Importance of the parental environment in determining offspring responses to toxicant stress

Parental effects are the effects of the parental phenotype and parental environment on offspring phenotype that occur without changes in the DNA sequence. Instead, they occur via transmission of epigenetic, cytoplasmic, somatic, nutritional, environmental, and behavioral factors from parents to offspring. To address the question of how biotic and abiotic conditions in the parental environment affect offspring responses to toxicants and other stressors, I used the snails Biomphalaria glabrata and Physa pomilia as model species. Exposure of parental Biomphalaria to predator cues altered offspring tolerance to cadmium but not malathion, while parental diet affected offspring tolerance to both cadmium and malathion. Additionally, within the parental diet experiment, cross-fertilizing parents produced offspring with greater cadmium and malathion tolerance than self-fertilizing parents. In another experiment, parental Physa exposure to cadmium at a high but environmentally relevant concentration of 20 µg/L increased the cadmium tolerance of their offspring compared to offspring of controls and parental snails exposed to 2 µg/L cadmium. However, subsequent exposure of egg masses to 2 µg/L cadmium during development negated the effects of parental exposure. Finally, exposure of parental Biomphalaria to 0, 16, or 50 µg/L cadmium resulted in alterations in tolerance to cadmium, heat, and cold in their hatchling offspring, but hatchling tolerances to copper and salinity were unaffected. When offspring of cadmium-exposed parents were raised from hatchlings to 8 weeks old in a factorial design of parental cadmium exposure (0, 16, or 50 µg/L cadmium)*offspring cadmium exposure (0 or 2 µg/L cadmium)*temperature (25 or 30°C), parental exposure to 50 µg/L cadmium increased offspring reproduction compared to controls. The effects of parental cadmium concentration and temperature also had an interactive effect on the reproductive output of offspring. Taken together, these data suggest several trends. First, parental stress may increase, decrease, or have no effect on offspring tolerance to abiotic stressors. Second, the occurrence of a parental effect may depend on the characteristics of both the parental stressor and the stressor to which offspring were exposed. Third, the toxicant concentration to which parents are exposed may play a role in determining whether a parental effect occurs. Fourth, toxicity of a contaminant to egg masses, as measured by hatching success and time to hatch, may not always be a good predictor of offspring tolerance to that contaminant. Overall, both biotic and abiotic conditions in the parental environment can significantly alter offspring stressor tolerance and life history traits. Because of their potential to influence offspring fitness and thereby the health and persistence of populations, parental effects constitute an important avenue of future ecotoxicology research.