The Origin and Ecological Function of an Ion Inducing Anti-Predator Behavior in Lithobates Tadpoles

Stress hormones mediate developmental plasticity in vertebrates with complex life cycles

The environment experienced by developing organisms can shape the timing and character of developmental processes, generating different phenotypes from the same genotype, each with different probabilities of survival and performance as adults. Chordates have two basic modes of development, indirect and direct. Species with indirect development, which includes most fishes and amphibians, have a complex life cycle with a free-swimming larva that is typically a growth stage, followed by a metamorphosis into the adult form. Species with direct development, which is an evolutionarily derived developmental mode, develop directly from embryo to the juvenile without an intervening larval stage. Among the best studied species with complex life cycles are the amphibians, especially the anurans (frogs and toads). Amphibian tadpoles are exposed to diverse biotic and abiotic factors in their developmental habitat. They have extensive capacity for developmental plasticity, which can lead to the expression of different, adaptive morphologies as tadpoles (polyphenism), variation in the timing of and size at metamorphosis, and carry-over effects on the phenotype of the juvenile/adult. The neuroendocrine stress axis plays a pivotal role in mediating environmental effects on amphibian development. Before initiating metamorphosis, if tadpoles are exposed to predators they upregulate production of the stress hormone corticosterone (CORT), which acts directly on the tail to cause it to grow, thereby increasing escape performance. When tadpoles reach a minimum body size to initiate metamorphosis they can vary the timing of transformation in relation to growth opportunity or mortality risk in the larval habitat. They do this by modulating the production of thyroid hormone (TH), the primary inducer of metamorphosis, and CORT, which synergizes with TH to promote tissue transformation. Hypophysiotropic neurons that release the stress neurohormone corticotropin-releasing factor (CRF) are activated in response to environmental stress (e.g., pond drying, food restriction, etc.), and CRF accelerates metamorphosis by directly inducing secretion of pituitary thyrotropin and corticotropin, thereby increasing secretion of TH and CORT. Although activation of the neuroendocrine stress axis promotes immediate survival in a deteriorating larval habitat, costs may be incurred such as reduced tadpole growth and size at metamorphosis. Small size at transformation can impair performance of the adult, reducing probability of survival in the terrestrial habitat, or fecundity. Furthermore, elevations in CORT in the tadpole caused by environmental stressors cause long term, stable changes in neuroendocrine function, behavior and physiology of the adult, which can affect fitness. Comparative studies show that the roles of stress hormones in developmental plasticity are conserved across vertebrate taxa including humans.

An inductively coupled plasma study and an ion cyclotron resonance matrix-assisted laser/desorption study at high mass resolution of two ancient Maya greenstone artifacts from Pacbitun, Belize, and two decorative jade specimens

RATIONALE

Two Early Classic period (ca 250-600 CE) Maya carved, greenstone jade pendants, Specimens A and B, were recovered from the Pacbitun site in Belize in 1987. Mass scans of resolution ~600,000 were obtained for the jade pendants and two British Columbia jade ornaments.

METHODS

Polyatomic ions from jade grit have been observed using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS), specifically the Bruker Daltonics, SolariX XR, combined with matrix-assisted laser desorption/ionization (MALDI); the matrix material was α-cyano-4-hydroxycinnamicacid.

RESULTS

Presented here is an interpretation of the mass spectrometric observations from multiplicities of silicate materials. Observation of novel polyatomic ion species of m/z 800-1400 indicate the ejection of charged ensembles composed of 60-180 atoms from jade grit. The charged ensembles represent minute regions of jade grit that have retained many of the spatial relationships of atoms within the jade; the required ejection energy is provided by laser photons accumulated in the matrix. Acid solutions of grit from Specimens A and ornament KMM were analyzed by inductively-coupled plasma mass spectrometry. The novelty of this application of mass spectrometry to jade was reported in the Journal of Archaeological Science: Reports.

CONCLUSIONS

Isotopic fine structure analysis can yield elemental compositions for ions composed of base isotopes together with stable isotope intensities arising from ions with one and two additional neutrons. The analysis is challenging as jade is a multiplicity of similar solid solutions; thus, a multiple isotopic fine structure approach has been explored. Attempts to compose ensembles of atoms, of those elements found normally in coloured silicates, to match the observed ion mass/charge ratios were unsuccessful without the addition of multiple hydrogen atoms. The production of polyatomic ions of siliceous material may permit impinging fine glass coatings onto construction materials for protection against fire and weather.

The Effects of Conspecific Alarm Cues on Larval Cane Toads (Rhinella marina)

Many aquatic organisms detect and avoid damage-released cues from conspecifics, but the chemical basis of such responses, and the effects of prolonged exposure to such cues, remain poorly understood. Injured tadpoles of the cane toad (Rhinella marina) produce chemical cues that induce avoidance by conspecific tadpoles; and chronic exposure to those cues decreases rates of tadpole survival and growth, and reduces body size at metamorphosis. Such effects suggest that we might be able to use the cane toads' alarm cue for biocontrol of invasive populations in Australia. In the present study, we examined behavioral and ecological effects of compounds that are present in cane toad tadpoles and thus, might trigger avoidance of crushed conspecifics. Four chemicals (L-Arg, L-Leu-L-Leu-OH, L-Leu-L-Ile-OH and suberic acid) induced behavioral avoidance in toad tadpoles at some (but not all) dosage levels, so we then exposed toad larvae to these chemicals over the entire period of larval development. Larval survival and size at metamorphosis were decreased by chronic exposure to crushed conspecifics (consistent with earlier studies), but not by exposure to any of the four chemicals. Indeed, L-Arg increased body size at metamorphosis. We conclude that the behavioral response to crushed conspecifics by cane toad tadpoles can be elicited by a variety of chemical cues, but that consistent exposure to these individual chemical cues does not affect tadpole viability or developmental trajectory. The optimal behavioral tactic of a tadpole may be to flee if it encounters even a single chemical cue likely to have come from an injured conspecific (indicative of predation risk), whereas the continuing presence of that single chemical (but no others) provides a less reliable signal of predation risk. Our data are consistent with results from studies on fish, that suggest a role for multiple chemicals in initiating alarm responses to damage-released cues.