Hypoxia inducible factor-1α knockout does not impair acute thermal tolerance or heat hardening in zebrafish

The rapid increase in critical thermal maximum (CT_max) in fish (or other animals) previously exposed to critically high temperature is termed 'heat hardening', which likely represents a key strategy to cope with increasingly extreme environments. The physiological mechanisms that determine acute thermal tolerance, and the underlying pathways facilitating heat hardening, remain debated. It has been posited, however, that exposure to high temperature is associated with tissue hypoxia and may be associated with the increased expression of hypoxia-inducible factor-1 (Hif-1). We studied acute thermal tolerance in zebrafish (Danio rerio) lacking functional Hif-1α paralogs (Hif-1aa and Hif-1ab double knockout; Hif-1α^-/-), which are known to exhibit markedly reduced hypoxia tolerance. We hypothesized that Hif-1α^-/- zebrafish would suffer reduced acute thermal tolerance relative to wild type and that the heat hardening ability would be lost. However, on the contrary, we observed that Hif-1α^-/- and wild-type fish did not differ in CT_max, and both genotypes exhibited heat hardening of a similar degree when CT_max was re-tested 48 h later. Despite exhibiting impaired hypoxia tolerance, Hif-1α^-/- zebrafish display unaltered thermal tolerance, suggesting that these traits are not necessarily functionally associated. Hif-1α is accordingly not required for short-term acclimation in the form of heat hardening.