatments with respect to the control and was significantly higher in triploids than diploids following multiple stress exposure (36.4% vs. 14.8%). Triploids within the multiple stressor treatment exhibited signs of energetic limitation, including metabolic depression, a significant reduction in ctenidium Na+ /K+ ATPase activity, and the dysregulated expression of genes associated with stress response, innate immunity, glucose metabolism, and mitochondrial function. Functional enrichment analysis of ploidy-specific gene sets identified that biological processes associated with metabolism, stress tolerance, and immune function were overrepresented within triploids across stress treatments. Our results suggest that triploidy impacts the transcriptional regulation of key processes that underly the stress response of Pacific oysters, resulting in downstream shifts in physiological tolerance limits that may increase susceptibility to extreme climate events that present multiple environmental stressors. The impact of chromosome set manipulation on the climate resilience of marine organisms has important implications for domestic food security within future climate scenarios, especially as triploidy induction becomes an increasingly popular tool to elicit reproductive control across a wide range of species used within marine aquaculture.

How do suboptimal temperatures affect polyploid sterlet Acipenser ruthenus during early development?

Sturgeons are ancient fish exhibiting unique genome plasticity and a high tendency to produce spontaneously autopolyploid genome states. The temperature profiles of the rivers in which sturgeon live and reproduce have been severely altered by human intervention, and the effect of global warming is expected to cause further temperature shifts, which may be detrimental for early developmental stages with narrow windows of thermal tolerance. The comparison of the performance of diploid and autopolyploid sturgeon kept at unfavourable temperatures contributes to scientific knowledge of the effects of polyploid genome states on organisms and can shed light on the ability of polyploids to cope with human-induced alterations to natural conditions. Using the sterlet Acipenser ruthenus as a model species, we carried out conventional artificial fertilization, as well as the induction of the second polar body retention (SPBR), of the first mitotic division suppression (FMDS) and of the second polar body retention followed by the first mitotic division suppression (SPBR+FMDS). Two experiments were conducted to evaluate the effect of polyploidy on two basic performance parameters, survival and growth. In Experiment 1, fish belonging to untreated, SPBR-, FMDS- and SPBR+FMDS-induced groups were kept at 10, 16 and 20°C from the neurula stage until the end of endogenous feeding. In Experiment 2, larvae from the untreated and SPBR-induced groups were reared at 10, 16 and 20°C after their endogenous feeding transition for 3 weeks. Based on our findings, we report that the embryos, prelarvae and larvae of triploid A. ruthenus do not differ from diploids in their ability to survive, grow and develop under suboptimal temperature conditions, while the survival of tetraploids was significantly reduced even at the optimal temperature and even more so at temperatures far from the optimum. This was also the case in the 2n/4n mosaics observed in FMDS-induced group. Thus, we assume that in tetraploid and 2n/4n individuals, the limits of thermal tolerance are closer to the optimum than in diploids. We also conclude that the hexaploid genome state is probably lethal in A. ruthenus since none of the hexaploids or 3n/6n mosaics arising from the SPBR+FMDS induction survived the prelarval period.

Seasonal Temperature Fluctuations Differently Affect the Immune and Biochemical Parameters of Diploid and Triploid Oncorhynchus mykiss Cage-Cultured in Temperate Latitudes

In the coming decades, and despite advances in the selection of resistant strains and the production of triploid organisms, the temperature could seriously affect salmonid aquaculture. Lower environmental tolerance has been hinted at for triploids, but the physiological mechanisms leading to such differences, and whether they are translated to the individual level, are poorly understood. This study aimed to evaluate the effects of seasonal variations on the humoral and immune status in the blood (peripheral blood leukocytes) and plasma (antiprotease, lysozyme and peroxidase activities), the oxidative stress (catalase, glutathione-S-transferase, total glutathione and lipid peroxidation) balance in the liver, and the energy budget (sugars, lipids, proteins and energy production) in the liver and muscle of diploid and triploid Oncorhynchus mykiss. Leukocytes’ numbers changed with the water temperature and differed between fish ploidies. Peroxidase activity was increased in the summer, but lysozyme and antiprotease activities were increased in the winter. Concomitantly, antioxidant defenses were significantly altered seasonally, increasing oxidative damage at higher temperatures. Moreover, warmer waters induced a reduction in the energy production measured in the liver. Differences in feed efficiency, which have been previously reported, were confirmed by the low lipid and protein contents of the muscle of the triploids. In sum, the inherent trade-offs to deal with the seasonal changes culminated in the higher growth observed for diploid fish.

Hybridization of Russian Sturgeon (Acipenser gueldenstaedtii, Brandt and Ratzeberg, 1833) and American Paddlefish (Polyodon spathula, Walbaum 1792) and Evaluation of Their Progeny

Two species from the families Acipenseridae and Polyodontidae, Russian sturgeon (Acipenser gueldenstaedtii, Brandt and Ratzeberg, 1833; functional tetraploid) and American paddlefish (Polyodon spathula, Walbaum 1792, functional diploid) were hybridized. The hybridization was repeated using eggs from three sturgeon and sperm from four paddlefish individuals. Survival in all hybrid family groups ranged from 62% to 74% 30 days after hatching. This was the first successful hybridization between these two species and between members of the family Acipenseridae and Polyodontidae. Flow cytometry and chromosome analysis revealed two ploidy levels in hybrids. The chromosome numbers of the hybrids ranged between 156-184 and 300-310, in "functional" triploids and "functional" pentaploids, respectively. The hybrid origin and the ploidy levels were also confirmed by microsatellite analyses. In hybrids, the size and the number of dorsal and ventral scutes correlated with the ploidy levels as well as with the calculated ratio of the maternal and paternal chromosome sets. An extra haploid cell lineage was found in three hybrid individuals irrespective of the ploidy level, suggesting polyspermy. Although the growth performance showed high variance in hybrids (mean: 1.2 kg, SD: 0.55), many individuals reached a size of approximately 1 kg by the age of one year under intensive rearing conditions.

The Innate Immune Response of Atlantic Salmon (Salmo salar) Is Not Negatively Affected by High Temperature and Moderate Hypoxia

Climate change is predicted to increase water temperatures and decrease oxygen levels in freshwater and marine environments, however, there is conflicting information regarding the extent to which these conditions may impact the immune defenses of fish. In this study, Atlantic salmon were exposed to: (1) normoxia (100–110% air saturation) at 12°C; (2) an incremental temperature increase (1°C per week from 12 to 20°C), and then held at 20°C for an additional 4 weeks; and (3) “2” with the addition of moderate hypoxia (~65–75% air saturation). These conditions realistically reflect what farmed salmon in some locations are currently facing, and future conditions in Atlantic Canada and Europe, during the summer months. The salmon were sampled for the measurement of head kidney constitutive anti-bacterial and anti-viral transcript expression levels, and blood parameters of humoral immune function. Thereafter, they were injected with either the multi-valent vaccine Forte V II (contains both bacterial and viral antigens) or PBS (phosphate-buffer-saline), and the head kidney and blood of these fish were sampled at 6, 12, 24, and 48 h post-injection (HPI). Our results showed that: (1) neither high temperature, nor high temperature + moderate hypoxia, adversely affected respiratory burst, complement activity or lysozyme concentration; (2) the constitutive transcript expression levels of the anti-bacterial genes il1β, il8-a, cox2, hamp-a, stlr5-a, and irf7-b were up-regulated by high temperature; (3) while high temperature hastened the peak in transcript expression levels of most anti-bacterial genes by 6–12 h following V II injection, it did not affect the magnitude of changes in transcript expression; (4) anti-viral (viperin-b, mx-b, and isg15-a) transcript expression levels were either unaffected, or downregulated, by acclimation temperature or V II injection over the 48 HPI; and (5) hypoxia, in addition to high temperature, did not impact immune transcript expression. In conclusion, temperatures up to 20°C, and moderate hypoxia, do not impair the capacity of the Atlantic salmon's innate immune system to respond to bacterial antigens. These findings are surprising, and highlight the salmon's capacity to mount robust innate immune responses (i.e., similar to control fish under optimal conditions) under conditions approaching their upper thermal limit.