Variations in tetrodotoxin levels in populations of Taricha granulosa are expressed in the morphology of their cutaneous glands

Pedro Luiz Mailho-Fontana,Marta Maria Antoniazzi,Daniel Carvalho Pimenta,Carlos Jared,Juliana Mozer Sciani,Amber N Stokes,Taran Grant,Edmund D Brodie,Edmund D Brodie

doi:10.1038/s41598-019-54765-z

Pedro Luiz Mailho-Fontana, Marta Maria Antoniazzi + Show 7 more

Open Access

https://doi.org/10.1038/s41598-019-54765-z

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Abstract

Tetrodotoxin (TTX), one of the most toxic substances in nature, is present in bacteria, invertebrates, fishes, and amphibians. Marine organisms seem to bioaccumulate TTX from their food or acquire it from symbiotic bacteria, but its origin in amphibians is unclear. Taricha granulosa can exhibit high TTX levels, presumably concentrated in skin poison glands, acting as an agent of selection upon predatory garter snakes (Thamnophis). This co-evolutionary arms race induces variation in T. granulosa TTX levels, from very high to undetectable. Using morphology and biochemistry, we investigated differences in toxin localization and quality between two populations at the extremes of toxicity. TTX concentration within poison glands is related to the volume of a single cell type in which TTX occurs exclusively in distinctive secretory granules, suggesting a relationship between granule structure and chemical composition. TTX was detected in mucous glands in both populations, contradicting the general understanding that these glands do not secrete defensive chemicals and expanding currently held interpretations of amphibian skin gland functionality. Skin secretions of the two populations differed in low-mass molecules and proteins. Our results demonstrate that interpopulation variation in TTX levels is related to poison gland morphology.

Highlights

Tetrodotoxin (TTX), one of the most toxic substances in nature, is present in bacteria, invertebrates, fishes, and amphibians
In an effort to understand the process(es) involved in TTX accumulation in amphibians, we investigated the skin, especially the cutaneous glands, of T. granulosa by comparing the morphology and immunohistochemistry of individuals sampled from two populations, one with undetectable TTX levels and the other with high levels of TTX
Taricha granulosa skin presents many, evenly distributed poison glands. These glands are composed of several secretory cells that completely fill the interior of the gland and do not form a lumen (Fig. 1a–e)

Summary

Introduction

Tetrodotoxin (TTX), one of the most toxic substances in nature, is present in bacteria, invertebrates, fishes, and amphibians. Taricha granulosa can exhibit high TTX levels, presumably concentrated in skin poison glands, acting as an agent of selection upon predatory garter snakes (Thamnophis). This co-evolutionary arms race induces variation in T. granulosa TTX levels, from very high to undetectable. The amount of TTX present in the skin of individual T. granulosa varies extensively across populations, from undetectable to levels sufficient to kill 25,000 mice[6] This extreme variation is driven by an evolutionary arms race with TTX-resistant garter snakes (Thamnophis spp.; Colubridae)[11,12,13]. We examined the broader toxin profile of skin secretions of these populations using a biochemical approach

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