The effects of hypoxia and temperature on metabolic aspects of embryonic development in the annual killifish Austrofundulus limnaeus

Abstract

Embryos of Austrofundulus limnaeus are exceptional in their ability to tolerate prolonged bouts of complete anoxia. Hypoxia and anoxia are a normal part of their developmental environment. Here, we exposed embryos to a range of PO2 levels at two different temperatures (25 and 30 °C) to study the combined effects of reduced oxygen and increased temperature on developmental rate, heart rate, and metabolic enzyme capacity. Hypoxia decreased overall developmental rate and caused a stage-specific decline in heart rate. However, the rate of early development prior to the onset of organogenesis is insensitive to PO2. Increased incubation temperature caused an increase in the developmental rate at high PO2s, but hindered developmental progression under severe hypoxia. Embryonic DNA content in pre-hatching embryos was positively correlated with PO2. Citrate synthase, lactate dehydrogenase, and phosphoenolpyruvate carboxykinase capacity were all reduced in embryos developing under hypoxic conditions. Embryos of A. limnaeus are able to develop normally across a wide range of PO2s and contrary to most other vertebrates severe hypoxia is not a teratogen. Embryos of A. limnaeus do not respond to hypoxia through an increase in the capacity for enzymatic activity of the metabolic enzymes lactate dehydrogenase, citrate synthase, or phosphoenolpyruvate carboxykinase. Instead they appear to adjust whole-embryo metabolic capacity to match oxygen availability. However, decreased DNA content in hypoxia-reared embryos suggests that cellular enzymatic capacity may remain unchanged in response to hypoxia, and the reduced capacity may rather indicate reduced cell number in hypoxic embryos.

Transcriptomic analysis of maternally provisioned cues for phenotypic plasticity in the annual killifish, Austrofundulus limnaeus

“…Embryos of A. limnaeus are the most anoxia-tolerant vertebrates [6, 36, 51] and can develop normally even under extreme hypoxia [68]. Thus, it may not be surprising that maternal provisioning of antioxidant systems would be elevated in this species.…”

Hit pause: Developmental arrest in annual killifishes and their close relatives

“…Diapause can be experimentally “broken” by increasing incubation temperature and exposing the embryos to continuous light or a long‐day photoperiod (Podrabsky and Hand, ). Diapause II is likely to be the most resistant stage to environmental stresses such as temperature and dehydration (Podrabsky et al, ), but it is worth noting that extreme hypoxia and dehydration do not seem to induce diapause or affect the length of diapause II under laboratory conditions in A. limnaeus (Podrabsky et al, ; Anderson and Podrabsky, ; Podrabsky and Wilson, ).…”

“…When incubated under aquatic conditions, embryos slowly reduce their metabolic rate over several weeks to rates that are depressed to less that 15% of peak pre‐diapause rates (Levels et al, ; Podrabsky and Hand, ). Heart rate also declines over the course of several weeks (Anderson and Podrabsky, ). When incubation occurs on moist peat moss, heart rate has been reported to be reduced in most embryos (Thompson and Ortí, ).…”

Annual killifish adaptations to ephemeral environments: Diapause i in two austrolebias species

“…Therefore, species survival in a given location is entirely embryo dependent (Wourms, 1972a–c; Errea and Danulat, ; Arezo et al, ; Berois et al, ; Blažek et al, ). Embryo survival during the rainy season is likely reliant on tolerance of long periods of hypoxia or anoxia (Podrabsky et al, ; Anderson and Podrabsky, ), whereas survival through the dry season is possibly supported by both tolerance of hypoxia/anoxia and the ability to resist environmental water loss (Podrabsky et al, ). Noteworthy, embryos are able to tolerate rather harsh environments while retaining the ability to sense and respond to critical environmental cues such as temperature and hypoxia (Podrabsky et al, )…”