Remarkable anoxia tolerance by stoneflies from a floodplain aquifer

Hemocyanin contributes to embryonic adaptation to hypoxia in the migratory locust

Ambient hypoxia can pose a major threat to the survival of metazoan organisms, especially insect embryos. Hemocyanin exhibits dominant expression in insect embryos, but its specific roles in hypoxia adaptation remain unexplored. Soil-dwelling locust eggs may frequently experience hypoxia during development. A comprehensive analysis of physiological and biochemical characters of hemocyanin was conducted in the embryos of migratory locust Locusta migratoria. Our results demonstrated that the revolution process was the critical hypoxia-sensitive event during locust embryogenesis. Hemocyanin presented a prominent expression in the revolution stage and exhibited strong responses to hypoxia. The relative duration of revolution was correlated negatively with the expression of hemocyanin subunit 2 (HC2), suggesting that HC2 might be closely associated with hypoxia adaptation of locust embryos. Furthermore, a HC2 mutant locust strain was established using the CRISPR/Cas9 technology, and higher hypoxia sensitivity was found for HC2-deficient locust embryos. Knockdown of HC2 increased anaerobic metabolism and oxidative stress while reducing oxidative metabolism. Overall, these findings clearly demonstrated the pivotal roles of hemocyanin in hypoxia adaptation of insect embryos.

Response of wild aquatic insect communities to thermal variation through comparative landscape transcriptomics

Fluctuations in temperature are recognized as a potent driver of selection pressure, fostering genomic variations that are crucial for the adaptation and survival of organisms under selection. Notably, water temperature is a pivotal factor influencing aquatic organism persistence. By comprehending how aquatic organisms respond to shifts in water temperature, we can understand their potential physiological adaptations to environmental change in one or multiple species. This, in turn, contributes to the formulation of biologically relevant guidelines for the landscape scale transcriptome profile of organisms in lotic systems. Here, we investigated the distinct responses of seven stream stonefly species, collected from four geographical regions across Japan, to variations in temperature, including atmospheric and water temperatures. We achieved this by assessing the differences in gene expression through RNA-sequencing within individual species and exploring the patterns of community-genes among different species. We identified 735 genes that exhibited differential expressions across the temperature gradient. Remarkably, the community displayed expression levels differences of respiration and metabolic genes. Additionally, the diversity in molecular functions appeared to be linked to spatial variation, with water temperature differences potentially contributing to the overall functional diversity of genes. We found 22 community-genes with consistent expression patterns among species in response to water temperature variations. These genes related to respiration, metabolism and development exhibited a clear gradient providing robust evidence of divergent adaptive responses to water temperature. Our findings underscore the differential adaptation of stonefly species to local environmental conditions, suggesting that shared responses in gene expression may occur across multiple species under similar environmental conditions. This study emphasizes the significance of considering various species when assessing the impacts of environmental changes on aquatic insect communities and understanding potential mechanisms to cope with such changes.

Exposure to copper increases hypoxia sensitivity and decreases upper thermal tolerance of giant salmonfly nymphs (Pteronarcys californica)

Many aquatic insects are exposed to the dual stressors of heavy metal pollution and rising water temperatures from global warming. These stresses may interact and have stronger impacts on aquatic organisms if heavy metals interfere with the ability of these organisms to handle high temperatures. Here we focus on the effect of copper on upper thermal limits of giant salmonfly nymphs (Order: Plecoptera, Pteronarcys californica), a stonefly species which is common in parts of western North America. Experimental exposure to copper reduced upper thermal limits by ∼ 10 °C in some cases and depressed the hypoxia tolerance (Pcrit) of nymphs by ∼ 0.5 mg L-1 DO. These results suggest that copper inhibits the delivery of oxygen, which may explain, in part, the strong reductions in CTMAX that we report. Fluorescence microscopy of Cu-exposed individuals indicated high levels of copper in chloride cells but no clear evidence of damage to or high levels of copper on the gills themselves. Our study indicates that populations of aquatic insects from copper-polluted environments may be further at risk to future warming than those from uncontaminated environments.

Plasticity of salmonfly (Pteronarcys californica) respiratory phenotypes in response to changes in temperature and oxygen

Like all taxa, populations of aquatic insects may respond to climate change by evolving new physiologies or behaviors, shifting their ranges, exhibiting physiological and behavioral plasticity, or by going extinct. We evaluated the importance of plasticity by measuring changes in growth, survival, and respiratory phenotypes of salmonfly nymphs (the stonefly Pteronarcys californica) in response to experimental combinations of dissolved oxygen and temperature. Overall, smaller individuals grew more rapidly during the six-week experimental period, and oxygen and temperature interacted to affect growth in complex ways. Survival was lower for the warm treatment, though only four mortalities occurred (91.6 vs 100%). Nymphs acclimated to warmer temperatures did not have higher critical thermal maxima (CTMAX), but those acclimated to hypoxia had CTMAX values (in normoxia) higher by approximately 1 °C. These results suggest possible adaptive plasticity of systems for taking up or delivering oxygen. We examined these possibilities by measuring the oxygen-sensitivity of metabolic rates and the morphologies of tracheal gill tufts located ventrally on thoracic and abdominal segments. Mass-specific metabolic rates of individuals acclimated to warmer temperatures were higher in acute hypoxia but lower in normoxia, regardless of their recent history of oxygen exposure during acclimation. The morphology of gill filaments, however, changed in ways that appeared to depress rates of oxygen delivery in functional hypoxia. Our combined results from multiple performance metrics indicate that rising temperatures and hypoxia may interact to magnify the risks to aquatic insects, but that physiological plasticity in respiratory phenotypes may offset some of these risks.

Tolerance of aquifer stoneflies to repeated hypoxia exposure and oxygen dynamics in an alluvial aquifer

Aquatic insects cope with hypoxia and anoxia using a variety of behavioral and physiological responses. Most stoneflies (Plecoptera) occur in highly oxygenated surface waters, but some species live underground in alluvial aquifers containing heterogeneous oxygen concentrations. Aquifer stoneflies appear to be supported by methane-derived food resources, which they may exploit using anoxia-resistant behaviors. We documented dissolved oxygen dynamics and collected stoneflies over 5 years in floodplain wells of the Flathead River, Montana. Hypoxia regularly occurred in two wells, and nymphs of Paraperla frontalis were collected during hypoxic periods. We measured mass-specific metabolic rates (MSMRs) at different oxygen concentrations (12, 8, 6, 4, 2, 0.5 mg l-1, and during recovery) for 111 stonefly nymphs to determine whether aquifer and benthic taxa differed in hypoxia tolerance. Metabolic rates of aquifer taxa were similar across oxygen concentrations spanning 2 to 12 mg l-1 (P>0.437), but the MSMRs of benthic taxa dropped significantly with declining oxygen (P<0.0001; 2.9-times lower at 2 vs. 12 mg l-1). Aquifer taxa tolerated short-term repeated exposure to extreme hypoxia surprisingly well (100% survival), but repeated longer-term (>12 h) exposures resulted in lower survival (38-91%) and lower MSMRs during recovery. Our work suggests that aquifer stoneflies have evolved a remarkable set of behavioral and physiological adaptations that allow them to exploit the unique food resources available in hypoxic zones. These adaptations help to explain how large-bodied consumers might thrive in the underground aquifers of diverse and productive river floodplains.