Population-Level Resistance to Chytridiomycosis is Life-Stage Dependent in an Imperiled Anuran

Amphibian declines caused by chytridiomycosis have been severe, but some susceptible populations have persisted or even recovered. Resistance to the causal agent Batrachochytrium dendrobatidis (Bd) could result from alleles of the adaptive immune system. During metamorphosis, however, immune systems may not be fully functional, implying that an effective immune response to Bd may be life-stage dependent. We evaluated the susceptibility of the relict leopard frog (Rana onca) sourced from two areas where Bd was present or absent, and where the populations appeared to show differences in pathogen resistance. We evaluated whether population-level resistance manifested across life stages using challenge experiments with late-stage tadpoles (Gosner stage 31-38), metamorphs (stage 45-46), and juvenile frogs. We used three different Bd isolates including one from wild R. onca to challenge juvenile frogs and focused on the isolate from R. onca to challenge tadpoles and resulting metamorphs. We found that juveniles from the Bd exposed population were 5.5 times more likely to survive Bd infection and 10 times more likely to clear infections than those from the area without Bd. In contrast, and regardless of the source area, we observed 98% survivorship of tadpoles, but only 19% survivorship of resulting metamorphs following re-exposure. Given the low survivorship of exposed metamorphs in the laboratory, we speculate on how resistance characteristics, whether adaptive or innate, that do not manifest at each life stage could develop in the wild. We suggest that seasonal high temperatures during times when metamorphosis appears common may modulate the effects of the pathogen during this most susceptible life stage.

Skin innate immunity of diskless-fingered odorous frogs (Odorrana grahami) with spatial-temporal variations

The skin innate immunities of diskless-fingered odorous frogs (Odorrana grahami) from three populations were investigated. The antimicrobial capacities of skin secretions against the 60 representative environmental bacterial strains were evaluated using the values of the minimum inhibitory concentration (MIC) equivalents, which were defined as the volumes of antimicrobial solution just inhibiting the tested bacteria per 1 cm² of surface area, from 0.06 to 9.10 mL/cm². Our results revealed significantly different skin antimicrobial capacities among the three populations: Mianning < Huili < Kunming. Within the frog population, the skin antimicrobial capacities are highly variable depending on the season: in Mianning frogs, summer < autumn and spring; in Huili frogs, spring < autumn < summer; in Kunming frogs, autumn < spring < summer. The animal density and body mass significantly impacted the skin antimicrobial capacity, while the sex ratio and soil or water bacterial counts did not.

Signatures of historical selection on MHC reveal different selection patterns in the moor frog (Rana arvalis)

MHC genes are key components in disease resistance and an excellent system for studying selection acting on genetic variation in natural populations. Current patterns of variation in MHC genes are likely to be influenced by past and ongoing selection as well as demographic fluctuations in population size such as those imposed by post-glacial recolonization processes. Here, we investigated signatures of historical selection and demography on an MHC class II gene in 12 moor frog populations along a 1700-km latitudinal gradient. Sequences were obtained from 207 individuals and consecutively assigned into two different clusters (northern and southern clusters, respectively) in concordance with a previously described dual post-glacial colonization route. Selection analyses comparing the relative rates of non-synonymous to synonymous substitutions (dN/dS) suggested evidence of different selection patterns in the northern and the southern clusters, with divergent selection prevailing in the south but uniform positive selection predominating in the north. Also, models of codon evolution revealed considerable differences in the strength of selection: The southern cluster appeared to be under strong selection while the northern cluster showed moderate signs of selection. Our results indicate that the MHC alleles in the north diverged from southern MHC alleles as a result of differential selection patterns.

Effects of road salt on larval amphibian susceptibility to parasitism through behavior and immunocompetence

Large quantities of road salts are used for de-icing in temperate climates but often leach into aquatic ecosystems where they can cause harm to inhabitants, including reduced growth and survival. However, the implications of road salt exposure for aquatic animal susceptibility to pathogens and parasites have not yet been examined even though infectious diseases can significantly contribute to wildlife population declines. Through a field survey, we found a range of NaCl concentrations (50-560mg/L) in ponds known to contain larval amphibians, with lower levels found in sites close to gravel- rather than hard-surfaced roads. We then investigated how chronic exposure to environmentally-realistic levels of road salt (up to 1140mg/L) affected susceptibility to infection by trematode parasites (helminths) in larval stages of two amphibian species (Lithobates sylvaticus - wood frogs, and L. pipiens - northern leopard frogs) by considering effects on host anti-parasite behavior and white blood cell profiles. Wood frogs exposed to road salt had higher parasite loads, and also exhibited reduced anti-parasite behavior in these conditions. In contrast, infection intensity in northern leopard frogs had a non-monotonic response to road salts even though lymphocytes were only elevated at the highest concentration. Our results indicate the potential for chronic road salt exposure to affect larval amphibian susceptibility to pathogenic parasites through alterations of behavior and immunocompetence, with further studies needed at higher concentrations, as well as that of road salts on free-living parasite infectious stages.

Heavy metal mediated innate immune responses of the Indian green frog, Euphlyctis hexadactylus (Anura: Ranidae): Cellular profiles and associated Th1 skewed cytokine response

Immune cell and cytokine profiles in relation to metal exposure though much studied in mammals has not been adequately investigated in amphibians, due mainly to lack of suitable reagents for cytokine profiling in non-model species. However, interspecies cross reactivity of cytokines permitted us to assay levels of IFNγ, TNFα, IL6 and IL10in a common anuran, the Indian green frog (Euphlyctis hexadactylus), exposed to heavy metals (Cd, Cr, Cu, Zn and Pb, at ~5ppm each) under field and laboratory settings in Sri Lanka. Enumeration of immune cells in blood and melanomacrophages in the liver, assay of serum and hepatic cytokines, and Th1/Th2 cytokine polarisation were investigated. Immune cell counts indicated overall immunosuppression with decreasing total WBC and splenocyte counts while neutrophil/lymphocyte ratio increased with metal exposure, indicating metal mediated stress. Serum IL6 levels of metal exposed frogs reported the highest (~9360pg/mL) of all cytokines tested. Significantly elevated IFNγ production (P<0.05) was evident in heavy metal exposed frogs. Th1/Th2 cytokine ratio in both serum and liver tissue homogenates was Th1 skewed due to significantly higher production of pro-inflammatory cytokines, IFNγ in serum and TNFα in the liver (P<0.01).Metal mediated aggregations of melanomacrophages in the liver were positively and significantly (P<0.05) correlated with the hepatic expression of TNFα, IL6 and IL10 activity. Overall, Th1 skewed response may well be due to oxidative stress mediated nuclear factor κ-light chain enhancer of activated B cells (NFκB) which enhances the transcription of pro-inflammatory cytokines. Xenobiotic stress has recently imposed an unprecedented level of threat to wildlife, particularly to sensitive species such as amphibians. Therefore, understanding the interactions between physiological stress and related immune responses is fundamental to conserve these environmental sentinels in the face of emerging eco-challenges.

Development of antimicrobial peptide defenses of southern leopard frogs, Rana sphenocephala, against the pathogenic chytrid fungus, Batrachochytrium dendrobatidis

Amphibian species face the growing threat of extinction due to the emerging fungal pathogen Batrachochytrium dendrobatidis, which causes the disease chytridiomycosis. Antimicrobial peptides (AMPs) produced in granular glands of the skin are an important defense against this pathogen. Little is known about the ontogeny of AMP production or the impact of AMPs on potentially beneficial symbiotic skin bacteria. We show here that Rana (Lithobates) sphenocephala produces a mixture of four AMPs with activity against B. dendrobatidis, and we report the minimum inhibitory concentration (MIC) of synthesized replicates of these four AMPs tested against B. dendrobatidis. Using mass spectrometry and protein quantification assays, we observed that R. sphenocephala does not secrete a mature suite of AMPs until approximately 12 weeks post-metamorphosis, and geographically disparate populations produce a different suite of peptides. Use of norepinephrine to induce maximal secretion significantly reduced levels of culturable skin bacteria.

Genetic polymorphism of major histocompatibility complex class IIB alleles and pathogen resistance in the giant spiny frog Quasipaa spinosa

Major histocompatibility complex (MHC) genes are candidates for determining disease susceptibility due to their pivotal role in both innate and adaptive immune responses. Accordingly, the association between the genetic variation of MHC genes and the pathogen resistance has been investigated in numerous vertebrates. To date, however, little is reported in amphibians. In this study, we investigate the genetic variation at the MHC class IIB gene in the giant spiny frog Quasipaa spinosa, which has high commercial value in China. The full length of MHC class IIB cDNA was cloned from Q. spinosa by homology cloning and rapid amplification of cDNA end-polymerase chain reaction (RACE-PCR). Two MHC class IIB loci were identified in Q. spinosa. We also developed PCR primers for a portion of the second exon of the MHC class IIB gene. A total of 26 MHC class IIB alleles were identified. The dN rate was significantly higher than the dS rate in the putative peptide-binding region, thereby proving the positive selection hypothesis. In addition, individuals intraperitoneally injected with Aeromonas hydrophila were used to study the association between MHC class IIB alleles and pathogen resistance/susceptibility, to explore the specific alleles in balancing selection. Eighty frogs were used after exposure to A. hydrophila infection. Nine alleles were used to study the association between the alleles and disease resistance. Two alleles, namely, Pasa-DAB(∗)1301 and Pasa-DAB(∗)0901, were significantly associated with resistance against A. hydrophila. This study provides valuable information on the structure of the MHC class IIB gene and confirms the association between MHC class IIB gene alleles and disease resistance to bacterial infection in Q. spinosa. Moreover, pathogen resistance-related MHC markers can be used for the selective breeding of the giant spiny frog.

Conservation and divergence in the frog immunome: pyrosequencing and de novo assembly of immune tissue transcriptomes

BACKGROUND

Frogs are a diverse group of vertebrates for which limited genomic resources are available. Natural frog populations face a multitude of threats, including habitat degradation, infectious disease, and environmental change. Characterizing the functional genomics of anuran tissues in general - and the immune system in particular - will enhance our knowledge of genetic and epigenetic responses to environmental threats and inform conservation and recovery efforts.

RESULTS

To increase the number of species with genomic datasets and characterize gene expression in immune-related tissues, we sequenced the transcriptomes of three tissues from two frogs (Espadarana prosoblepon and Lithobates yavapaiensis) on the Roche 454 GS FLX platform. Our sequencing produced 8881 E. prosoblepon and 5428 L. yavapaiensis annotated gene products after de novo assembly and Gene Ontology classification. Transcripts of the innate and acquired immune system were expressed in all three tissues. Inflammatory response and acquired immunity transcripts were significantly more diverged between E. prosoblepon and L. yavapaiensis compared to innate immunity and immune system development transcripts. Immune-related transcripts did not show an overall elevated rate of functional evolution, with the exception of glycosyl proteases, which include lysozymes, central bacterial and fungal-killing enzymes of the innate immune system.

CONCLUSIONS

The three frog transcriptomes provide more than 600 Mbp of new genomic data, and will serve as a valuable framework for future comparative studies of non-model anurans. Additionally, we show that immune gene divergence varies by functional group and that transcriptome studies can be useful in comparing rates of evolutionary change across gene families.

Larval exposure to predator cues alters immune function and response to a fungal pathogen in post-metamorphic wood frogs

For the past several decades, amphibian populations have been decreasing around the globe at an unprecedented rate. Batrachochytrium dendrobatidis (Bd), the fungal pathogen that causes chytridiomycosis in amphibians, is contributing to amphibian declines. Natural and anthropogenic environmental factors are hypothesized to contribute to these declines by reducing the immunocompetence of amphibian hosts, making them more susceptible to infection. Antimicrobial peptides (AMPs) produced in the granular glands of a frog's skin are thought to be a key defense against Bd infection. These peptides may be a critical immune defense during metamorphosis because many acquired immune functions are suppressed during this time. To test if stressors alter AMP production and survival of frogs exposed to Bd, we exposed wood frog (Lithobates sylvaticus) tadpoles to the presence or absence of dragonfly predator cues crossed with a single exposure to three nominal concentrations of the insecticide malathion (0, 10, or 100 parts per billion [ppb]). We then exposed a subset of post-metamorphic frogs to the presence or absence of Bd zoospores and measured frog survival. Although predator cues and malathion had no effect on survival or size at metamorphosis, predator cues increased the time to metamorphosis by 1.5 days and caused a trend of a 20% decrease in hydrophobic skin peptides. Despite this decrease in peptides determined shortly after metamorphosis, previous exposure to predator cues increased survival in both Bd-exposed and unexposed frogs several weeks after metamorphosis. These results suggest that exposing tadpoles to predator cues confers fitness benefits later in life.

Natural stressors and ranavirus susceptibility in larval wood frogs (Rana sylvatica)

Chronic exposure to stressors has been shown to suppress immune function in vertebrates, making them more susceptible to pathogens. It is less clear, however, whether many natural stressors are immunosuppressive. Moreover, whether stressors make disease more likely or more severe in populations is unclear because animals respond to stressors both behaviorally and physiologically. We tested whether chronic exposure to three natural stressors of wood frog tadpoles-high-densities, predator-cues, and low-food conditions-influence their susceptibility to a lethal ranavirus both individually in laboratory experiments, and collectively in outdoor mesocosms. Prior to virus exposure, we observed elevated corticosterone only in low-food treatments, although other treatments altered rates of growth and development as well as tadpole behavior. None of the treatments, however, increased susceptibility to ranavirus as measured by the proportion of tadpoles that became infected or died, or the time to death compared to controls. In fact, mortality in the mesocosms was actually lower in the high-density treatment even though most individuals became infected, largely because of increased rates of metamorphosis. Overall we find no support for the hypothesis that chronic exposure to common, ecologically relevant challenges necessarily elevates corticosterone levels in a population or leads to more severe ranaviral disease or epidemics. Conditions may, however, conspire to make ranavirus infection more common in metamorphosing amphibians.

Dietary protein restriction impairs growth, immunity, and disease resistance in southern leopard frog tadpoles

The immune system is a necessary, but potentially costly, defense against infectious diseases. When nutrition is limited, immune activity may consume a significant amount of an organism's energy budget. Levels of dietary protein affect immune system function; high levels can enhance disease resistance. We exposed southern leopard frog [Lithobates sphenocephalus (=Rana sphenocephala)] tadpoles to high and low protein diets crossed with the presence or absence of the pathogenic amphibian chytrid fungus (Batrachochytrium dendrobatidis; Bd) and quantified: (1) tadpole resistance to Bd; (2) tadpole skin-swelling in response to phytohaemagglutinin (PHA) injection (a measure of the T cell-mediated response of the immune system); (3) bacterial killing ability (BKA) of tadpole blood (a measure of the complement-mediated cytotoxicity of the innate immune system); and (4) tadpole growth and development. Tadpoles raised on a low-protein diet were smaller and less developed than tadpoles on a high-protein diet. When controlled for developmental stage, tadpoles raised on a low-protein diet had reduced PHA and BKA responses relative to tadpoles on a high-protein diet, but these immune responses were independent of Bd exposure. High dietary protein significantly increased resistance to Bd. Our results support the general hypothesis that host condition can strongly affect disease resistance; in particular, fluctuations in dietary protein availability may change how diseases affect populations in the field.

Identification and structure-activity relationship of an antimicrobial peptide of the palustrin-2 family isolated from the skin of the endangered frog Odorrana ishikawae

Recently, we identified nine novel antimicrobial peptides from the skin of the endangered anuran species, Odorrana ishikawae, to assess its innate immune system. In this study an additional antimicrobial peptide was initially isolated based on antimicrobial activity against Escherichia coli. The new antimicrobial peptide belonging to the palustrin-2 family was named palustrin-2ISb. It consists of 36 amino acid residues including 7 amino acids C-terminal to the cyclic heptapeptide Rana box domain. The peptide's primary structure suggests a close relationship with the Chinese odorous frog, Odorrana grahami. The cloned cDNA encoding the precursor protein contained a signal peptide, an N-terminal acidic spacer domain, a Lys-Arg processing site and the C-terminal precursor antimicrobial peptide. It also contained 3 amino acid residues at the C-terminus not found in the mature peptide. Finally, the antimicrobial activities against four microorganisms (E. coli, Staphylococcus aureus, methicillin-resistant S. aureus and Candida albicans) were investigated using several synthetic peptides. A 29 amino acid truncated form of the peptide, lacking the 7 amino acids C-terminal to the Rana box, possessed greater antimicrobial activities than the native structure.

The fungicide chlorothalonil is nonlinearly associated with corticosterone levels, immunity, and mortality in amphibians

BACKGROUND

Contaminants have been implicated in declines of amphibians, a taxon with vital systems similar to those of humans. However, many chemicals have not been thoroughly tested on amphibians or do not directly kill them.

OBJECTIVE

Our goal in this study was to quantify amphibian responses to chlorothalonil, the most commonly used synthetic fungicide in the United States.

METHODS

We reared Rana sphenocephala (southern leopard frog) and Osteopilus septentrionalis (Cuban treefrog) in outdoor mesocosms with or without 1 time (1×) and 2 times (2×) the expected environmental concentration (EEC) of chlorothalonil (~ 164 μg/L). We also conducted two dose-response experiments on O. septentrionalis, Hyla squirella (squirrel treefrog), Hyla cinerea (green treefrog), and R. sphenocephala and evaluated the effects of chlorothalonil on the stress hormone corticosterone.

RESULTS

For both species in the mesocosm experiment, the 1× and 2× EEC treatments were associated with > 87% and 100% mortality, respectively. In the laboratory experiments, the approximate EEC caused 100% mortality of all species within 24 hr; 82 μg/L killed 100% of R. sphenocephala, and 0.0164 μg/L caused significant tadpole mortality of R. sphenocephala and H. cinerea. Three species showed a nonmonotonic dose response, with low and high concentrations causing significantly greater mortality than did intermediate concentrations or control treatments. For O. septentrionalis, corticosterone exhibited a similar nonmonotonic dose response and chlorothalonil concentration was inversely associated with liver tissue and immune cell densities (< 16.4 μg/L).

CONCLUSIONS

Chlorothalonil killed nearly every amphibian at the approximate EEC; at concentrations to which humans are commonly exposed, it increased mortality and was associated with elevated corticosterone levels and changes in immune cells. Future studies should directly quantify the effects of chlorothalonil on amphibian populations and human health.

Structural diversity and species distribution of host-defense peptides in frog skin secretions

Cationic peptides that adopt an amphipathic α-helical conformation in a membrane-mimetic environment are synthesized in the skins of many frog species. These peptides often display cytolytic activities against bacteria and fungi consistent with the idea that they play a role in the host's system of defense against pathogenic microorganisms, but their importance in the survival strategy of the animal is not clearly understood. Despite the common misconception that antimicrobial peptides are synthesized in the skins of all anurans, the species distribution is sporadic, suggesting that their production may confer some evolutionary advantage to the organism but is not necessary for survival. The low potency of many frog skin antimicrobial peptides is consistent with the hypothesis that cutaneous symbiotic bacteria may provide the major system of defense against pathogenic microorganisms in the environment with antimicrobial peptides assuming a supplementary role in some species.

Host defense peptides in skin secretions of the Oregon spotted frog Rana pretiosa: implications for species resistance to chytridiomycosis

Population declines due to chytridiomycosis among frogs belonging to the Amerana (Rana boylii) species group from western North America have been particularly severe. Norepinephrine-stimulated skin secretions from the Oregon spotted frog Rana pretiosa Baird and Girard, 1853 were collected from individuals that had been previously infected with the causative agent Batrachochytrium dendrobatidis but had proved resistant to developing chytridiomycosis. These secretions contained a more diverse array of antimicrobial peptides than found in other species from the Amerana group and 14 peptides were isolated in pure form. Determination of their primary structures identified the peptides as esculentin-2PRa and -2PRb; ranatuerin-2PRa, -2PRb, -2PRc, -2PRd, and -2PRe; brevinin-1PRa, -1PRb, -1PRc, and -1PRd; and temporin-PRa, -PRb, and -PRc. The strongly cationic ranatuerin-2PRd and the esculentin-2 peptides, which have not been identified in the secretions of other Amerana species except for the closely related R. luteiventris, showed the highest growth inhibitory potency against microorganisms. The strongly hydrophobic brevinin-1PRd was the most cytotoxic to erythrocytes. Although no clear correlation exists between production of dermal antimicrobial peptides by a species and its resistance to fatal chytridiomycosis, the diversity of these peptides in R. pretiosa may be pivotal in defending the species against environmental pathogens such as B. dendrobatidis.

Variations in the expressed antimicrobial peptide repertoire of northern leopard frog (Rana pipiens) populations suggest intraspecies differences in resistance to pathogens

The northern leopard frog (Rana pipiens or Lithobates pipiens) is historically found in most of the provinces of Canada and the northern and southwest states of the United States. In the last 50 years, populations have suffered significant losses, especially in the western regions of the species range. Using a peptidomics approach, we show that the pattern of expressed antimicrobial skin peptides of frogs from three geographically separated populations are distinct, and we report the presence of four peptides (brevinin-1Pg, brevinin-1Pl, ranatuerin-2Pb, and ranatuerin-2Pc) that have not previously been found in skin secretions. The differences in expressed peptides reflect differences in the distribution of alleles for the newly described Brevinin1.1 locus in the three populations. When enriched peptide mixtures were tested for their ability to inhibit growth of the pathogenic amphibian chytrid (Batrachochytrium dendrobatidis), peptides from Minnesota or Vermont frogs were more effective that peptides from Michigan frogs. Four of the purified peptides were tested for their ability to inhibit growth of two bacterial pathogens (Aeromonas hydrophila and Staphylococcus epidermidis) and B. dendrobatidis. Three of the four were effective inhibitors of B. dendrobatidis and S. epidermidis, but none inhibited A. hydrophila. We interpret these differences in expression and activity of antimicrobial peptides as evidence to suggest that each population may have been selected to express a suite of peptides that reflects current and past encounters with skin microbes.

The first antimicrobial peptide from sea amphibian

The crab-eating frog, Rana cancrivora, is one of only a handful of amphibians worldwide that tolerates saline waters. It typically inhabits brackish water of mangrove forests of Southeast Asia. A large amount of antimicrobial peptides belonging to different families have been identified from skins of amphibians inhabiting freshwater. No antimicrobial peptide from sea amphibians has been reported. In this paper, we firstly reported the antimicrobial peptide and its cDNA cloning from skin secretions of the crab-eating frog R. cancrivora. The antimicrobial peptide was named cancrin with an amino acid sequence of GSAQPYKQLHKVVNWDPYG. By BLAST search, cancrin had no significant similarity to any known peptides. The cDNA encoding cancrin was cloned from the cDNA library of the skin of R. cancrivora. The cancrin precursor is composed of 68 amino acid residues including a signal peptide, acidic spacer peptide, which are similar to other antimicrobial peptide precursors from Ranid amphibians and mature cancrin. The overall structure is similar to other amphibian antimicrobial peptide precursors although mature cancrin is different from known peptides. The current results reported a new family of amphibian antimicrobial peptide and the first antimicrobial peptide from sea amphibian.

Antimicrobial peptide defenses of the mountain yellow-legged frog (Rana muscosa)

The mountain yellow-legged frog (Rana muscosa) inhabits high elevation lakes in California that are largely undisturbed by human activities. In spite of this habitation in remote sites, populations continue to decline. Although predation by non-native fish is one cause for declines, some isolated populations in fishless lakes are suffering new declines. One possible cause of the current wave of declines is the introduction of the pathogenic chytrid fungus (Batrachochytrium dendrobatidis) which invades the adult skin to cause chytridiomycosis. In many amphibian species, the skin is protected by antimicrobial peptides secreted into the mucous. Here we show that R. muscosa produces three previously unknown antimicrobial peptides belonging to the ranatuerin-2 and temporin-1 families of antimicrobial peptides. These three peptides, along with bradykinin, are the most abundant peptides in the skin secretions detected by mass spectrometry. Natural mixtures of peptides and individual purified peptides strongly inhibit chytrid growth. The concentration of total peptides recovered from the skin of frogs following a mild norepinephrine induction is sufficient to inhibit chytrid growth in vitro. A comparison of the species susceptibility to chytridiomycosis and the antichytrid activity of peptides between R. muscosa and R. pipiens suggest that although R. muscosa produces more total skin peptides, it appears to be more vulnerable to B. dendrobatidis in nature. Possible differences in the antimicrobial peptide repertoires and life history traits of the two species that may account for differences in susceptibility are discussed.

Host-defense peptides isolated from the skin secretions of the Northern red-legged frog Rana aurora aurora

Antimicrobial peptides in the skin secretions of anurans constitute a component of the innate immunity that protects the organism against invading pathogens. Four peptides with antimicrobial activity were isolated in high yield from norepinephrine-stimulated skin secretions of the Northern red-legged frog Rana aurora aurora and their primary structures determined. Ranatuerin-2AUa (GILSSFKGVAKGVAKNLAGKLLDELKCKITGC) showed potent growth-inhibitory activity against a range of Gram-positive and Gram-negative bacteria (minimum inhibitory concentrations < 20 microM) but low hemolytic activity against human erythrocytes (50% hemolysis at 290 microM). Brevinin-1AUa (FLPILAGLAAKLVPKVFCSITKKC) and brevinin-1AUb (FLPILAGLAANILPKVFCSITKKC) also showed potent antimicrobial activity but were strongly hemolytic (HC50 < 10 microM). Temporin-1AUa (FLPIIGQLLSGLL.NH2) atypically lacked a basic amino acid residue and showed very weak antimicrobial and hemolytic activity. Its biological function remains to be established. The primary structures of the antimicrobial peptides are consistent with a close phylogenetic relationship between R. aurora, Rana boylii and Rana luteiventris.

Activity of antimicrobial skin peptides from ranid frogs against Batrachochytrium dendrobatidis, the chytrid fungus associated with global amphibian declines

Accumulating evidence suggests that a chytrid fungus, Batrachochytrium dendrobatidis, is responsible for recent declines in amphibian populations in Australia, Central America, Europe, and North America. Because the chytrid infects the keratinized epithelium of the skin, we investigated the possible role of antimicrobial peptides produced in the skin as inhibitors of infection and growth. We show here that 10 peptides representing eight families of peptides derived from North American ranid frogs can effectively inhibit growth of this chytrid. The peptides are members of the ranatuerin-1, ranatuerin-2, esculentin-1, esculentin-2, brevinin-2, temporin, palustrin-3, and ranalexin families. All the tested peptides inhibit growth of mature fungal cells at concentrations above 25 microM, and some of them inhibit at concentrations as low as 2 microM. A comparison of the sensitivity of infectious zoospores with that of mature cells showed that the zoospores are inhibited at significantly lower concentrations of peptides. To determine whether cold temperature interferes with the inhibitory effects of these peptides, we tested their effectiveness at both 22 and 10 degrees C. Although the peptides inhibit at both temperatures, they appear to be more effective against zoospores at the lower temperature. These results suggest that the ranid frogs have, within their repertoire of antimicrobial substances, a number of skin peptides that should be a deterrent to chytrid infection. This may provide some natural resistance to infection, but if environmental factors inhibit the synthesis and release of the skin peptides, the pathogen could gain the advantage.

Neuroendocrine-immune system interactions in amphibians: implications for understanding global amphibian declines

Amphibians are ancient creatures valued by biologists and naturalists around the world. They share with all other vertebrates a complex neuroendocrine system that enables them to flourish in a variety of aquatic and semiaquatic environments. Studies from a number of laboratories have demonstrated that the immune system of amphibian species is nearly as complex as that of mammals. Yet for reasons that are not well understood, amphibian species are facing greater survival challenges than in the recent past. This article will review our current understanding of the neuroendocrine immune system interactions in amphibians and address the question of whether environmental stressors may contribute to immunosuppression and amphibian declines.

Temperature but not season affects the transplantation immunity of anuran amphibians

The effects of ambient temperature (22 degrees C/10 degrees C) and season (summer/winter) on anuran skin allograft and xenograft rejection was tested in frogs (Rana temporaria and R. esculenta) and toads (Bufo bufo, Bombina variegata, and Bombina bombina). Mean graft survival times were significantly prolonged at the low temperature in a species-specific manner, the edible frog (R. esculenta) being the most sensitive and the common toad (Bufo bufo) relatively resistant. Allografts were more temperature-dependent than xenografts; in the latter case, temperature sensitivity was specific to each donor-host combination. Rejection of second-set grafts in R. esculenta was accelerated both in warmth and in cold, but second-set grafts were less temperature-sensitive than sensitizing ones. Both in summer and in winter, R. esculenta rejected allografts promptly at 22 degrees C but slowly at 10 degrees C. In both seasons, Bombina variegata kept at 22 degrees C rejected allografts in a chronic manner. This indicates that amphibian transplantation immunity depends on the donor-host genetic disparity and ambient temperature but is independent of season.

The immune system of ectothermic vertebrates

The adaptive immune system, as defined by T cell receptors, immunoglobulins, and the major histocompatibility complex (MHC), has been described definitively at the level of teleost fish. Cartilaginous fish, which display many of the hallmarks of such an adaptive system, nevertheless have several features of their responses that seem primitive. Data are presented suggesting that some adaptive mechanisms in cartilaginous fish, including MHC restriction and somatic diversification, are present to the same "degree' as compared to mammals, and that these animals may possess other molecules and functions previously overlooked. MHC linkage studies in amphibians suggest that the entire genetic complex, including class I, class II, and class III genes, arose early in the vertebrate line (at least 350 x 10(6) years ago) and has been maintained intact, at least for those genes involved in immunity. Studies of MHC in polyploid Xenopus have demonstrated that there is a maximal number of expressed MHC genes 'permitted' to be expressed in any individual, regardless of the number of potential MHC-bearing chromosomes present in the species. A speculative hypothesis is presented on the origins of adaptive immunity based on ectothermic models.

Immunological characterization of rat cardiac gap junctions: presence of common antigenic determinants in heart of other vertebrate species and in various organs

Antibodies to the following synthetic peptide, SALGKLLDKVQAY, were purified by affinity chromatography and characterized by ELISA and immunoblotting. These antibodies, shown to be specific to the major protein constituent of isolated rat heart junctions: connexin 43, cross-reacted with a homologous protein in immunoreplicas of whole heart fractions of trout, frog, chicken, guinea pig, mouse and rat, suggesting a phylogenic conservation of connexin 43 in vertebrates. By immunoblotting of whole organ fractions it was also demonstrated that these antibodies cross-reacted with major proteins of Mr 32 and 22 kD in rat and mouse liver, of Mr 41 kD in rat cerebellum, of Mr 43 kD in uterus, stomach and kidney of rat, of Mr 46 and 70 kD in rat lens, suggesting that these proteins share common or related epitopes with the synthetic peptide and connexin 43.

Isolation and partial characterization of IgG-like immunoglobulins from frog (Rana ridibunda Pall.) and tortoise (Testudo graeca Pall.) serum

1. By successive application of gel-filtration and ion-exchange chromatography IgG fraction have been isolated from frog (Rana ridibunda Pall.) and tortoise (Testudo graeca Pall.) sera. 2. The homogeneity of the fractions has been confirmed and a similar electrophoretic mobility has been revealed by polyacrylamide gel electrophoresis and immunoelectrophoresis. 3. In rabbits polyclonal antibodies giving specific reaction with the corresponding IgG immunoglobulin have been raised.

Isolation and partial characterization of IgM-like immunoglobulins from the serum of carp (Cyprinus carpio L.), frog (Rana ridibunda Pall.) and tortoise (Testudo graeca Pall.)

1. IgM-like immunoglobulin fractions have been isolated by combinations of precipitation procedures and gel-filtration from the serum of three vertebrate species, representative of the classes of Pisces (Cyprinus carpio L.), Amphibia (Rana ridibunda Pall.) and Reptilia (Testudo graeca Pall.). 2. The homogeneity of the isolated fractions has been confirmed electrophoretically and immunoelectrophoretically. 3. Electrophoretic mobility of IgM, similar for the three species studied has been observed in polyacrylamide gel; in agar gel immunoelectrophoresis carp and frog IgM mobilities are almost identical, in contrast to the markedly slower cathodic migration of tortoise IgM.

Anti-striated muscle antibody activity produced by Trypanosoma cruzi

We have previously shown that Trypanosoma cruzi shares antigenic determinants with preparations of the calcium-sequestering adenosine triphosphatase of sarcoplasmic reticulum. The cross-reacting antigen (SRA) is also apparently present on the sarcolemma of cardiac myofibers. Using highly specific reference antisera to either the small membranes of T. cruzi or to a tryptic fragment of striated muscle SRA, it was shown that SRA is present in the striated muscle of animals representative of the evolutionary scale ranging from nonhuman primate to fish. The small membranes of nine different T. cruzi strains isolated from widely divergent areas of the American continents also reacted with the reference antisera. This indicates that SRA is present in these T. cruzi strains and may be prevalent among all T. cruzi strains. The shared T. cruzi-striated muscle antigen, SRA, may be a heteroantigen present in all T. cruzi strains and in the striated muscle of all classes of animals. Immunization of rabbits (three of five) or chickens (five pairs of five pairs) with striated muscle membrane preparations of different classes of animals, particularly those of nonhuman primate, chicken, and turtle, gave rise to IgG anti-allogeneic striated muscle antibody activity. Immunization of rabbits (four of nine) and chickens (five pairs of six pairs) with the small membranes of different T. cruzi strains also produced IgG anti-allogeneic striated muscle. These data indicate that T. cruzi shares cross-immunogenicity with striated muscle SRA. Since SRA is apparently present on the sarcolemma of cardiac myofibers, it may be implicated in the immunopathogenesis of Chagas' disease.

Immunochemical investigations on water frog eggs: comparative studies on Rana ridibunda, Rana lessonae and their hybrid Rana esculenta

Eggs of Rana ridibunda, Rana lessonae and their hybrid Rana esculenta were examined for their immunochemical properties. The reaction spectra obtained confirm the hybrid origin of Rana esculenta.

[Phylogenetic studies on some Japanese amphibia by means of immunoelectrophoresis. II. Relationships of some amphibians to Japanese pond frogs (author's transl)]

Since the Japanese pond frogs (Rana nigromaculata and R. brevipoda) are known to be very closely allied with each other in morphological, ecological, physiological or immunological characters, the phylogenetical relationships between the Japanese pond frogs and other 10 species of Japanese amphibians were investigated by means of immunoelectrophoretic analysis of liver extract. The results obtained are as follows: (1) There are conservative antigens which are commonly found in all species of Anura. (2) The Japanese pond frogs have specific antigens. (3) R. nigromaculata and R. brevidpoda are very closely allied with each other. (4) Four species of the genus Rana (R. rugosa, R. catesbeiana, R. ornativentris and R. japonica) are closely related to the Japanese pond frogs. (5) Two species of the genus Rhacophorus (Rh. arboreus and Rh. burergeri) are related to the Japanese pond frogs. (6) R. limnocharis is related to the Japanese pond frogs at the same extent as the genus Rhcophorus is. (7) Tow species of the suborder Procoela (Hyla arbored and Bufo bufo) are only partially related to the Japanese pond frogs. (8) Cynops pyrrhogaster pyrrhogaster of Urodera had only a few common antigens with the Japanese pond frogs.

Albumin evolution in frogs: a test of the evolutionary clock hypothesis

Frogs are an ancient group compared to placental mammals. Yet, although there are about as many species of frogs as there are of mammals, zoologists consider that frogs have undergone only limited morphological divergence, while placental mammals have diversified greatly in morphology and way of life. The serum albumins of numerous frog species were compared by the quantitative microcomplement fixation technique. Frogs that are morphologically similar enough to merit taxonomic distinction at only the species level often exhibit differences in the serological properties of their albumins larger than those usually seen between mammals placed in distinct families or suborders. Thus, there seems to be a contrast between albumin evolution and evolution at the organismal level. The large differences between albumins among frogs can be explained by the hypothesis that albumin evolution has proceeded at the same rate in frogs as in mammals.