Development of integument and cutaneous glands in larval, juvenile and adult toads (Rhinella granulosa): a morphological and morphometric study

Spatio-temporal analysis of toxigenic genes expression in the growing Bufo gargarizans based on RNA sequencing data

BACKGROUND

Bufo gargarizans Cantor, a widely distributed amphibian species in Asia, produces and releases toxins through its retroauricular and granular glands. Although various tissues have been sequenced, the molecular mechanisms underlying the toxin production remain unclear. To elucidate these mechanisms, abdominal skin (non-toxic secretory glands) and retroauricular gland (toxic secreting glands) samples were collected at different time points (3, 6, 12, 24, and 36 months) for RNA sequencing (RNA-seq) and analysis.

RESULTS

In comparison to the S group during the same period, a total of 3053, 3026, 1516, 1028, and 2061 differentially expressed genes (DEGs) were identified across five developmental stages. Gene Ontology (GO) analysis revealed that DEGs were primarily enriched in biological processes including cellular processes, single-organism processes, metabolic processes, and biological regulation. In terms of cellular components, the DEGs were predominantly localized in the cell and cell parts, whereas molecular function indicated significant enrichment in binding and catalytic activity. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that the metabolism and synthesis of various substances, such as lipid metabolism, cofactor and vitamin metabolism, tryptophan metabolism, steroid biosynthesis, and primary bile acid biosynthesis, were accompanied by the development of toads. Additionally, using trend analysis, we discovered candidate genes that were upregulated in the retroauricular glands during development, and the abundance of these genes in the abdominal skin was extremely low. Finally, we identified 26 genes that are likely to be involved in toxin production and that are likely to be involved in toxin anabolism.

CONCLUSION

Overall, these results provide new insights into the genes involved in toxin production in B. gargarizans, which will improve our understanding of the molecular mechanisms underlying toxigenic gene expression.

The adaptive microbiome hypothesis and immune interactions in amphibian mucus

The microbiome is known to provide benefits to hosts, including extension of immune function. Amphibians are a powerful immunological model for examining mucosal defenses because of an accessible epithelial mucosome throughout their developmental trajectory, their responsiveness to experimental treatments, and direct interactions with emerging infectious pathogens. We review amphibian skin mucus components and describe the adaptive microbiome as a novel process of disease resilience where competitive microbial interactions couple with host immune responses to select for functions beneficial to the host. We demonstrate microbiome diversity, specificity of function, and mechanisms for memory characteristic of an adaptive immune response. At a time when industrialization has been linked to losses in microbiota important for host health, applications of microbial therapies such as probiotics may contribute to immunotherapeutics and to conservation efforts for species currently threatened by emerging diseases.

Evo Devo of the Vertebrates Integument

All living jawed vertebrates possess teeth or did so ancestrally. Integumental surface also includes the cornea. Conversely, no other anatomical feature differentiates the clades so readily as skin appendages do, multicellular glands in amphibians, hair follicle/gland complexes in mammals, feathers in birds, and the different types of scales. Tooth-like scales are characteristic of chondrichthyans, while mineralized dermal scales are characteristic of bony fishes. Corneous epidermal scales might have appeared twice, in squamates, and on feet in avian lineages, but posteriorly to feathers. In contrast to the other skin appendages, the origin of multicellular glands of amphibians has never been addressed. In the seventies, pioneering dermal-epidermal recombination between chick, mouse and lizard embryos showed that: (1) the clade type of the appendage is determined by the epidermis; (2) their morphogenesis requires two groups of dermal messages, first for primordia formation, second for appendage final architecture; (3) the early messages were conserved during amniotes evolution. Molecular biology studies that have identified the involved pathways, extending those data to teeth and dermal scales, suggest that the different vertebrate skin appendages evolved in parallel from a shared placode/dermal cells unit, present in a common toothed ancestor, c.a. 420 mya.

Histological changes of the skin during postembryonic development of the crested newt Triturus ivanbureschi (Urodela, Salamandridae)

BACKGROUND

Amphibian skin has been studied for many decades, especially the metamorphic changes in the skin of frogs. Less attention has been paid to salamander skin. Here, we describe changes in the skin structure during postembryonic development in a salamandrid species, the Balkan crested newt Triturus ivanbureschi.

METHOD

Using traditional histological techniques we examined the skin in the trunk region of three premetamorphic larval stages (hatchling, mid larval and late larval) and two postmetamorphic stages (juvenile, just after metamorphosis, and adult).

RESULTS

In larval stages, skin consists only of the epidermis, which gradually develops from the single epithelial cell layer in hatchlings, to a stratified epidermis with gland nests and characteristic Leydig cells at the late larval stage. During metamorphosis, Leydig cells disappear, and the dermal layer develops. In postmetamorphic stages, skin is differentiated on stratified epidermis and the dermis with well-developed glands. Three types of glands were observed in the skin of the postmetamorphic stages: mucous, granular and mixed. Gland composition appears to be stage- and sex-specific, with juveniles and adult female being more similar to each other. In juveniles and adult female, there are a similar proportion of glands in both dorsal and ventral skin, whereas in adult male granular glands dominated the dorsal skin, while mixed glands dominated the ventral skin.

CONCLUSION

Our results provide a baseline for future comparative research of skin anatomy in salamanders.

Development of the venom delivery system in Elapidae snake species: Naja siamensis and Oxyuranus microlepidotus

Many advanced snakes possess a unique venom delivery system which they utilise to subdue prey and for defence. Despite extensive efforts, the evolutionary differences in this key system between advanced snake families remains enigmatic. The current study has investigated the development of the venom delivery system using two oviparous Elapidae models, Naja siamensis and Oxyuranus microlepidotus. The development stages of the embryos in both models were detailed using previously standardised characterisation. Variations in the days post-oviposition between these stages was observed, despite a continuous development trajectory. These differences also translated to the development of the venom delivery system.

Morphological and biochemical characterization of the cutaneous poison glands in toads (Rhinella marina group) from different environments

Background

Amphibian defence against predators and microorganisms is directly related to cutaneous glands that produce a huge number of different toxins. These glands are distributed throughout the body but can form accumulations in specific regions. When grouped in low numbers, poison glands form structures similar to warts, quite common in the dorsal skin of bufonids (toads). When accumulated in large numbers, the glands constitute protuberant structures known as macroglands, among which the parotoids are the most common ones. This work aimed at the morphological and biochemical characterization of the poison glands composing different glandular accumulations in four species of toads belonging to group Rhinella marina (R. icterica, R. marina, R. schneideri and R. jimi). These species constitute a good model since they possess other glandular accumulations together with the dorsal warts and the parotoids and inhabit environments with different degrees of water availability.

Results

We have observed that the toads skin has three types of poison glands that can be differentiated from each other through the morphology and the chemical content of their secretion product. The distribution of these different glands throughout the body is peculiar to each toad species, except for the parotoids and the other macroglands, which are composed of an exclusive gland type that is usually different from that composing the dorsal warts. Each type of poison gland presents histochemical and biochemical peculiarities, mainly regarding protein components.

Conclusions

The distribution, morphology and chemical composition of the different types of poison glands, indicate that they may have different defensive functions in each toad species.

Body size influences energetic and osmoregulatory costs in frogs infected with Batrachochytrium dendrobatidis

Sloughing maintains the skins integrity and critical functionality in amphibians. Given the behavioural, morphological and osmoregulatory changes that accompany sloughing, this process is likely to be physiologically costly. Chytridiomycosis, a cutaneous disease of amphibians caused by the fungus Batrachochytrium dendrobatidis (Bd), disrupts skin function and increases sloughing rates. Moreover, mortality rates from chytridiomycosis are significantly higher in juveniles and so we hypothesised that smaller individuals maybe more susceptible to chytridiomycosis because of allometric scaling effects on the energetic and osmoregulatory costs of sloughing. We measured in-vivo cutaneous ion loss rates and whole animal metabolic rate (MR) of Green tree frogs, Litoria caerulea, over a range of body sizes both infected and uninfected frogs during sloughing. Infected animals had a greater rate of ion loss and mass-specific MR during non-sloughing periods but there were no additional effects of sloughing on either of these parameters. There were also significant interactions with body size and Bd load indicating that smaller animals with higher Bd loads have greater rates of ion loss and higher energetic demands. Our results shed light on why smaller Bd-infected anurans often exhibit greater physiological disruption than larger individuals.

Age- and environment-dependent changes in chemical defences of larval and post-metamorphic toads

BACKGROUND

Chemical defences are widespread in animals, but how their production is adjusted to ecological conditions is poorly known. Optimal defence theory predicts that inducible defences are favoured over constitutive defences when toxin production is costly and the need for it varies across environments. However, if some environmental changes occur predictably (e.g. coupled to transitions during ontogeny), whereas others are unpredictable (e.g. predation, food availability), changes in defences may have constitutive as well as plastic elements. To investigate this phenomenon, we raised common toad (Bufo bufo) tadpoles with ad libitum or limited food and in the presence or absence of chemical cues on predation risk, and measured their toxin content on 5 occasions during early ontogeny.

RESULTS

The number of compounds showed limited variation with age in tadpoles and was unaffected by food limitation and predator cues. The total amount of bufadienolides first increased and later decreased during development, and it was elevated in young and mid-aged tadpoles with limited food availability compared to their ad libitum fed conspecifics, whereas it did not change in response to cues on predation risk. We provide the first evidence for the active synthesis of defensive toxin compounds this early during ontogeny in amphibians. Furthermore, the observation of increased quantities of bufadienolides in food-restricted tadpoles is the first experimental demonstration of resource-dependent induction of elevated de novo toxin production, suggesting a role for bufadienolides in allelopathy.

CONCLUSIONS

Our study shows that the evolution of phenotypic plasticity in chemical defences may depend on the ecological context (i.e. predation vs. competition). Our results furthermore suggest that the age-dependent changes in the diversity of toxin compounds in developing toads may be fixed (i.e., constitutive), timed for the developmental stages in which they are most reliant on their chemical arsenal, whereas inducible plasticity may prevail in the amount of synthesized compounds.

Parotoid, radial, and tibial macroglands of the frog Odontophrynus cultripes: Differences and similarities with toads

Anuran integument is characterized by the presence of glands, some of which are responsible for toxin production. In some species these glands accumulate in parts of the body strategically located against predators, forming structures known as macroglands. This is the case for parotoid macroglands, on the dorsum of the head, tibial macroglands, on the rear limbs, and radial macroglands, on the forelimbs of toads and some other anurans. The toad Rhinella jimi, for example, simultaneously displays all three types of macroglands, which is unusual even among bufonids. Interestingly, considering the phylogenetic distance, the frog Odontophrynus cultripes (Odontophrynidae) also presents these three macroglandular types. In this study we analyze the morphology of O. cultripes macroglands and the chemical composition of their poison using an interdisciplinary approach. In this species, the parotoid, tibial, and radial macroglands consist of aggregates of elongated and juxtaposed poison glands, arranged in a honeycomb style, very similar to that of toads. Comparative analysis of these three macrogland types shows significant differences in both the morphology of secretory granules and biochemical composition. The present work on O. cultripes contributes to the evidence that amphibians, or at least anurans, share a basic design for all cutaneous glandular accumulations. The determinant factor for macroglandular formation may be the selective pressure for defense against predators.

Post-metamorphic development of skin glands in a true toad: Parotoids versus dorsal skin

Chemical defenses in amphibians are a common antipredatory and antimicrobial strategy related to the presence of dermal glands that synthesize and store toxic or unpalatable substances. Glands are either distributed throughout the skin or aggregated in multiglandular structures, being the parotoids the most ubiquitous macrogland in toads of Bufonidae. Even though dermal glands begin to develop during late-larval stages, many species, including Rhinella arenarum, have immature glands by the end of metamorphosis, and their post-metamorphic growth is unknown. Herein, we compared the post-metamorphic development of parotoids and dorsal glands by histological and allometric studies in a size series of R. arenarum. Histological and histochemical studies to detect proteins, acidic glycoconjugates, and catecholamines, showed that both, parotoids and dorsal glands, acquire characteristics of adults in individuals larger than 50 mm; that is, a moment in which the cryptic coloration disappears. Parotoid height increased allometrically as a function of body size, whereas the size of small dorsal glands decreased with body size. The number of glands in the dorsum was not linearly related to body size, appearing to be an individual characteristic. Only adult specimens had intraepithelial granular glands in the duct of the largest glands of the parotoids. Since toxic secretions accumulate in the central glands of parotoids, allometric growth of parotoids may translate into greater protection from predators in the largest animals. Conversely, large glands in the dorsum, which produce a proteinaceous secretion of unknown function, grow isometrically to body size. Some characteristics, like intraepithelial glands in the ducts and basophilic glands in the dorsum, are limited to adults.