The effects of early-life and intergenerational stress on the brain

Stress experienced during ontogeny can have profound effects on the adult phenotype. However, stress can also be experienced intergenerationally, where an offspring's phenotype can be moulded by stress experienced by the parents. Although early-life and intergenerational stress can alter anatomy, physiology, and behaviour, nothing is known about how these stress contexts interact to affect the neural phenotype. Here, we examined how early-life and intergenerational stress affect the brain in eastern fence lizards (Sceloporus undulatus). Some lizard populations co-occur with predatory fire ants, and stress from fire ant attacks exerts intergenerational physiological and behavioural changes in lizards. However, it is unclear if intergenerational stress, or the interaction between intergenerational and early-life stress, modulates the brain. To test this, we captured gravid females from fire ant invaded and uninvaded populations, and subjected offspring to three early-life stress treatments: (1) fire ant attack, (2) corticosterone, or (3) a control. Corticosterone and fire ant attack decreased some aspects of the neural phenotype while population of origin and the interaction of early-life stress and population had no effects on the brain. These results suggest that early-life stressors may better predict adult brain variation than intergenerational stress in this species.

Population history with invasive predators predicts innate immune function response to early life glucocorticoid exposure

Early life stress can suppress immune function, but it is unclear if transgenerational stress exposure modulates the immune consequences of early stress. In populations where, historically, the immune system is frequently activated, e.g. persistent stressors that cause injury, it may be maladaptive to suppress immune function after early life stress. Thus, the relationship between early life stress and immune function may vary with population-level historical stressor exposure. We collected gravid fence lizards (Sceloporus undulatus) from populations that naturally differ in long-term exposure to invasive fire ants (Solenopsis invicta). We manipulated early life stress in resulting offspring via weekly exposure to fire ants, application of the stress-relevant hormone corticosterone, or control treatment from 2 to 43 weeks of age. We quantified adult immune function in these offspring with baseline and antigen-induced hemagglutination and plasma bacterial killing ability. Early life corticosterone exposure suppressed baseline hemagglutination in offspring of lizards from populations without fire ants but enhanced hemagglutination in those from populations with fire ants. This enhancement may prepare lizards for high rates of wounding, toxin exposure, and infection associated with fire ant attack. Adult bacterial killing ability and hemagglutination were not affected by early life exposure to fire ants, but the latter was higher in offspring of lizards from invaded sites. A population's history of persistent wounding may thus alter individual long-term immunological responses to early life stressors. Further consideration of historical stressor exposure (type and duration) may be important to better understand how early life stressors affect adult physiology.

Ancestry trumps experience: Transgenerational but not early life stress affects the adult physiological stress response

Exposure to stressors can affect an organism's physiology and behavior as well as that of its descendants (e.g. through maternal effects, epigenetics, and/or selection). We examined the relative influence of early life vs. transgenerational stress exposure on adult stress physiology in a species that has populations with and without ancestral exposure to an invasive predator. We raised offspring of eastern fence lizards (Sceloporus undulatus) from sites historically invaded (high stress) or uninvaded (low stress) by predatory fire ants (Solenopsis invicta) and determined how this different transgenerational exposure to stress interacted with the effects of early life stress exposure to influence the physiological stress response in adulthood. Offspring from these high- and low-stress populations were exposed weekly to either sub-lethal attack by fire ants (an ecologically relevant stressor), topical treatment with a physiologically-appropriate dose of the stress-relevant hormone, corticosterone (CORT), or a control treatment from 2 to 43weeks of age. Several months after treatments ended, we quantified plasma CORT concentrations at baseline and following restraint, exposure to fire ants, and adrenocorticotropic hormone (ACTH) injection. Exposure to fire ants or CORT during early life did not affect lizard stress physiology in adulthood. However, offspring of lizards from populations that had experienced multiple generations of fire ant-invasion exhibited more robust adult CORT responses to restraint and ACTH-injection compared to offspring from uninvaded populations. Together, these results indicate that transgenerational stress history may be at least as important, if not more important, than early life stress in affecting adult physiological stress responses.

Epigenetic response to environmental change: DNA methylation varies with invasion status

Epigenetic mechanisms may be important for a native species' response to rapid environmental change. Red Imported Fire Ants (Solenopsis invicta Santschi, 1916) were recently introduced to areas occupied by the Eastern Fence Lizard (Sceloporus undulatus Bosc & Daudin, 1801). Behavioral, morphological and physiological phenotypes of the Eastern Fence Lizard have changed following invasion, creating a natural biological system to investigate environmentally induced epigenetic changes. We tested for variation in DNA methylation patterns in Eastern Fence Lizard populations associated with different histories of invasion by Red Imported Fire Ants. At methylation sensitive amplified fragment length polymorphism loci, we detected a higher diversity of methylation in Eastern Fence Lizard populations from Fire Ant uninvaded versus invaded sites, and uninvaded sites had higher methylation. Our results suggest that invasive species may alter methylation frequencies and the pattern of methylation among native individuals. While our data indicate a high level of intrinsic variability in DNA methylation, DNA methylation at some genomic loci may underlie observed phenotypic changes in Eastern Fence Lizard populations in response to invasion of Red Imported Fire Ants. This process may be important in facilitating adaptation of native species to novel pressures imposed by a rapidly changing environment.