A novel larval diet interacts with nutritional stress to modify juvenile behaviors and glucocorticoid responses

Diet-induced plasticity modifies relationships between larval growth rate and post-metamorphic behavior and physiology in spadefoot toads

It has frequently been hypothesized that among-individual variation in behavior and physiology will correlate with life history traits, yet the nature of these correlations can vary. Such variability may arise from plasticity in trait development, which can amplify or attenuate trait correlations across different environments. Using the Mexican spadefoot toad (Spea multiplicata), we tested whether relationships between larval growth rate and post-metamorphic behavior or physiology are influenced by a key mediator of developmental plasticity: larval diet type. Spea multiplicata larvae develop on two alternative diets, with slower growing omnivores feeding on detritus and faster growing carnivores consuming live fairy shrimp. We found that correlations between larval growth rate and post-metamorphic behavior and physiology differed by diet type. Among detritus feeders, faster growing larvae developed into juvenile frogs that were not only bolder but also had higher hypothalamic-pituitary-interrenal axis reactivity (an indicator of stress responsiveness) and longer telomeres, suggesting greater somatic maintenance. In contrast, among shrimp feeders - which exhibited faster growth overall - larval growth rate was less strongly correlated with juvenile behavior and physiology, indicating that a shift from omnivory to carnivory can attenuate trait correlations among individuals. Overall, our study suggests that developmental plasticity induced by different diet types can modify relationships between life history traits and individual behavior or physiology.

Early development of the glucocorticoid stress response in dyeing poison frog tadpoles

In vertebrates, the glucocorticoid "stress" response (corticosterone or cortisol) through the hypothalamic-pituitary-adrenal (HPA) axis influences many essential functions, including behavior, metabolism, immunity, and ontogenetic transitions. During development, stress responses can be adaptive if they facilitate antipredator behavior and modulate developmental speed to adjust to environmental conditions; however, these same responses can be maladaptive when energetic costs become too high and developmental speed trades-off with size and health at maturity. Thus, the timing of HPA-axis development may be aligned with specific developmental challenges and opportunities presented by a species' life history strategy. In anurans (frogs and toads), corticosterone plays critical roles in development and behavior, and concentrations can fluctuate in response to environmental stressors. Given the role of corticosterone in ontogenetic changes and behaviors, we studied the development of the HPA axis in tadpoles of the dyeing poison frog ( Dendrobates tinctorius ), a species with a unique life history that includes transport to water after hatching on land and aggressive and cannibalistic behavior. We measured the excretion rate and whole-body concentration of corticosterone and the corticosterone response to adrenocorticotropic hormone (ACTH) in free-swimming tadpoles after transport and throughout metamorphosis. We found no significant differences across development in excretion rates or whole-body concentration of corticosterone, nor corticosterone response to ACTH, indicating that that the glucocorticoid response develops early in ontogeny. This pattern differs from those in other species of tadpoles, suggesting the unique ecological pressures faced by D. tinctorius have shaped the development of its HPA axis. More broadly, this study illustrates how life history strategies and tradeoffs impact the timing of the HPA axis development.

Early life nutrient restriction affects hypothalamic-pituitary-interrenal axis gene expression in a diet type-specific manner

Stressful experiences in early life can alter phenotypic expression later in life. For instance, in vertebrates, early life nutrient restriction can modify later life activity of the hypothalamic-pituitary-adrenal/interrenal axis (the HPI in amphibians), including the up- and downstream regulatory components of glucocorticoid signaling. Early life nutrient restriction can also influence later life behavior and metabolism (e.g., fat accumulation). Yet, less is known about whether nutrient stress-induced carryover effects on HPA/HPI axis regulation can vary across environmental contexts, such as the type of diet on which nutrient restriction occurs. Here, we experimentally address this question using the plains spadefoot toad (Spea bombifrons), whose larvae develop in ephemeral habitats that impose intense competition over access to two qualitatively distinct diet types: detritus and live shrimp prey. Consistent with diet type-specific carryover effects of early life nutrient restriction on later life HPI axis regulation, we found that temporary nutrient restriction at the larval stage reduced juvenile (i.e., post-metamorphic) brain gene expression of an upstream glucocorticoid regulator (corticotropin-releasing hormone) and two downstream regulators (glucocorticoid and mineralocorticoid receptors) only on the shrimp diet. These patterns are consistent with known diet type-specific effects of larval nutrient restriction on juvenile corticosterone and behavior. Additionally, larval nutrient restriction increased juvenile body fat levels. Our study indicates that HPA/HPI axis regulatory responses to nutrient restriction can vary remarkably across diet types. Such diet type-specific regulation of the HPA/HPI axis might provide a basis for developmental or evolutionary decoupling of stress-induced carryover effects.

Genetic regulators of a resource polyphenism interact to couple predatory morphology and behaviour

Phenotypic plasticity often requires the coordinated response of multiple traits observed individually as morphological, physiological or behavioural. The integration, and hence functionality, of this response may be influenced by whether and how these component traits share a genetic basis. In the case of polyphenism, or discrete plasticity, at least part of the environmental response is categorical, offering a simple readout for determining whether and to what degree individual components of a plastic response can be decoupled. Here, we use the nematode Pristionchus pacificus, which has a resource polyphenism allowing it to be a facultative predator of other nematodes, to understand the genetic integration of polyphenism. The behavioural and morphological consequences of perturbations to the polyphenism's genetic regulatory network show that both predatory activity and ability are strongly influenced by morphology, different axes of morphological variation are associated with different aspects of predatory behaviour, and rearing environment can decouple predatory morphology from behaviour. Further, we found that interactions between some polyphenism-modifying genes synergistically affect predatory behaviour. Our results show that the component traits of an integrated polyphenic response can be decoupled and, in principle, selected upon individually, and they suggest that multiple routes to functionally comparable phenotypes are possible.

Corticosterone Contributes to Diet-Induced Reprogramming of Post-Metamorphic Behavior in Spadefoot Toads

Stressful experiences in early life can have phenotypic effects that persist into, or manifest during, adulthood. In vertebrates, such carryover effects can be driven by stress-induced secretion of glucocorticoid hormones, such as corticosterone, which can lead to developmental reprogramming of hypothalamic-pituitary-adrenal/interrenal axis activity and behavior. Nutritional stress in the form of early life nutrient restriction is well known to modify later life behaviors and stress activity through corticosterone-related mechanisms. However, it is not known whether corticosterone is also mechanistically involved in carryover effects induced by a different form of nutritional variation: the use of alternate or entirely novel types of dietary resources. The plains spadefoot (Spea bombifrons) presents an excellent system for testing this question, since larvae of this species have evolved to use 2 alternate diet types: an ancestral detritus-based diet and a more novel diet of live shrimp. While previous work has shown that feeding on the novel shrimp diet influences juvenile (i.e., post-metamorphic) behavior and corticosterone levels, it is unclear whether these diet-induced carryover effects are mediated by diet-induced corticosterone itself. To test for the mechanistic role of corticosterone in diet-induced carryover effects, we experimentally treated S. bombifrons larvae with exogenous corticosterone and measured subsequent effects on juvenile behavior and corticosterone levels. We found that while shrimp-fed larvae had elevated corticosterone levels, treatment of larvae with corticosterone itself had effects on juvenile behavior that partially resembled those carryover effects induced by the shrimp diet, such as altered food seeking and higher locomotor activity. However, unlike carryover effects caused by the shrimp diet, larval corticosterone exposure did not affect juvenile corticosterone levels. Overall, our study shows that corticosterone-related mechanisms are likely involved in carryover effects induced by a novel diet, yet such diet-induced carryover effects are not driven by corticosterone alone.