Developmental morphology of granular skin glands in pre-metamorphic egg-eating poison frogs

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.

Intrinsic Factors Associated with Dietary Toxin Quantity and Concentration in the Nuchal Glands of a Natricine Snake Rhabdophis Tigrinus

The snake Rhabdophis tigrinus sequesters cardiotonic steroids, bufadienolides (BDs), from ingested toads and stores them in the nuchal glands as defensive toxins. It has previously been shown that there are individual differences in the total quantity of BDs stored in the nuchal glands of adult R. tigrinus and that BD quantities and profiles of R. tigrinus exhibit geographic variation. However, no previous study has examined the total quantity of BDs as a percentage of body mass (relative BD quantity) and the concentration of BDs in the nuchal gland fluid (BD gland concentration). In addition, intrinsic factors that are associated with relative BD quantity and BD concentration have not been examined within a single population. We collected 158 adult snakes from an area of central Japan from May to October and analyzed their BD quantities by UV analysis. We assessed individual differences in BD quantity, relative BD quantity and BD gland concentration. We found that 1) in approximately 60% of the 158 individuals, the BD gland concentration was greater than 50%; 2) body length and body condition are positively correlated with relative BD quantity and BD gland concentration; 3) even in a single population, individual differences of BD quantity are large, and are greater in females than in males; and 4) relative BD quantity and BD gland concentration of females during the gestation season are lower than those during the non-gestation season.

Maternal chemical defenses predict offspring defenses in a dendrobatid poison frog

Within and among populations, alkaloid defenses of the strawberry poison frog (Oophaga pumilio) vary spatially, temporally, and with life history stage. Natural variation in defense has been implicated as a critical factor in determining the level of protection afforded against predators and pathogens. Oophaga pumilio tadpoles sequester alkaloids from nutritive eggs and are, thus, entirely dependent on their mothers for their defense. However, it remains unclear how tadpole alkaloid composition relates to that of its mother and how variation in maternally provisioned defenses might result in varying levels of protection against predators. Here, we demonstrate that natural variation in the alkaloid composition of a mother frog is reflected as variation in her tadpole's alkaloid composition. Tadpoles, like mother frogs, varied in their alkaloid composition but always contained the identical alkaloids found in their mother. Alkaloid quantity in tadpoles was highly correlated with alkaloid quantity in their mothers. Additionally, alkaloid quantity was the best predictor of tadpole palatability, wherein tadpoles with higher alkaloid quantities were less palatable. Mother frogs with greater quantities of alkaloids are, thus, providing better protection for their offspring by provisioning chemical defenses during one of the most vulnerable periods of life.

Maternal Provisioning of Alkaloid Defenses are Present in Obligate but not Facultative Egg Feeding Dendrobatids

Poison frogs sequester alkaloid defenses from a diet of largely mites and ants. As a result, frogs are defended against certain predators and microbial infections. Frogs in the genus Oophaga exhibit complex maternal care, wherein mothers transport recently hatched tadpoles to nursery pools and return regularly to supply developing tadpoles with unfertilized (nutritive) eggs. Developing tadpoles are obligate egg feeders. Further, female O. pumilio and O. sylvatica maternally provision their nutritive eggs with alkaloid defenses, providing protection to their developing tadpoles at a vulnerable life-stage. In another genus of poison frog, Ranitomeya, tadpoles only receive and consume eggs facultatively, and it is currently unknown if mothers also provision these eggs (and thus their tadpoles) with alkaloid defenses. Here, we provide evidence that mother frogs of another species in the genus Oophaga (Oophaga granulifera) also provision alkaloid defenses to their tadpoles. We also provide evidence that Ranitomeya imitator and R. variabilis eggs and tadpoles do not contain alkaloids, suggesting that mother frogs in this genus do not provision alkaloid defenses to their offspring. Our findings suggest that among dendrobatid poison frogs, maternal provisioning of alkaloids may be restricted to the obligate egg-feeding members of Oophaga.

Meta-analysis of tadpole taste tests: consumption of anuran prey across development and predator strategies

The risk of predation and the costs and benefits of diverse anti-predator strategies can shift across the life stages of an organism. Yet, empirical examples of ontogenetic switches in defense mechanisms are scarce. Anurans represent an alleged exception; previous meta-analytic work suggests that unpalatability of developing anurans is "rare", whereas adult anurans in many lineages are well defended by toxic and/or unpalatable skin secretions. Here, we revisit the question of the unpalatability of anuran young in a meta-analysis of the relative proportion of prey consumed within 922 predation tests, including 135 anuran species. We tested the hypotheses that a predator's propensity to consume anuran young depends on (1) prey family, (2) predator manipulation strategy, and (3) prey ontogenetic stage. We used a binomial mixed model approach with considerations for multiple effect sizes within studies to evaluate the log odds ratio of the proportion of prey consumed by individual predators. Prey consumption was highly variable, but toads (Bufonidae) were consumed in lower proportions. Chewing invertebrates consumed more anuran prey than biting vertebrates. Late stage tadpoles were more vulnerable to predation than other stages of anuran ontogeny. However, more studies are needed to unravel the roles of development and evolutionary history in the chemical ecology of anuran young. This synthesis provides clear meta-analytic evidence that relative unpalatability is an important component in the anti-predator defenses of young in some anuran families, calling into question the degree to which chemically defended anuran families undergo ontogenetic switches in anti-predator strategies.

Experimental evidence for maternal provisioning of alkaloid defenses in a dendrobatid frog

Dendrobatid frogs sequester alkaloid defenses from dietary arthropods. Here, we provide experimental evidence that mother strawberry poison frogs (Oophaga pumilio) provision alkaloids to tadpoles. Captive-raised females were fed the synthetic alkaloid decahydroquinoline (DHQ), which we subsequently quantified in their skin, eggs, and developing tadpoles. DHQ quantity was positively associated with tadpole mass/development, suggesting high sequestration rates by tadpoles. These data confirm that tadpoles obtain nutrition and alkaloids by feeding exclusively on maternally provisioned eggs.

Frog Skin Innate Immune Defences: Sensing and Surviving Pathogens

Amphibian skin is a mucosal surface in direct and continuous contact with a microbially diverse and laden aquatic and/or terrestrial environment. As such, frog skin is an important innate immune organ and first line of defence against pathogens in the environment. Critical to the innate immune functions of frog skin are the maintenance of physical, chemical, cellular, and microbiological barriers and the complex network of interactions that occur across all the barriers. Despite the global decline in amphibian populations, largely as a result of emerging infectious diseases, we understand little regarding the cellular and molecular mechanisms that underlie the innate immune function of amphibian skin and defence against pathogens. In this review, we discuss the structure, cell composition and cellular junctions that contribute to the skin physical barrier, the antimicrobial peptide arsenal that, in part, comprises the chemical barrier, the pattern recognition receptors involved in recognizing pathogens and initiating innate immune responses in the skin, and the contribution of commensal microbes on the skin to pathogen defence. We briefly discuss the influence of environmental abiotic factors (natural and anthropogenic) and pathogens on the immunocompetency of frog skin defences. Although some aspects of frog innate immunity, such as antimicrobial peptides are well-studied; other components and how they contribute to the skin innate immune barrier, are lacking. Elucidating the complex network of interactions occurring at the interface of the frog's external and internal environments will yield insight into the crucial role amphibian skin plays in host defence and the environmental factors leading to compromised barrier integrity, disease, and host mortality.