Mechanisms underlying tolerance of severe hypoxia in sulfide spring fish

Isolating the causative effects of single sources of selection in nature is challenging because environments are often complex with different selective pressures covarying along environmental gradients. For instance, sulfide springs are characterized by high levels of toxic hydrogen sulfide (H₂S) and low levels of oxygen in comparison to nearby non-sulfidic streams, which are normoxic. Previous studies on Poecilia mexicana in these sulfide spring systems have focused on the effects of H₂S on gene expression, but hypoxia could also impact gene expression. We compared hypoxia tolerance, baseline gene expression of hypoxia related genes, and gene expression after exposure to acute hypoxia across populations from non-sulfidic and sulfidic habitats to make inferences about how these fish survive chronic hypoxia. Overall gene expression patterns were strongly separated by geography (Mexico vs. Costa Rica) and habitat type (sulfidic vs. non-sulfidic), while treatment (hypoxic vs. normoxic) explained less variation. While most gene expression responses were population specific, there were also shared gene expression responses between non-sulfidic and sulfidic populations. Additionally, aquatic surface respiration was required for hypoxia tolerance for fish from both habitat types, suggesting ancestral behavioral and gene expression responses to hypoxia. Fish in sulfide springs exhibited constitutive differential expression of many hypoxia related genes under control conditions compared to non-sulfidic fish, and hypoxia tolerance was greater in fish from sulfidic sites. Our study sheds light on variation in constitutive and plastic hypoxia responses and sets the stage for future studies to consider interactions between hypoxia and H₂S.