There is growing evidence that an organism's environment can have profound impacts on the expression of its genes, without necessarily altering its underlying genetic code. These 'epigenetic' changes can be passed down from generation to generation. Environmental factors that can lead to epigenetic modifications include external substances, such as toxins or pollutants, as well as exposure to sources of stress. One such increasingly common environmental stressor is hypoxia. Hypoxia refers to a condition found in aquatic ecosystems where dissolved oxygen in the water is depleted to detrimental concentrations; it can occur naturally, but pollution exacerbates the condition and has expanded its prevalence, particularly in estuaries and coastal ecosystems. Although variable in scale, duration, and severity, hypoxic events are problematic throughout the globe. The impacts to individuals that are directly exposed to hypoxia have been frequently studied in numerous species, but it was previously unknown if those effects were inheritable and could continue to impair future generations. Simon Wang and his colleagues in Hong Kong explored the transgenerational effects of hypoxia in a recently published study (Nat. Commun. 7, 12114; 2016).

The research team tracked reproductive impairments across three generations of male marine medaka fish (Oryzias melastigma) in three treatments: a normoxic group, in which all fish had sufficiently oxygenated, or normoxic, water; a hypoxic group chronically exposed to low dissolved oxygen; and a transgenerational group where the parent experienced hypoxia but subsequent generations were raised in normoxic water after they were spawned. Compared to the normoxic control generations, both the hypoxic and transgenerational fish displayed decreased sperm motility, lowered sperm production, and reduced fertilization success. Underlying those impairments were changes in DNA methylation—an epigenetic mechanism capable of modifying gene expression—to genes involved in spermatogenesis. Importantly, the observed changes in the transgenerational treatment offspring, which were never directly exposed to hypoxia, provides evidence that deleterious modifications induced by hypoxia in an individual can be inherited by its offspring.

Wang's study is the first to establish that low dissolved oxygen can adversely impact the reproductive potential of subsequent generations in addition to the individual directly exposed, suggesting that environmental managers and regulatory bodies may need to consider longer-term costs when evaluating the harm caused by aquatic hypoxia. It also raises questions about whether hypoxia in other species, including humans, can lead to transgenerational consequences.