Cutaneous Tail Glands, Noxious Skin Secretions, and Scent Marking in a Terrestrial Salamander (Plethodon shermani)

Histological, histochemical, and morphometric analysis of epidermal Leydig cells and histochemical characterization of epidermal apical cells in juvenile and adult axolotls (Ambystoma mexicanum)

Ambystoma mexicanum, also known as the axolotl, is a paedomorphic urodele. Metamorphosis can be induced experimentally, and the most significant changes occur in the skin. These include thinning of the epidermis, increased keratinization of the stratified squamous epithelium, and loss of Leydig cells (LCs). Similar epidermal changes are observed in other metamorphic urodeles. Epidermal cells are responsible for the secretory function of the skin in juvenile amphibians, whereas dermal glands perform this function in adults after metamorphosis. In the axolotl, this occurrence is still partially understood. The only recognized epidermal secretory cells in juvenile A. mexicanum are the LCs, whose specific secretion products have not yet been characterized from the histochemical standpoint. Additionally, the persistence of LCs in adulthood, when mucous and serous (granular-protein secretion) glands are abundant, remains a matter of debate. The present study aims to describe the morphological and histochemical changes in the epidermis of 10 cutaneous regions from juvenile (4 months old) and adult (24 and 48 months old) non-metamorphic A. mexicanum, with a particular focus on the amount and histochemical characteristics of LCs. Results indicate that the juvenile epidermis is a stratified cuboidal epithelium formed by three strata: basal, spinosum (containing the LCs), and apical. The most superficial layer contains cuboidal cells that lack the characteristics of a true stratum corneum. In adults, the stratum apical is also formed by squamous cells, suggesting a transition to a cornified and squamous layer as age increases. Histochemical methods demonstrated that LCs are most likely serous and not mucous cells. On the other hand, cuboidal cells of the juvenile apical stratum would be responsible for producing mucous secretion components. Morphometric analysis revealed a significant decrease in both LCs and the epidermal thickness in the 24-month-old adult axolotl compared to the juvenile. While LC count and epidermal thickness in the 48-month-old adult showed a slight increase compared to the 24-month-old adult, these differences were not statistically significant and far lower than those observed in the juvenile axolotl, which exhibited the highest number of LCs and a thicker epidermis. These natural axolotl epidermal changes indicate a gradual transition toward a morphology resembling metamorphic skin as age advances. The decreased number of LCs and the transition from cuboid cells to squamous cells in the stratum apical suggest that both cell types may naturally disappear entirely at some point during development.

Insights into caudate amphibian skin secretions with a focus on the chemistry and bioactivity of derived peptides

Amphibians are well-known for their ability to produce and secrete a mixture of bioactive substances in specialized skin glands for the purpose of antibiotic self-protection and defense against predators. Some of these secretions contain various small molecules, such as the highly toxic batrachotoxin, tetrodotoxin, and samandarine. For some time, the presence of peptides in amphibian skin secretions has attracted researchers, consisting of a diverse collection of - to the current state of knowledge - three to 104 amino acid long sequences. From these more than 2000 peptides many are known to exert antimicrobial effects. In addition, there are some reports on amphibian skin peptides that can promote wound healing, regulate immunoreactions, and may serve as antiparasitic and antioxidative substances. So far, the focus has mainly been on skin peptides from frogs and toads (Anura), eclipsing the research on skin peptides of the ca. 700 salamanders and newts (Caudata). Just recently, several novel observations dealing with caudate peptides and their structure-function relationships were reported. This review focuses on the chemistry and bioactivity of caudate amphibian skin peptides and their potential as novel agents for clinical applications.

Enhancing predictive performance for spectroscopic studies in wildlife science through a multi-model approach: A case study for species classification of live amphibians

Near infrared spectroscopy coupled with predictive modeling is a growing field of study for addressing questions in wildlife science aimed at improving management strategies and conservation outcomes for managed and threatened fauna. To date, the majority of spectroscopic studies in wildlife and fisheries applied chemometrics and predictive modeling with a single-algorithm approach. By contrast, multi-model approaches are used routinely for analyzing spectroscopic datasets across many major industries (e.g., medicine, agriculture) to maximize predictive outcomes for real-world applications. In this study, we conducted a benchmark modeling exercise to compare the performance of several machine learning algorithms in a multi-class problem utilizing a multivariate spectroscopic dataset obtained from live animals. Spectra obtained from live individuals representing eleven amphibian species were classified according to taxonomic designation. Seven modeling techniques were applied to generate prediction models, which varied significantly (p < 0.05) with regard to mean classification accuracy (e.g., support vector machine: 95.8 ± 0.8% vs. K-nearest neighbors: 89.3 ± 1.0%). Through the use of a multi-algorithm approach, candidate algorithms can be identified and applied to more effectively model complex spectroscopic data collected for wildlife sciences. Other key considerations in the predictive modeling workflow that serve to optimize spectroscopic model performance (e.g., variable selection and cross-validation procedures) are also discussed.

In Vitro Investigation of the Antibacterial Activity of Salamander Skin Peptides

Given the current and future costs of antibiotic-resistant bacteria to human health and economic productivity, there is an urgent need to develop new antimicrobial compounds. Antimicrobial peptides are a promising alternative to conventional antibiotics and other antimicrobials. Amphibian skin is a rich source of bioactive compounds, but the antibacterial properties of salamander skin peptides have been neglected. Here, we examined the in vitro ability of skin peptides from 9 species of salamander representing 6 salamander families to inhibit the growth of ESKAPE pathogens, which are bacteria that have developed resistance to conventional antibiotics. We also examined whether the skin peptides caused lysis of human red blood cells. Skin peptides from Amphiuma tridactylum had the greatest antimicrobial properties, completely inhibiting the growth of all bacterial strains except for Enterococcus faecium. Likewise, skin peptides from Cryptobranchus alleganiensis completely inhibited the growth of several of the bacterial strains. In contrast, skin peptide mixtures from Ambystoma maculatum, Desmognathus fuscus, Eurycea bislineata, E. longicauda, Necturus beyeri, N. maculosus, and Siren intermedia did not completely inhibit bacterial growth even at the highest concentrations. Finally, none of the skin peptide mixtures caused lysis of human red blood cells. Together, we demonstrate that salamander skin produces peptides with potent antibacterial properties. It remains to elucidate the peptide sequences and their antibacterial mechanisms.

Biofluorescent sexual dimorphism revealed in a southern Appalachian endemic salamander, Plethodon metcalfi

Biofluorescence occurs when a living organism absorbs high energy light and reemits it at longer wavelengths. Many species within clades of vertebrates are known to fluoresce including mammals, reptiles, birds, and fish. Most, if not all, amphibians exhibit biofluorescence when exposed to either blue (440-460 nm) or ultra-violet (360-380 nm) wavelengths of light. Salamanders (Lissamphibia: Caudata) appear to consistently fluoresce in green wavelengths (520-560 nm) when excited by blue light. Biofluorescence is theorized to have many ecological functions including mate signaling, camouflage, and mimicry. Despite the discovery of their biofluorescence, its role in salamander ecology and behavior remains unresolved. In this study we present the first case of biofluorescent sexual dimorphism within Amphibia and the first documentation of the biofluorescent pattern of a salamander within the Plethodon jordani species complex. This sexually dimorphic trait was discovered in the southern Appalachian endemic species, Southern Gray-Cheeked Salamander (Plethodon metcalfi, Brimley in Proc Biol Soc Wash 25:135-140, 1912), and may extend into other species within the Plethodon jordani and Plethodon glutinosus species complexes. We propose that this sexually dimorphic trait could be related to fluorescence of ventral modified granular glands used in plethodontid chemosensory communication.

A New Perspective on Female-to-Male Communication in Salamander Courtship

Courtship behavior in salamanders is often complex and involves well-documented communication from males to females in multiple sensory modalities. Historically, behaviors exhibited during the major stages of courtship have been predominately framed as a male acting and signaling to "persuade" a passive female to participate in courtship and remain with him until sperm release is completed. In this review, we use courtship descriptions for lungless salamanders (Plethodontidae) as a case study to illustrate this historical bias of a male-centered perspective. We then re-examine the literature and summarize the many ways females are active participants during plethodontid courtships. We also relate female behaviors to the types of female-to-male communication that may occur. For example, females have been documented to approach a male and initiate courtship, participate in mutual head rubbing, and step astride the male's tail to begin the tail-straddling walk (a key courtship behavior observed in all plethodontids). Additionally, females have glands that may produce chemical signals that males respond to during courtship. We conclude that communication during courtship is more accurately described as a two-way interaction where each partner's behavior is coordinated with the other's via multi-modal signaling. Shifting the lens through which we view courtship and behavior provides insight into which female behaviors and anatomical features are most likely to be used for communication with males.

Pheromonal communication in urodelan amphibians

Pheromonal communication is an ancient and pervasive sensory modality in urodelan amphibians. One family of salamander pheromones (the sodefrin precursor-like factor (SPF) family) originated 300 million years ago, at the origin of amphibians. Although salamanders are often thought of as relatively simple animals especially when compared to mammals, the pheromonal systems are varied and complex with nuanced effects on behavior. Here, we review the function and evolution of pheromonal signals involved in male-female reproductive interactions. After describing common themes of salamander pheromonal communication, we describe what is known about the rich diversity of pheromonal communication in each salamander family. Several pheromones have been described, ranging from simple, invariant molecules to complex, variable blends of pheromones. While some pheromones elicit overt behavioral responses, others have more nuanced effects. Pheromonal signals have diversified within salamander lineages and have experienced rapid evolution. Once receptors have been matched to pheromonal ligands, rapid advance can be made to better understand the olfactory detection and processing of salamander pheromones. In particular, a large number of salamander species deliver pheromones across the skin of females, perhaps reflecting a novel mode of pheromonal communication. At the end of our review, we list some of the many intriguing unanswered questions. We hope that this review will inspire a new generation of scientists to pursue work in this rewarding field.

Emergent functions of proteins in non-stoichiometric supramolecular assemblies

Proteins are the basic functional units of the cell, carrying out myriads of functions essential for life. There are countless reports in molecular cell biology addressing the functioning of proteins under physiological and pathological conditions, aiming to understand life at the atomistic-molecular level and thereby being able to develop remedies against diseases. The central theme in most of these studies is that the functional unit under study is the protein itself. Recent rapid progress has radically challenged and extended this protein-function paradigm, by demonstrating that novel function(s) may emerge when proteins form dynamic and non-stoichiometric supramolecular assemblies. There is an increasing number of cases for such collective functions, such as targeting, localization, protection/shielding and filtering effects, as exemplified by signaling complexes and prions, biominerals and mucus, amphibian adhesions and bacterial biofilms, and a broad range of membraneless organelles (bio-condensates) formed by liquid-liquid phase separation in the cell. In this short review, we show that such non-stoichiometric organization may derive from the heterogeneity of the system, a mismatch in valency and/or geometry of the partners, and/or intrinsic structural disorder and multivalency of the component proteins. Either way, the resulting functional features cannot be simply described by, or predicted from, the properties of the isolated single protein(s), as they belong to the collection of proteins.

Skin glands of an aquatic salamander vary in size and distribution and release antimicrobial secretions effective against chytrid fungal pathogens

Amphibian skin is unique among vertebrate classes, containing a large number of multicellular exocrine glands that vary among species and have diverse functions. The secretions of skin glands contain a rich array of bioactive compounds including antimicrobial peptides (AMPs). Such compounds are important for amphibian innate immune responses and may protect some species from chytridiomycosis, a lethal skin disease caused by the fungal pathogens Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal). While the bioactivity of skin secretions against Bd has been assessed for many amphibian taxa, similar studies are lacking for Bsal, a chytrid fungus that is especially pathogenic for salamanders. We studied the skin glands and their potential functions in an aquatic salamander, the three-toed amphiuma (Amphiuma tridactylum). Skin secretions of captive adult salamanders were analyzed by RP-HPLC and tested against the growth of Bd and Bsal using in vitro assays. We found that compounds within collected skin secretions were similar between male and female salamanders and inhibited the growth of Bd and Bsal. Thus, skin secretions that protect against Bd may also provide protection against Bsal. Histological examination of the skin glands of preserved salamanders revealed the presence of enlarged granular glands concentrated within caudal body regions. A site of potential gland specialization was identified at the tail base and may indicate specialized granular glands related to courtship and communication.

Chemical characterization of the adhesive secretions of the salamander Plethodon shermani (Caudata, Plethodontidae)

Salamanders have developed a wide variety of antipredator mechanisms, including tail autotomy, colour patterns, and noxious skin secretions. As an addition to these tactics, the red-legged salamander (Plethodon shermani) uses adhesive secretions as part of its defensive strategy. The high bonding strength, the fast-curing nature, and the composition of the biobased materials makes salamander adhesives interesting for practical applications in the medical sector. To understand the adhesive secretions of P. shermani, its components were chemically analysed by energy dispersive X-ray spectroscopy (EDX), inductively coupled plasma mass spectrometry (ICP-MS), amino acid analysis, and spectroscopy (ATR-IR, Raman). In addition, proteins were separated by gel-electrophoresis and selected spots were characterised by peptide mass fingerprinting. The salamander secretion contains a high amount of water and predominantly proteins (around 77% in the dry stage). The gel-electrophoresis and peptide mass fingerprint analyses revealed a de novo set of peptides/proteins, largely with a pI between 5.0 and 8.0 and a molecular mass distribution between 10 and 170 kDa. Only low homologies with other proteins present in known databases could be identified. The results indicate that the secretions of the salamander Plethodon clearly differ chemically from those shown for other glue-producing terrestrial or marine species and thus represent a unique glue system.

Morphological characterization of the glandular system in the salamander Plethodon shermani (Caudata, Plethodontidae)

Amphibians have evolved a wide variety of mechanisms that provide a certain degree of protection against predators, including camouflage, tail autonomy, encounter behavior and noxious or toxic skin secretions. In addition to these strategies, some amphibians release a glue-like secretion onto the surface of their skin when threatened. While some information regarding the origin and production of these adhesive secretions is available for frogs such as Notaden bennetti, these aspects are only partially understood in salamanders. We contribute to an earlier study and provide additional information regarding the origin, production, and characterization of the adhesive secretion in the red-legged salamander (Plethodon shermani) at a microanatomical level. When stressed, this salamander secretes a milky, viscous liquid from its dorsal and ventral skin. This secretion is extremely adhesive and hardens within seconds upon exposure to air. This study describes two cutaneous gland types (mucous and granular) in the dorsal and ventral epithelial tissue that differ considerably in their secretory content. While the smaller mucous glands contains flocculent to granular material, mostly acidic glycoproteins, the granular glands synthesize various granules of differing size and density that consist of basic proteinaceous material. The results strongly indicate that the secretions of both gland types from the dorsal as well as the ventral side form the adhesive mucus in Plethodon shermani, consisting of basic and acidic glycoproteins, glycoconjugates with mannose and α-L-fucose residues as well as lipid components.

Chemosignals, hormones, and amphibian reproduction

This article is part of a Special Issue "Chemosignals and Reproduction". Amphibians are often thought of as relatively simple animals especially when compared to mammals. Yet the chemosignaling systems used by amphibians are varied and complex. Amphibian chemosignals are particularly important in reproduction, in both aquatic and terrestrial environments. Chemosignaling is most evident in salamanders and newts, but increasing evidence indicates that chemical communication facilitates reproduction in frogs and toads as well. Reproductive hormones shape the production, dissemination, detection, and responsiveness to chemosignals. A large variety of chemosignals have been identified, ranging from simple, invariant chemosignals to complex, variable blends of chemosignals. Although some chemosignals elicit straightforward responses, others have relatively subtle effects. Review of amphibian chemosignaling reveals a number of issues to be resolved, including: 1) the significance of the complex, individually variable blends of courtship chemosignals found in some salamanders, 2) the behavioral and/or physiological functions of chemosignals found in anuran "breeding glands", 3) the ligands for amphibian V2Rs, especially V2Rs expressed in the main olfactory epithelium, and 4) the mechanism whereby transdermal delivery of chemosignals influences behavior. To date, only a handful of the more than 7000 species of amphibians has been examined. Further study of amphibians should provide additional insight to the role of chemosignals in reproduction.

The adhesive skin exudate of Notaden bennetti frogs (Anura: Limnodynastidae) has similarities to the prey capture glue of Euperipatoides sp. velvet worms (Onychophora: Peripatopsidae)

The dorsal adhesive secretion of the frog Notaden bennetti and the prey-capture "slime" ejected by Euperipatoides sp. velvet worms look and handle similarly. Both consist largely of protein (55-60% of dry weight), which provides the structural scaffold. The major protein of the onychophoran glue (Er_P1 for Euperipatoides rowelli) and the dominant frog glue protein (Nb-1R) are both very large (260-500 kDa), and both give oddly "turbulent" electrophoresis bands. Both major proteins, which are rich in Gly (16-17 mol%) and Pro (7-12 mol%) and contain 4-hydroxyproline (Hyp, 4 mol%), have the composition of intrinsically unstructured proteins. Their propensities for elastomeric or amyloid structures are discussed in light of Er_P1's large content of intrinsically disordered long tandem repeats. The low carbohydrate content of both glues is consistent with conventional protein glycosylation, which in the N. bennetti adhesive was explored by 2D PAGE. The N-linked sugars of Nb-1R appear to prevent inappropriate self-aggregation. Some peptide sequences from Nb-1R are presented. Overall, there are enough similarities between the frog and the velvet worm glues to suspect that they employ related mechanisms for setting and adhesion. A common paradigm is proposed for amphibian and onychophoran adhesives, which, if correct, points to convergent evolution.

Pheromonal communication in amphibians

Pheromonal communication is widespread in salamanders and newts and may also be important in some frogs and toads. Several amphibian pheromones have been behaviorally, biochemically and molecularly identified. These pheromones are typically peptides or proteins. Study of pheromone evolution in plethodontid salamanders has revealed that courtship pheromones have been subject to continual evolutionary change, perhaps as a result of co-evolution between the pheromonal ligand and its receptor. Pheromones are detected by the vomeronasal organ and main olfactory epithelium. Chemosensory neurons express vomeronasal receptors or olfactory receptors. Frogs have relatively large numbers of vomeronasal receptors that are transcribed in both the vomeronasal organ and the main olfactory epithelium. Salamander vomeronasal receptors apparently are restricted to the vomeronasal organ. To date, no chemosensory ligands have been matched to vomeronasal receptors or olfactory receptors so it is unknown whether particular receptor types are (1) specialized for detection of pheromones versus other chemosignals, or (2) specialized for detection of volatile, nonvolatile, or water-borne chemosignals. Despite progress in understanding amphibian pheromonal communication, only a small fraction of amphibian species have been examined. Study of additional species of amphibians will indicate which traits related to pheromonal communication are evolutionarily conserved and which traits have diverged over time.

Exposure to pheromones increases plasma corticosterone concentrations in a terrestrial salamander

Sensory cues involved in social interactions can influence plasma steroid hormone concentrations. Although pheromonal communication is common in amphibians, it is unknown whether pheromones can alter hormone levels in amphibians as they do in mammals. We tested whether courtship pheromones would alter steroid hormone concentrations in male and female terrestrial salamanders (Plethodon shermani). Plasma corticosterone concentrations were elevated in male salamanders exposed to mental gland courtship pheromones, as compared to males exposed to female skin secretions or a saline control. Chemosensory cues had no effect on testosterone levels in males or on corticosterone or estradiol levels in females. These results provide the first evidence that pheromones have priming effects on the endocrine system in amphibians.