Temperature and the microbial environment alter brain morphology in a larval amphibian

Understanding how the global climate impacts the physiology of wildlife animals is of importance. Amphibians are particularly sensitive to climate change, and it is hypothesized that rising temperatures impair their neurodevelopment. Temperature influences the composition of the gut microbiota, which is critical to host neurodevelopment through the microbiota-gut-brain (MGB) axis. Most research investigating the link between the gut microbiota and neurodevelopment occurs in germ-free mammalian model systems, leaving the nature of the MGB axis in non-mammalian wildlife unclear. Here, we tested the hypothesis that the temperature and the microbial environment in which tadpoles were raised shapes neurodevelopment, possibly through the MGB axis. Newly hatched green frog tadpoles (Lithobates clamitans) were raised in natural pond water or autoclaved pond water, serving as an experimental manipulation of the microbiota by reducing colonizing microbes, at three different water temperatures: 14, 22 and 28°C. Neurodevelopment was analyzed through measures of relative brain mass and morphology of brain structures of interest. We found that tadpole development in warmer temperatures increased relative brain mass and optic tectum width and length. Further, tadpole development in autoclaved pond water increased relative optic tectum width and length. Additionally, the interaction of treatments altered relative diencephalon length. Lastly, we found that variation in brain morphology was associated with gut microbial diversity and the relative abundance of individual bacterial taxa. Our results indicate that both environmental temperature and microbial communities influence relative brain mass and shape. Furthermore, we provide some of the first evidence for the MGB axis in amphibians.

An integrated physiological perspective on anthropogenic stressors in the Gulf coast toad (Incilius nebulifer)

Anthropogenic environmental change, including climate change and urbanization, results in warmer temperatures in both terrestrial and aquatic habitats and changes in community assemblages including invasive species introductions, among many other alterations. Anurans are particularly susceptible to these changes because generally they have a biphasic lifecycle and rely on aquatic and terrestrial habitats for survival. Changes such as warmer water temperature can result in direct and carryover effects, after metamorphosis that decrease fitness. However, Gulf Coast toads (Incilius (Bufo) nebulifer) are expanding their range, including into anthropogenically disturbed areas. We hypothesize that I. nebulifer copes with warmer water, reduced water levels, and invasive species by altering their physiology and/or behavior. Corticosterone is the primary glucocorticoid in amphibians, and it modulates many aspects of physiology and behavior, potentially including lipid storage and hop performance, during unpredictable (stressful) events. As a true toad, I. nebulifer also produces bufadienolide toxins that aid in its antipredator defense and may have tradeoffs with corticosterone. In a fully factorial design, we measured baseline corticosterone levels in tadpoles in response to two treatments: decreased water levels and increased water temperatures. After metamorphosis, we measured the corticosterone profile and other associated responses to exposure to the predatory red imported fire ant (Solenopsis invicta; RIFA). We found that tadpoles had elevated baseline corticosterone release rates when reared in warmer water and reduced water levels. Toadlets also had elevated baseline corticosterone release rates when exposed to any combination of two of the three treatments but when exposed to all three treatments toadlets instead showed elevated magnitude of their stress response. Predator avoidance (as measured by hop performance) was reduced after exposure to RIFA. Tadpoles from warmer water developed more quickly and were smaller in mass after metamorphosis. Toadlets had reduced production of two of the three detected bufadienolides and increased energy storage (lipids) after exposure to warmer water and reduced growth after exposure to reduced water levels. We found direct and carryover effects of common anthropogenic changes in I. nebulifer that may aid in their ability to persist despite these changes.

Developmental Exposure to Trace Concentrations of Chlorpyrifos Results in Nonmonotonic Changes in Brain Shape and Behavior in Amphibians

Despite regulations and improved design, pesticides remain ubiquitous in the environment at relatively low, trace concentrations. To understand how prolonged exposure to trace pesticide concentrations impacts vertebrate brain development and behavior, we raised larval amphibians (northern leopard frogs, Lithobates pipiens) in 0, 1, or 10 μg/L of the organophosphorus pesticide chlorpyrifos (CPF) from hatching to metamorphosis. Tadpoles exposed to 1 μg/L CPF, but not 10 μg/L CPF, had changes in relative brain mass, relative telencephalon shape, and behavioral responses to a novel visual cue. Tadpoles exposed to 10 μg/L CPF had altered behavioral responses to predator-associated olfactory cues. After metamorphosis, frogs raised in 1 μg/L CPF, but not 10 μg/L CPF, had changes in the shape of their optic tectum and medulla. Thus, we provide robust evidence that even trace, yet ecologically realistic, concentrations of CPF have neurodevelopmental and behavioral effects that carry over to later life-history stages, further emphasizing the potent effects of trace levels of CPF on vertebrate development. Also, some but not all effects were nonmonotonic, meaning that effects were evident at the lowest but not at the higher concentration of CPF.