Intergenerational plasticity to cycling high temperature and hypoxia affects offspring stress responsiveness and tolerance in zebrafish.

Predicted climate change-induced increases in heat waves and hypoxic events will have profound effects on fishes, yet the capacity of parents to alter offspring phenotype via non-genetic inheritance and buffer against these combined stressors is not clear. This study tested how prolonged adult zebrafish exposure to combined diel cycles of thermal stress and hypoxia affect offspring early survival and development, parental investment of cortisol and heat shock proteins (HSPs), larval offspring stress responses, and both parental and offspring heat and hypoxia tolerance. Parental exposure to the combined stressor did not affect fecundity, but increased mortality, produced smaller embryos and delayed hatching. The combined treatment also reduced maternal deposition of cortisol and increased embryo hsf1, hsp70a, HSP70, hsp90aa and HSP90 levels. In larvae, basal cortisol levels did not differ between treatments, but acute exposure to combined heat stress and hypoxia increased cortisol levels in control larvae with no effect on larvae from exposed parents. In contrast, whereas larval basal hsf1, hsp70a and hsp90aa levels differed between parental treatments, the combined acute stressor elicited similar transcriptional responses across treatments. Moreover, the combined acute stressor only induced a marked increase in HSP47 levels in the larvae derived from exposed parents. Finally, combined hypoxia and elevated temperatures increased both thermal and hypoxia tolerance in adults and conferred an increase in offspring thermal but not hypoxia tolerance. These results demonstrate that intergenerational acclimation to combined thermal stress and hypoxia elicit complex carryover effects on stress responsiveness and offspring tolerance with potential consequences for resilience.

Zebrafish parental progeny investment in response to cycling thermal stress and hypoxia: deposition of heat shock proteins but not cortisol.

The maternal match hypothesis predicts that maternal exposure to a stressor may help prepare offspring to cope with the same disturbance in later life. Although there is support for this hypothesis, the signals involved in non-genetic inheritance are unclear. In this study, we tested how adult zebrafish exposure to diel cycles of thermal stress (27-36°C), hypoxia (20-85% dissolved oxygen) or the combined treatment affects maternal and embryonic levels of cortisol and heat shock proteins (HSPs). While parental exposure to the thermal, hypoxic or combined treatment for 2 weeks did not affect whole-body cortisol levels, the combined exposure increased ovarian cortisol levels by 4-fold and reduced embryonic cortisol content by 60%. The combined treatment also elicited 3- and 19-fold increases in embryo transcripts involved in cortisol breakdown (11bhsd2) and export (abcb4), respectively. The thermal stress and combined exposure also elicited marked increases in ovary and embryo hsp70a (20- to 45-fold) and HSP70 (3- to 7-fold), and smaller increases in ovary and embryo hsp90aa and hsp47 (2- to 4-fold) and in embryo HSP90 and HSP47 (2- to 6-fold). In contrast, except for increases in ovary hsp90aa (2-fold) and embryo HSP90 (3-fold), the hypoxia treatment had little effect on HSP expression and transfer. Overall, while the embryonic deposition of HSPs largely paralleled the ovarian cellular stress response, the inverse relationship between ovary and embryo cortisol levels suggests the existence of barriers against cortisol deposition in response to environmental stressors. We conclude that the endocrine and cellular stress responses make stressor-specific and distinct contributions to non-genetic inheritance.

The effects of fluctuating temperature regimes on the embryonic development of lake whitefish (Coregonus clupeaformis).

Fluctuating incubation temperatures may have significant effects on fish embryogenesis; yet most laboratory-based studies use constant temperatures. For species that experience large, natural seasonal temperature changes during embryogenesis, such as lake whitefish (Coregonus clupeaformis), seasonal temperature regimes are likely optimal for development. Anthropogenic activities can increase average and/or variability of natural incubation temperatures over large (e.g. through climate change) or smaller (e.g. thermal effluent discharge) geographic scales. To investigate this, we incubated lake whitefish embryos under constant (2, 5, or 8°C) and fluctuating temperature regimes. Fluctuating temperature regimes had a base temperature of 2°C with: 1) seasonal temperature changes that modeled natural declines/inclines; 2) tri-weekly +3°C, 1h temperature spikes; or 3) both seasonal temperature changes and temperature spikes. We compared mortality to hatch, morphometrics, and heart rate at three developmental stages. Mortality rate was similar for embryos incubated at constant 2°C, constant 5°C, or with seasonal temperatures, but was significantly greater at constant 8°C. Embryos incubated constantly at >2°C had reduced body growth and yolk consumption compared to embryos incubated with seasonal temperature changes. When measured at the common base temperature of 2°C, embryos incubated at constant 2°C had lower heart rates than embryos incubated with both seasonal temperature changes and temperature spikes. Our study suggests that incubating lake whitefish embryos with constant temperatures may significantly alter development, growth, and heart rate compared to incubating with seasonal temperature changes, emphasizing the need to include seasonal temperature changes in laboratory-based studies.

Impacts of temperature, morpholine, and chronic radiation on the embryonic development of round whitefish (Prosopium cylindraceum).

During incubation, round whitefish embryos may experience fluctuating or elevated temperatures from natural (e.g., seasonal temperature changes) and/or anthropogenic sources. Anthropogenic sources like once-through cooling discharges from nuclear power plants can also expose embryos to chemicals (e.g., morpholine) and/or radiation. To examine the effects of these potential stressors on embryogenesis, round whitefish were incubated under fluctuating or constant temperatures, with morpholine or 137 Cs gamma rays. We report the percentage of prehatch and posthatch mortality, developmental rate, hatch dynamics, and morphometrics at 4 development stages. Embryos reared at constant temperatures had delayed developmental stage onset and median hatch, higher mortality at constant 8 °C, and lower mortality at ≤5 °C, compared with embryos reared under seasonal temperature regimes. Embryos incubated with ≥500 mg L-1 morpholine (>200× regulatory limits) had advanced hatch, reduced body size, and increased prehatch (100% at 1000 mg L-1 ) and posthatch (≈95% at 500 mg L-1 ) mortality compared with controls. Relative to controls, embryos irradiated with ≥0.16 mGy/d had larger body mass early in development, and all irradiated embryos had decreased posthatch mortality; the lowest dose was >300× discharge limits. Our study suggests that fluctuating or elevated temperatures and high-dose morpholine can alter development rate, hatch dynamics, and growth, and/or increase mortality compared with embryos reared at constant temperatures of ≤5 °C; conversely, low-dose irradiation had transient developmental effects but may benefit early posthatch survival. Environ Toxicol Chem 2018;37:2593-2608. © 2018 SETAC.