Ecological adaptations of Amazonian fishes acquired during evolution under environmental variations in dissolved oxygen: A review of responses to hypoxia in fishes, featuring the hypoxia-tolerant Astronotus spp

Effects of fasting and environmental factors on appetite regulators in pond loach Misgurnus anguillicaudatus

The pond loach (Misgurnus anguillicaudatus) is an important aquaculture freshwater species, used as an ornamental fish, food source for humans and angling bait. Pond loaches are resistant to fasting and extreme environmental conditions, including temperature and low oxygen levels. Little is known about how these factors affect the feeding physiology and the endocrine regulation of feeding of loaches. In this study, we examined the effects of fasting, as well as increased temperature and decreased oxygen levels on food intake and transcript levels of appetite regulators. Fasted fish had lower blood glucose levels, and lower expression levels of intestine CCK and PYY, and brain CART1, but had higher levels of brain orexin and ghrelin than fed fish. Fish held at 30 °C had higher food intake, glucose levels, and mRNA levels of intestine CCK and PYY, and brain CART2, but lower brain orexin levels than fish at 20 °C. Fish held at low oxygen levels had a lower food intake, higher intestine CCKa and ghrelin, and brain orexin, CART2 and ghrelin mRNA expression levels than fish held at high O2 levels. Our results suggest that fasting and high temperatures increase the expression of orexigenic and anorexigenic factors respectively, whereas the increase in expression of both orexigenic and anorexigenic factors in low O2 environments might not be related to their role in feeding, but possibly to protection from tissue damage. The results of our study might shed new light on how pond loaches are able to cope with extreme environmental conditions such as low food availability, extreme temperatures and hypoxia.

A novel hypoxic lncRNA, LOC110520012 sponges miR-206-y to regulate angiogenesis and liver cell proliferation in rainbow trout (Oncorhynchus mykiss) by targeting vegfaa

Long non-coding RNA (lncRNA) is a novel emerging type of competitive endogenous RNA (ceRNA) that performs key functions in multiple biological processes. However, little is known about the roles of lncRNA under hypoxia stress in fish. Here, vascular endothelial growth factor-Aa (vegfaa) was cloned in rainbow trout (Oncorhynchus mykiss), with the complete cDNA sequence of 2914 bp, encoding 218 amino acids. The molecular weight of the protein was approximately 25.33 kDa, and contained PDGF and VEGF_C domains. Time-course and spatial expression patterns revealed that LOC110520012 was a key regulator of rainbow trout in response to hypoxia stress, and LOC110520012, miR-206-y and vegfaa exhibited a ceRNA regulatory relationship in liver, gill, muscle and rainbow trout liver cells treated with acute hypoxia. Subsequently, the targeting relationship of LOC110520012 and vegfaa with miR-206-y was confirmed by dual-luciferase reporter analysis, and overexpression of LOC110520012 mediated the inhibition of miR-206-y expression in rainbow trout liver cells, while the opposite results were obtained after LOC110520012 silencing with siRNA. We also proved that vegfaa was a target of miR-206-y in vitro and in vivo, and the vegfaa expression and anti-proliferative effect on rainbow trout liver cells regulated by miR-206-y mimics could be reversed by LOC110520012. These results suggested that LOC110520012 can positively regulate vegfaa expression by sponging miR-206-y under hypoxia stress in rainbow trout, which facilitate in-depth understanding of the molecular mechanisms of fish adaptation and tolerance to hypoxia.

Evolution of a novel regulatory mechanism of hypoxia inducible factor in hypoxia-tolerant electric fishes

Hypoxia is a significant source of metabolic stress that activates many cellular pathways involved in cellular differentiation, proliferation, and cell death. Hypoxia is also a major component in many human diseases and a known driver of many cancers. Despite the challenges posed by hypoxia, there are animals that display impressive capacity to withstand lethal levels of hypoxia for prolonged periods of time and thus offer a gateway to a more comprehensive understanding of the hypoxic response in vertebrates. The weakly electric fish genus Brachyhypopomus inhabits some of the most challenging aquatic ecosystems in the world, with some species experiencing seasonal anoxia, thus providing a unique system to study the cellular and molecular mechanisms of hypoxia tolerance. In this study, we use closely related species of Brachyhypopomus that display a range of hypoxia tolerances to probe for the underlying molecular mechanisms via hypoxia inducible factors (HIFs)-transcription factors known to coordinate the cellular response to hypoxia in vertebrates. We find that HIF1⍺ from hypoxia tolerant Brachyhypopomus species displays higher transactivation in response to hypoxia than that of intolerant species, when overexpressed in live cells. Moreover, we identified two SUMO-interacting motifs near the oxygen-dependent degradation and transactivation domains of the HIF1⍺ protein that appear to boost transactivation of HIF1, regardless of the genetic background. Together with computational analyses of selection, this shows that evolution of HIF1⍺ are likely to underlie adaptations to hypoxia tolerance in Brachyhypopomus electric fishes, with changes in two SUMO-interacting motifs facilitating the mechanism of this tolerance.

Gut flora alterations among aquatic firefly Aquatica leii inhabiting various dissolved oxygen in fresh water

Knowledge about the impact of different dissolved oxygen (DO) on the composition and function of gut bacteria of aquatic insects is largely unknown. Herein, we constructed freshwater environments with different DOs (hypoxia: 2.50 ± 0.50, normoxia: 7.00 ± 0.50, and hyperoxia: 13.00 ± 0.50 mg/L) where aquatic firefly Aquatica leii larvae lived for three months. Their gut flora was analyzed using the combination of 16S rRNA amplicon sequencing and metagenomics. The results showed no difference in alpha diversity of the gut flora between A. leii inhabiting various DOs. However, the relative abundance of several bacterial lineages presented significant changes, such as Pseudomonas. In addition, bacterial genes with an altered relative abundance in response to various DOs were primarily related to metabolism. The alteration of these functions correlated with the DO change. This is the first to uncover structure of gut flora under various DOs in aquatic insect larvae.

"Bet hedging" against climate change in developing and adult animals: roles for stochastic gene expression, phenotypic plasticity, epigenetic inheritance and adaptation

Animals from embryos to adults experiencing stress from climate change have numerous mechanisms available for enhancing their long-term survival. In this review we consider these options, and how viable they are in a world increasingly experiencing extreme weather associated with climate change. A deeply understood mechanism involves natural selection, leading to evolution of new adaptations that help cope with extreme and stochastic weather events associated with climate change. While potentially effective at staving off environmental challenges, such adaptations typically occur very slowly and incrementally over evolutionary time. Consequently, adaptation through natural selection is in most instances regarded as too slow to aid survival in rapidly changing environments, especially when considering the stochastic nature of extreme weather events associated with climate change. Alternative mechanisms operating in a much shorter time frame than adaptation involve the rapid creation of alternate phenotypes within a life cycle or a few generations. Stochastic gene expression creates multiple phenotypes from the same genotype even in the absence of environmental cues. In contrast, other mechanisms for phenotype change that are externally driven by environmental clues include well-understood developmental phenotypic plasticity (variation, flexibility), which can enable rapid, within-generation changes. Increasingly appreciated are epigenetic influences during development leading to rapid phenotypic changes that can also immediately be very widespread throughout a population, rather than confined to a few individuals as in the case of favorable gene mutations. Such epigenetically-induced phenotypic plasticity can arise rapidly in response to stressors within a generation or across a few generations and just as rapidly be "sunsetted" when the stressor dissipates, providing some capability to withstand environmental stressors emerging from climate change. Importantly, survival mechanisms resulting from adaptations and developmental phenotypic plasticity are not necessarily mutually exclusive, allowing for classic "bet hedging". Thus, the appearance of multiple phenotypes within a single population provides for a phenotype potentially optimal for some future environment. This enhances survival during stochastic extreme weather events associated with climate change. Finally, we end with recommendations for future physiological experiments, recommending in particular that experiments investigating phenotypic flexibility adopt more realistic protocols that reflect the stochastic nature of weather.

Experiment for Effect of Attack Angle and Environmental Condition on Hydrodynamics of Near-Surface Swimming Fish-Like Robot

Fish-like robot is a special autonomous underwater vehicle with broad application prospects. Some previous studies concentrated on the hydrodynamics of free-swimming fish-like robots. But the hydrodynamic performance of fish-like robot swimming with a tilt angle in constrained space has not been well studied, and the influence of environmental wave and current on its is also still unclear. In this paper, the experiment devices, including a physical fish-like robot, a hydrodynamics measurement platform, and a six-axis force sensor, are used to study the effect of attack angle and environmental condition on the hydrodynamics of near-surface swimming fish-like robot. Nine attack angles, five oscillating amplitudes, and three environmental conditions are analyzed in the experiments. It shows that thrust force decreases when caudal fin passes above water surface, but the increased difference between gravity force and buoyancy force will compensate the decreased force generated by caudal fin when fish-like robot swims with certain dive angle. The extra reaction force generated by solid bottom boundary will promote the thrust force and vertical force. The surface water wave condition or surface water current condition also has obvious effects on hydrodynamic performance. This paper provides a new perspective to the research on the hydrodynamic performance of fish-like robot and will do favor in the development of fish-like robot.

The interplay between prior selection, mild intermittent exposure, and acute severe exposure in phenotypic and transcriptional response to hypoxia

Hypoxia has profound and diverse effects on aerobic organisms, disrupting oxidative phosphorylation and activating several protective pathways. Predictions have been made that exposure to mild intermittent hypoxia may be protective against more severe exposure and may extend lifespan. Here we report the lifespan effects of chronic, mild, intermittent hypoxia, and short‐term survival in acute severe hypoxia in four clones of Daphnia magna originating from either permanent or intermittent habitats. We test the hypothesis that acclimation to chronic mild intermittent hypoxia can extend lifespan through activation of antioxidant and stress‐tolerance pathways and increase survival in acute severe hypoxia through activation of oxygen transport and storage proteins and adjustment to carbohydrate metabolism. Unexpectedly, we show that chronic hypoxia extended the lifespan in the two clones originating from intermittent habitats but had the opposite effect in the two clones from permanent habitats, which also showed lower tolerance to acute hypoxia. Exposure to chronic hypoxia did not protect against acute hypoxia; to the contrary, Daphnia from the chronic hypoxia treatment had lower acute hypoxia tolerance than normoxic controls. Few transcripts changed their abundance in response to the chronic hypoxia treatment in any of the clones. After 12 h of acute hypoxia treatment, the transcriptional response was more pronounced, with numerous protein‐coding genes with functionality in oxygen transport, mitochondrial and respiratory metabolism, and gluconeogenesis, showing upregulation. While clones from intermittent habitats showed somewhat stronger differential expression in response to acute hypoxia than those from permanent habitats, contrary to predictions, there were no significant hypoxia‐by‐habitat of origin or chronic‐by‐acute treatment interactions. GO enrichment analysis revealed a possible hypoxia tolerance role by accelerating the molting cycle and regulating neuron survival through upregulation of cuticular proteins and neurotrophins, respectively.