Theories have been widely proposed and tested for impacts of soil nitrogen (N) on phytochemical defenses. Among the hundreds of distinct cardenolide toxins produced by milkweeds (Asclepias spp.), few contain N, yet these appear to be the most toxic against specialist herbivores. Because N- and non-N-cardenolides coexist in milkweed leaves and likely have distinct biosynthesis, they present an opportunity to address hypotheses about drivers of toxin expression. We tested effects of soil N and herbivore-damage on cardenolide profiles of two milkweed species differing in life-history strategies (Asclepias syriaca and A. curassavica), and the toxicity of their leaves. In particular leaf extracts were tested against the target enzymes (Na+/K+-ATPase extracted from neural tissue) from both monarch butterflies (Danaus plexippus) as well as less cardenolide-resistant queen butterflies, D. gilippus. Increasing soil N enhanced biomass of Asclepias syriaca but had weak effects on cardenolides, including causing a significant reduction in the N-cardenolide labriformin; feeding by monarch caterpillars strongly induced N-cardenolides (labriformin), its precursors, and total cardenolides. Conversely, soil N had little impact on A. curassavica biomass, but was the primary driver of increasing N-cardenolides (voruscharin, uscharin and their precursors); caterpillar induction was weak. Butterfly enzyme assays revealed damage-induced cardenolides substantially increased toxicity of both milkweeds to both butterflies, swamping out effects of soil N on cardenolide concentration and composition. Although these two milkweed species differentially responded to soil N with allocation to growth and specific cardenolides, leaf toxicity to butterfly Na+/K+-ATPases was primarily driven by herbivore-induced defense. Thus, both biotic and abiotic factors shape the composition of phytochemical defense expression, and their relative importance may be dictated by plant life-history differences.

Iron is a common and essential element for maintaining life in bacteria, plants and animals and is found in soil, fresh waters and marine waters; however, over exposure is toxic to organisms. Iron is used in electron transport complexes within mitochondria as well as a co-factor in many essential proteins. It is also established that iron accumulation in the central nervous system in mammals is associated with various neurological disorders. Ample studies have investigated the long-term effects of iron overload in the nervous system. However, its acute effects in nervous tissue and additional organ systems warrant further studies. This study investigates the effects of iron overload on development, behavior, survival, cardiac function, and glutamatergic synaptic transmission in the Drosophila melanogaster. Additionally, physiological responses in crayfish were examined following Fe3+ exposure. Fe3+ reduced neuronal excitability in proprioceptive neurons in a crayfish model. Thus, Fe3+ may block stretch activated channels (SACs) as well as voltage-gated Na+ channels. Exposure also rapidly reduces synaptic transmission but does not block ionotropic glutamatergic receptors, suggesting a blockage of pre-synaptic voltage-gated Ca2+ channels in both crustacean and Drosophila models. The effects are partly reversible with acute exposure, indicating the cells are not rapidly damaged. This study is relevant in demonstrating the effects of Fe3+ on various physiological functions in different organisms in order to further understand the acute and long-term consequences of overload.