What could you accomplish in just 4 h? Run a marathon, perhaps, or bake a pie? How about permanently alter the behavioral phenotype of a zebrafish? It turns out that depriving a zebrafish embryo of oxygen for just a few hours has a big impact on that fish's future. For example, a previous study from Nicholas Bernier's lab at the University of Guelph, Canada, showed that 4 h at zero oxygen (anoxia) during a critical developmental window resulted in larval zebrafish that were better able to tolerate low oxygen environments and adult zebrafish that were disproportionately male. In this follow-up study, students Catherine Ivy and Cayleih Robertson from Bernier's lab tested whether this early exposure to anoxia also affected the behavior of these fish when they were reared to adults.

Ivy and Robertson began by cutting the O2 to nil for one group of 36 h old zebrafish embryos, then returning them to fully oxygenated water (normoxia) 4 h later. They also kept a separate group of embryos in normoxic water throughout development. A few months later, when all of the fish were mature adults, the researchers ran a series of experiments to compare the behaviors of the anoxia- and normoxia-exposed fish. First, they isolated pairs of sex-matched fish – one from each of the developmental treatments – to see which treatment produced the feistier fish. Compared with the normoxia-reared fish, fish that were exposed to anoxia during development more frequently chased and bit their tankmate to establish themselves as the dominant fish in the pair. Even when the researchers used a mirror in the tank instead of a second fish, to simulate a social interaction, the anoxia-exposed fish bit at their reflection almost twice as much as their normoxia-exposed cohort. This means that those 4 h of anoxia – way back when the fish weren't even hatched – produced a more aggressive adult fish, and this might help the fish out-compete others for limited resources like food and mates.

Next, Ivy and Robertson wanted to understand the physiology that might underlie this punchy personality, so they looked at the sex hormone testosterone, which is well known for its ability to turn a docile string bean into a raging muscle head when taken in excess. Testosterone is produced in the gonads of both male and female fish, but the amount released into the circulation depends on the activity of aromatase, an enzyme that helps convert testosterone into estrogen. When the researchers measured gonad aromatase expression, they found that fish exposed to anoxia as embryos had lower levels than those in the normoxia group, meaning that the anoxia group could secrete more testosterone from their gonads. In fact, the team found higher testosterone levels in the bodies of the fish from the embryonic anoxia exposure, and this could be responsible for the higher aggression in these fish. It also helps to explain the male-biased population that results from embryonic anoxia exposure, which the group discovered in their previous study, because high levels of testosterone can masculinize a female fish's ovaries.

This study from the Bernier group breathes new life into the topic of how early life environments influence future phenotypes. As the incidence of environmental stressors, including aquatic hypoxia, continues to increase, understanding these latent effects is critical to predicting how species will weather climate change.

C. M.
C. E.
N. J.
Acute embryonic anoxia exposure favours the development of a dominant and aggressive phenotype in adult zebrafish
Proc. R. Soc. B