Amphibians used in research and teaching

Amphibian myelopoiesis

Macrophage-lineage cells are indispensable to immunity and physiology of all vertebrates. Amongst these, amphibians represent a key stage in vertebrate evolution and are facing decimating population declines and extinctions, in large part due to emerging infectious agents. While recent studies indicate that macrophages and related innate immune cells are critically involved during these infections, much remains unknown regarding the ontogeny and functional differentiation of these cell types in amphibians. Accordingly, in this review we coalesce what has been established to date about amphibian blood cell development (hematopoiesis), the development of key amphibian innate immune cells (myelopoiesis) and the differentiation of amphibian macrophage subsets (monopoiesis). We explore the current understanding of designated sites of larval and adult hematopoiesis across distinct amphibian species and consider what mechanisms may lend to these species-specific adaptations. We discern the identified molecular mechanisms governing the functional differentiation of disparate amphibian (chiefly Xenopus laevis) macrophage subsets and describe what is known about the roles of these subsets during amphibian infections with intracellular pathogens. Macrophage lineage cells are at the heart of so many vertebrate physiological processes. Thus, garnering greater understanding of the mechanisms responsible for the ontogeny and functionality of these cells in amphibians will lend to a more comprehensive view of vertebrate evolution.

Differential sensitivity of aquatic life stages of Pelophylax perezi to an acidic metal-contaminated effluent

Acid mine drainage (AMD) involves complex mixtures of metals and hydrogen ions that can be highly toxic to the biota. Assessing the effects of AMD to aquatic stages of amphibians is key, as this group constitutes the vertebrate class with the highest proportion of species considered as threatened. Thus, the present work aimed at assessing the sensitivity of two aquatic life stages of the green frog Pelophylax perezi to an AMD originated from a cupric-pyrite mine. Embryos (Gosner stages 10-11) and tadpoles (Gosner stages 20-21) of P. perezi were exposed, for 96 h, to six AMD dilutions (1.39% to 7.5%). Endpoints involving responses at different levels of biological organization were monitored: mortality, malformations, hatching rates, body length and growth rate, enzymatic activity related with detoxification metabolism (glutathione S-transferase), and histopathologies (anatomical structures of the digestive, respiratory, and excretory systems). Embryos presented high mortality and malformation rates at AMD levels equal or above 5.36%, as well as premature hatching at 1.95% of AMD or higher. A significant reduction in body length and growth rate occurred in embryos and tadpoles exposed to 1.95% or higher levels of AMD, respectively. At the histological level, several abnormalities were observed for AMD-exposed tadpoles in a variety of tissues. One of the most noticeable histological changes occurred in the intestine that exhibited papillary epithelial hyperplasia along with a yellowish content and was more pronounced in tadpoles exposed to higher AMD levels. FEmbryos were more sensitive to lethal levels of AMD than tadpoles, suggesting embryos as a useful model life stage when performing amphibian risk assessment of mine drainage. Furthermore, AMD was highly toxic for P. perezi aquatic life stages since levels as low as 1.95% induced lethal effects. These results emphasize the importance of implementing efficient remediation methodologies for AMD, given its high toxicity.

Prevalence of Batrachochytrium dendrobatidis in Amphibians From 2000 to 2021: A Global Systematic Review and Meta-Analysis

Chytridiomycosis is an amphibian fungal disease caused by Batrachochytrium dendrobatidis (Bd), which has caused large-scale death and population declines on several continents around the world. To determine the current status of Bd infection in amphibians, we conducted a global meta-analysis. Using PubMed, ScienceDirect, SpringerLink, China National Knowledge Infrastructure (CNKI) and Wanfang database searches, we retrieved a total of 111 articles from 2000 to 2021. Based on these, we estimated the Bd prevalence to be 18.54% (95% CI: 13.76–20.52) in current extent amphibians. Among these populations, the prevalence of Bd in Asia was the lowest at 7.88% (95% CI: 1.92–8.71). Further, no Bd infection was found in Vietnam. However, the prevalence of Bd in Oceania was the highest at 36.34% (95% CI: 11.31–46.52). The Bd prevalence in Venezuela was as high as 49.77% (95% CI: 45.92–53.62). After 2009, the global Bd prevalence decreased to 18.91% (95% CI: 13.23–21.56). The prevalence of Bd in epizootic populations was significantly higher than enzootic populations. The highest prevalence of Bd was detected with real-time PCR at 20.11% (95% CI: 13.12–21.38). The prevalence of Bd in frogs was the highest at 20.04% (95% CI: 13.52–21.71), and this different host was statistically significant (P < 0.05). At the same time, we analyzed the geographic factors (longitude, latitude, elevation, rainfall and temperature) that impacted the fungal prevalence in amphibians. Our meta-analysis revealed that factors including region, disease dynamic, detection method, host and climate may be sources of the observed heterogeneity. These results indicate that chytridiomycosis was a consistent threat to amphibians from 2000 to 2021. Based on different habitat types and geographical conditions, we recommend formulating corresponding control plans and adopting reasonable and efficient biological or chemical methods to reduce the severity of such diseases.

Understanding the codon usage patterns of mitochondrial CO genes among Amphibians

A universal phenomenon of using synonymous codons unequally in coding sequences known as codon usage bias (CUB) is observed in all forms of life. Mutation and natural selection drive CUB in many species but the relative role of evolutionary forces varies across species, genes and genomes. We studied the CUB in mitochondrial (mt) CO genes from three orders of Amphibia using bioinformatics approach as no work was reported yet. We observed that CUB of mt CO genes of Amphibians was weak across different orders. Order Caudata had higher CUB followed by Gymnophiona and Anura for all genes and CUB also varied across genes. Nucleotide composition analysis showed that CO genes were AT-rich. The AT content in Caudata was higher than that in Gymnophiona while Anura showed the least content. Multiple investigations namely nucleotide composition, correspondence analysis, parity plot analysis showed that the interplay of mutation pressure and natural selection caused CUB in these genes. Neutrality plot suggested the involvement of natural selection was more than the mutation pressure. The contribution of natural selection was higher in Anura than Gymnophiona and the lowest in Caudata. The codons CGA, TGA, AAA were found to be highly favoured by nature across all genes and orders.

Bioinformatic Analysis of 1000 Amphibian Antimicrobial Peptides Uncovers Multiple Length-Dependent Correlations for Peptide Design and Prediction

Amphibians are widely distributed on different continents, except for the polar regions. They are important sources for the isolation, purification and characterization of natural compounds, including peptides with various functions. Innate immune antimicrobial peptides (AMPs) play a critical role in warding off invading pathogens, such as bacteria, fungi, parasites, and viruses. They may also have other biological functions such as endotoxin neutralization, chemotaxis, anti-inflammation, and wound healing. This article documents a bioinformatic analysis of over 1000 amphibian antimicrobial peptides registered in the Antimicrobial Peptide Database (APD) in the past 18 years. These anuran peptides were discovered in Africa, Asia, Australia, Europe, and America from 1985 to 2019. Genomic and peptidomic studies accelerated the discovery pace and underscored the necessity in establishing criteria for peptide entry into the APD. A total of 99.9% of the anuran antimicrobial peptides are less than 50 amino acids with an average length of 24 and a net charge of +2.5. Interestingly, the various amphibian peptide families (e.g., temporins, brevinins, esculentins) can be connected through multiple length-dependent relationships. With an increase in length, peptide net charge increases, while the hydrophobic content decreases. In addition, glycine, leucine, lysine, and proline all show linear correlations with peptide length. These correlations improve our understanding of amphibian peptides and may be useful for prediction and design of new linear peptides with potential applications in treating infectious diseases, cancer and diabetes.

Commonly Used Animal Models

This chapter provides an introduction to animals that are commonly used for research. It presents information on basic care topics such as biology, behavior, housing, feeding, sexing, and breeding of these animals. The chapter provides some insight into the reasons why these animals are used in research. It also gives an overview of techniques that can be utilized to collect blood or to administer drugs or medicine. Each section concludes with a brief description of how to recognize abnormal signs, in addition to lists of various diseases.

Effects of Transgenic cry1Ca Rice on the Development of Xenopus laevis

In fields of genetically modified, insect-resistant rice expressing Bacillus thuringiensis (Bt) proteins, frogs are exposed to Bt Cry proteins by consuming both target and non-target insects, and through their highly permeable skin. In the present study, we assessed the potential risk posed by transgenic cry1Ca rice (T1C-19) on the development of a frog species by adding purified Cry1Ca protein or T1C-19 rice straw into the rearing water of Xenopus laevis tadpoles, and by feeding X. laevis froglets diets containing rice grains of T1C-19 or its non-transformed counterpart MH63. Our results showed that there were no significant differences among groups receiving 100 μg L^–1 or 10 μg L^–1 Cry1Ca and the blank control in terms of time to completed metamorphosis, survival rate, body weight, body length, organ weight and liver enzyme activity after being exposed to the Cry1Ca (P > 0.05). Although some detection indices in the rice straw groups were significantly different from those of the blank control group (P < 0.05), there was no significant difference between the T1C-19 and MH63 rice straw groups. Moreover, there were no significant differences in the mortality rate, body weight, daily weight gain, liver and fat body weight of the froglets between the T1C-19 and MH63 dietary groups after 90 days, and there were no abnormal pathological changes in the stomach, intestines, livers, spleens and gonads. Thus, we conclude that the planting of transgenic cry1Ca rice will not adversely affect frog development.

Metamorphic remodeling of the olfactory organ of the African clawed frog, Xenopus laevis

The amphibian olfactory system undergoes massive remodeling during metamorphosis. The transition from aquatic olfaction in larvae to semiaquatic or airborne olfaction in adults requires anatomical, cellular, and molecular modifications. These changes are particularly pronounced in Pipidae, whose adults have secondarily adapted to an aquatic life style. In the fully aquatic larvae of Xenopus laevis, the main olfactory epithelium specialized for sensing water-borne odorous substances lines the principal olfactory cavity (PC), whereas a separate olfactory epithelium lies in the vomeronasal organ (VNO). During metamorphosis, the epithelium of the PC is rearranged into the adult "air nose," whereas a new olfactory epithelium, the adult "water nose," forms in the emerging middle cavity (MC). Here we performed a stage-by-stage investigation of the anatomical changes of the Xenopus olfactory organ during metamorphosis. We quantified cell death in all olfactory epithelia and found massive cell death in the PC and the VNO, suggesting that the majority of larval sensory neurons is replaced during metamorphosis in both sensory epithelia. The moderate cell death in the MC shows that during the formation of this epithelium some cells are sorted out. Our results show that during MC formation some supporting cells, but not sensory neurons, are relocated from the PC to the MC and that they are eventually eliminated during metamorphosis. Together our findings illustrate the structural and cellular changes of the Xenopus olfactory organ during metamorphosis.

Microbiota and mucosal immunity in amphibians

We know that animals live in a world dominated by bacteria. In the last 20 years, we have learned that microbes are essential regulators of mucosal immunity. Bacteria, archeas, and viruses influence different aspects of mucosal development and function. Yet, the literature mainly covers findings obtained in mammals. In this review, we focus on two major themes that emerge from the comparative analysis of mammals and amphibians. These themes concern: (i) the structure and functions of lymphoid organs and immune cells in amphibians, with a focus on the gut mucosal immune system; and (ii) the characteristics of the amphibian microbiota and its influence on mucosal immunity. Lastly, we propose to use Xenopus tadpoles as an alternative small-animal model to improve the fundamental knowledge on immunological functions of gut microbiota.

Cytochemical and immunocytochemical characterization of blood cells and immunohistochemical analysis of spleen cells from 2 species of frog, Rana (Aquarana) catesbeiana and Xenopus laevis

BACKGROUND

Mechanisms of amphibian diseases are not characterized as well as those in domestic mammalian species. Antemortem laboratory testing is limited in frogs, presenting a diagnostic challenge to zoos, laboratories, and exotic veterinarians.

OBJECTIVE

This study aimed to characterize blood cells and splenic cells from 2 anuran species based on characteristics identified by Wright staining, cytochemical staining, and immunochemical analysis and on histologic examination of spleens.

METHODS

Blood specimens and spleens were obtained from 2 species of frog, the American bullfrog (Rana [Aquarana] catesbeiana) and the African clawed frog (Xenopus laevis). Blood smears were evaluated after Wright staining and cytochemical staining for α-naphthyl butyrate esterase (NBE), chloroacetate esterase (CAE), myeloperoxidase (PER), Sudan black B (SBB), and leukocyte alkaline phosphatase (LAP) reactions and for immunoreactivity for antibodies against CD3ε, CD79a, and BLA.36 antigens. Histologic sections of spleen were evaluated after staining with H&E and for immunoreactivity for CD3ε, CD79a, and BLA.36 antigens.

RESULTS

In bullfrogs, neutrophils, eosinophils, and monocytes were positive for some or all of the following: NBE, CAE, PER, and SBB; lymphocytes occasionally were positive for CAE. In clawed frogs, neutrophils, basophils, and monocytes were positive for some or all of the following: NBE, CAE, PER, and SBB; eosinophils occasionally were positive for CAE and PER, and lymphocytes were negative for all cytochemical stains. LAP was not a useful marker for any leukocyte type. In both species, peripheral blood lymphocytes were strongly immunoreactive for CD3ε, CD79a, and BLA.36. In splenic tissue, histologic patterns varied and there was diffuse immunoreactivity for CD79a and BLA.36 with focal reactivity for CD3ε, but with different distribution patterns in each species.

CONCLUSION

Cytochemical and immunochemical analysis of cells may be helpful in identification and characterization of amphibian blood cells and splenic cells for evaluation of the health of these animals.

Amphibian Biology and Husbandry

Extant amphibians comprise three lineages-- salamanders (Urodela or Caudata), frogs and toads (Anura), and caecilians (Gymnophiona, Apoda, or Caecilia)--which contain more than 6,000 species. Fewer than a dozen species of amphibians are commonly maintained in laboratory colonies, and the husbandry requirements for the vast majority of amphibians are poorly known. For these species, a review of basic characteristics of amphibian biology supplemented by inferences drawn from the morphological and physiological characteristics of the species in question provides a basis for decisions about housing and feeding. Amphibians are ectotherms, and their skin is permeable to water, ions, and respiratory gases. Most species are secretive and, in many cases, nocturnal. The essential characteristics of their environment include appropriate levels of humidity, temperature, and lighting as well as retreat sites. Terrestrial and arboreal species require moist substrates, water dishes, and high relative humidity. Because temperature requirements for most species are poorly known, it is advisable to use a temperature mosaic that will allow an animal to find an appropriate temperature within its cage. Photoperiod may affect physiology and behavior (especially reproduction and hibernation), and although the importance of ultraviolet light for calcium metabolism by amphibians is not yet known, ecological observations suggest that it might be important for some species of frogs. Some amphibians are territorial, and some use olfactory cues to mark their territory and to recognize other individuals of their species. All amphibians are carnivorous as adults, and the feeding response of many species is elicited by the movement of prey. Diets should include a mixture of prey species, and it may be advisable to load prey with vitamins and minerals.

Amphibians as Animal Models for Laboratory Research in Physiology

The concept of animal models is well honored, and amphibians have played a prominent part in the success of using key species to discover new information about all animals. As animal models, amphibians offer several advantages that include a well-understood basic physiology, a taxonomic diversity well suited to comparative studies, tolerance to temperature and oxygen variation, and a greater similarity to humans than many other currently popular animal models. Amphibians now account for approximately 1/4 to 1/3 of lower vertebrate and invertebrate research, and this proportion is especially true in physiological research, as evident from the high profile of amphibians as animal models in Nobel Prize research. Currently, amphibians play prominent roles in research in the physiology of musculoskeletal, cardiovascular, renal, respiratory, reproductive, and sensory systems. Amphibians are also used extensively in physiological studies aimed at generating new insights in evolutionary biology, especially in the investigation of the evolution of air breathing and terrestriality. Environmental physiology also utilizes amphibians, ranging from studies of cryoprotectants for tissue preservation to physiological reactions to hypergravity and space exploration. Amphibians are also playing a key role in studies of environmental endocrine disruptors that are having disproportionately large effects on amphibian populations and where specific species can serve as sentinel species for environmental pollution. Finally, amphibian genera such as Xenopus, a genus relatively well understood metabolically and physiologically, will continue to contribute increasingly in this new era of systems biology and "X-omics."