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PubMed1987

Time courses of thermal acclimation for critical thermal minima in the salamanders Desmognathus quadramaculatus, Desmognathus monticola, Desmognathus ochrophaeus, and Plethodon jordani.

1. We examined the time courses of thermal acclimation for critical thermal minima (CTMin) in the salamanders Desmognathus quadramaculatus, D. monticola, D. ochrophaeus, and Plethodon jordani. 2. D. quadramaculatus showed no annual differences in their time courses, but we noted seasonal differences in the CTMin with these salamanders having lower CTMin in the spring. 3. Gain in CTMin (25 degrees C to 5 degrees C transfer) was faster than its corresponding loss (5 degrees C to 25 degrees C transfer) in D. quadramaculatus. 4. Comparison of all three Desmognathus species revealed no differences in the CTMin for the 5 degrees C to 25 degrees C time courses, but P. jordani had lower CTMin than corresponding D. quadramaculatus. 5. Our results indicate that the pattern of CTMin acclimation correlates with the constraints acting on salamanders in their natural environment.

Acclimatization↗
PubMed1991

A double-label study demonstrating that enkephalin and somatostatin are localized in separate populations of amacrine cells in the larval tiger salamander retina.

Previous studies have localized enkephalin and somatostatin to amacrine cell populations in the larval tiger salamander retina. Double-label immunocytochemistry was utilized to examine if enkephalin- and somatostatin-like immunoreactivities are colocalized to tiger salamander amacrine cells. Of the more than 2000 labelled cells observed in double-labelled preparations, none were found to express both enkephalin and somatostatin immunoreactivity. Therefore, these studies demonstrate that in the larval tiger salamander retina, enkephalin and somatostatin are localized to separate populations of amacrine cells.

Ambystoma↗
PubMed1992

A double-label analysis demonstrating the non-coexistence of tyrosine hydroxylase-like and GABA-like immunoreactivities in amacrine cells of the larval tiger salamander retina.

Previous studies have localized tyrosine hydroxylase, the rate-limiting enzyme for the production of dopamine, and gamma-aminobutyric acid (GABA) to amacrine cell populations in the larval tiger salamander retina. Double-label immunocytochemistry was used to examine if tyrosine hydroxylase-like and GABA-like immunoreactivities colocalize in tiger salamander amacrine cells. A total of 2,162 tyrosine hydroxylase-like immunoreactive amacrine cells were observed in double-labelled sections. None of these cells were observed to express GABA-like immunoreactivity. Therefore, the present study demonstrates that dopamine and GABA are localized to distinct neuronal populations in the larval tiger salamander retina.

Animals↗
PubMed2006

Effects of competition and coal-combustion wastes on recruitment and life history characteristics of salamanders in temporary wetlands.

Amphibians in natural systems must cope with a number of biotic and abiotic stressors that can potentially interact with pollutants to influence toxicity. Although interactive effects of short-lived pesticides with various environmental stressors have been studied, how persistent and bioaccumulative compounds such as metals interact with natural stressors to influence amphibians remains unexplored. We exposed the salamander Ambystoma talpoideum to coal-combustion wastes (a complex mixture of metals and metalloids, hereafter referred to as CCW) at low and high larval density throughout aquatic development in mesocosms simulating temporary wetlands. CCW and high density reduced survival to metamorphosis by 57-77% and 85-92%, respectively, and the effects of these two factors together were additive. Reduced metamorphosis was due in part to larval mortality prior to initiation of pond drying, but CCW and high density also extended the larval period, causing mortality of larvae that were not ready to metamorphose before the pond dried. To our knowledge, this is the first demonstration of a pollutant-induced extension of larval period leading to reduction in amphibian recruitment. Metamorphs were smaller in high density compared to low in reference ponds, but those from CCW emerged at similar sizes irrespective of density, suggesting less-than-additive effects of density and CCW on metamorph size. The adverse responses of salamanders to CCW were likely due to direct toxicity, as A. talpoideum metamorphs accumulated high concentrations of a suite of trace elements (As, Se, Sr, and V), and also to indirect effects on the community food web. We conclude that in no case did the addition of a natural stressor (high density) exacerbate CCW-related effects, but that the effects of CCW alone can be detrimental to larvae of salamanders that breed in temporary ponds.

Ambystoma↗
PubMed2006

Microarray analysis identifies keratin loci as sensitive biomarkers for thyroid hormone disruption in the salamander Ambystoma mexicanum.

Ambystomatid salamanders offer several advantages for endocrine disruption research, including genomic and bioinformatics resources, an accessible laboratory model (Ambystoma mexicanum), and natural lineages that are broadly distributed among North American habitats. We used microarray analysis to measure the relative abundance of transcripts isolated from A. mexicanum epidermis (skin) after exogenous application of thyroid hormone (TH). Only one gene had a >2-fold change in transcript abundance after 2 days of TH treatment. However, hundreds of genes showed significantly different transcript levels at days 12 and 28 in comparison to day 0. A list of 123 TH-responsive genes was identified using statistical, BLAST, and fold level criteria. Cluster analysis identified two groups of genes with similar transcription patterns: up-regulated versus down-regulated. Most notably, several keratins exhibited dramatic (1000 fold) increases or decreases in transcript abundance. Keratin gene expression changes coincided with morphological remodeling of epithelial tissues. This suggests that keratin loci can be developed as sensitive biomarkers to assay temporal disruptions of larval-to-adult gene expression programs. Our study has identified the first collection of loci that are regulated during TH-induced metamorphosis in a salamander, thus setting the stage for future investigations of TH disruption in the Mexican axolotl and other salamanders of the genus Ambystoma.

Ambystoma↗
PubMed1992

Localization of GABAA receptor subtypes in the tiger salamander retina.

Dry autoradiography was used to determine the distribution of GABAA binding sites in tiger salamander retina. High-affinity binding of [3H]-flunitrazepam [( 3H]-FNZ) was used to localize benzodiazepine receptors (BZR) and [3H]-muscimol was used to localize the GABAA recognition site. Specific [3H]-FNZ binding was present only in the inner retina, primarily in the inner plexiform layer (IPL). Co-incubation with GABA enhanced [3H]-FNZ binding by 20-50%. [3H]-muscimol binding was found throughout the IPL and in the outer plexiform layer (OPL). Mouse monoclonal antibodies 62-3G1 and BD-17, that recognize the GABAA beta 2, beta 3 polypeptides, and BD-24, that recognizes the GABAA alpha 1 polypeptide, did not label either the OPL or IPL, despite numerous variations in the fixation and immunoprocessing methods. GABAA receptor location, as revealed by [3H]-muscimol binding, matches the distribution of presumed GABAergic terminals in the OPL and IPL. We suggest that there are at least two subtypes of GABAA receptor in the tiger salamander retina: one type is present only in the inner retina, primarily in the IPL and is functionally coupled to BZRs; the other type is located in the OPL and is not coupled to the BZRs. Furthermore, GABAA receptors in the tiger salamander retina appear to be of a different epitope than GABAA receptors in numerous other preparations that are recognized by mAbs 62-3G1, BD-17, and BD-24.

Ambystoma↗
PubMedDate not supplied

Similar effects of carbachol and dopamine on neurons in the distal retina of the tiger salamander.

Though there is considerable evidence that dopamine is an important retinal neuromodulator that mediates many of the changes in the properties of retinal neurons that are normally seen during light adaptation, the mechanism by which dopamine release is controlled remains poorly understood. In this paper, we present evidence which indicates that dopamine release in the retina of the tiger salamander, Ambystoma tigrinum, is driven excitatorily by a cholinergic input. We compared the effects of applying carbachol to those of dopamine application on the responses of rods, horizontal cells, and bipolar cells recorded intracellularly from the isolated, perfused retina of the tiger salamander. Micromolar concentrations of dopamine reduced the amplitudes of rod responses throughout the rods' operating range. The ratio of amplitudes of the cone-driven to rod-driven components of the responses of both horizontal and bipolar cells was increased by activation of both D1 and D2 dopamine receptors. Dopamine acted to uncouple horizontal cells and also off-center bipolar cells, the mechanism in the case of horizontal cells depending only upon activation of D1 receptors. Carbachol, a specific cholinomimetic, applied in five- to ten-fold higher concentrations, produced effects that were essentially identical to those of dopamine. These effects of carbachol were blocked by application of specific dopamine blockers, however, indicating that they are mediated secondarily by dopamine. We propose that the dopamine-releasing amacrine cells in the salamander are under the control of cells, probably amacrine cells, which secrete acetylcholine as their transmitter.

Ambystoma↗
PubMedDate not supplied

Identification and distribution of photoreceptor subtypes in the neotenic tiger salamander retina.

The neotenic tiger salamander retina is a major model system for the study of retinal physiology and circuitry, yet there are unresolved issues regarding the organization of the photoreceptors and the photoreceptor mosaic. The rod and cone subtypes in the salamander retina were identified using a combination of morphological and immunocytochemical markers for specific rod and cone opsin epitopes. Because the visual pigment mechanisms present in the tiger salamander retina are well characterized and the antibodies employed in these studies are specific for particular rod and cone opsin epitopes, we also were able to identify the spectral class of the various rod and cone subtypes. Two classes of rods corresponding to the "red" and "green" rods previously reported in amphibian retinas were identified. In serial semithin section analyses, rods and cones comprised 62.4+/-1.4% and 37.6+/-1.4% of all photoreceptors, respectively. One rod type comprising 98.0+/-0.7% of all rods showed the immunological and morphological characteristics of "red" rods, which are maximally sensitive to middle wavelengths. The second rod subtype comprised 2.0+/-0.7% of all rods and possessed the immunological and morphological characteristics of "green" rods, which are maximally sensitive to short wavelengths. By morphology four cone types were identified, showing three distinct immunological signatures. Most cones (84.8+/-1.5% of all cones), including most large single cones, the accessory and principal members of the double cone, and some small single cones, showed immunolabeling by antisera that recognize long wavelength-sensitive cone opsins. A subpopulation of small single cones (8.4+/-1.7% of all cones) showed immunolabeling for short wavelength-sensitive cone opsin. A separate subpopulation of single cones which included both large and small types (6.8+/-1.4% of all cones) was identified as the UV-Cone population and showed immunolabeling by antibodies that recognize rod opsin epitopes. Analysis of flatmounted retinas yielded similar results. All photoreceptor types appeared to be distributed in all retinal regions. There was no obvious crystalline organization of the various photoreceptor subtypes in the photoreceptor mosaic.

Animals↗
PubMed2006

Fine-scale population differentiation and gene flow in a terrestrial salamander (Plethodon cinereus) living in continuous habitat.

Several recent studies have shown that amphibian populations may exhibit high genetic subdivision in areas with recent fragmentation and urban development. Less is known about the potential for genetic differentiation in continuous habitats. We studied genetic differentiation of red-backed salamanders (Plethodon cinereus) across a 2-km transect through continuous forest in Virginia, USA. Mark-recapture studies suggest very little dispersal for this species, whereas homing experiments and post-Pleistocene range expansion both suggest greater dispersal abilities. We used six microsatellite loci to examine genetic population structure and differentiation between eight subpopulations of red-backed salamanders at distances from 200 m to 2 km. We also used several methods to extrapolate dispersal frequencies and test for sex-biased dispersal. We found small, but detectable differentiation among populations, even at distances as small as 200 m. Differentiation was closely correlated with distance and both Mantel tests and assignment tests were consistent with an isolation-by-distance model for the population. Extrapolations of intergenerational variance in spatial position (sigma(2)<15 m(2)) and pair-wise dispersal frequencies (4 Nm < 25 for plots separated by 300 m) both suggest limited gene flow. Additionally, tests for sex-biased dispersal imply that dispersal frequency is similarly low for both sexes. We suggest that these low levels of gene flow and the infrequent dispersal observed in mark-recapture studies may be reconciled with homing ability and range expansion if dispersing animals rarely succeed in breeding in saturated habitats, if dispersal is flexible depending on the availability of habitat, or if dispersal frequency varies across the geographic range of red-backed salamanders.

Animals↗
PubMed2000

Candidate gene analysis of thyroid hormone receptors in metamorphosing vs. nonmetamorphosing salamanders.

We used two different experimental approaches to test the hypothesis that thyroid hormone receptor (TR) variation is associated with alternate life cycles modes in ambystomatid salamanders. In the first experiment, the inheritance of TRalpha and TRbeta genotypes was determined for metamorphic and non metamorphic offspring from backcrosses between Ambystoma mexicanum (an obligate metamorphic-failure species) and metamorphic F1 hybrids (A. mexicanum x A. tigrinum tigrinum). The segregation of TR genotype was independent of the expression of life cycle mode phenotype, and neither TR locus was linked to DNA markers that flank a major-effect locus for life cycle mode. In the second experiment, a portion of the ligand-binding domain of TRalpha and TRbeta was cloned and sequenced for DNA samples from 14 different ambystomatid salamander populations, including obligate metamorphic, facultative metamorphic, and obligate metamorphic-failure taxa. Nucleotide sequence variation was found for both TRalpha and TRbeta, with several nonsynonomous substitutions that presumably code for nonconservative amino acid replacements. However, no general relationship was found between TR allelic variation and life cycle mode among populations or species. These data do not implicate TRs as candidate loci involved in the current maintenance or past evolution of alternate life cycle modes in members of the tiger salamander complex.

Alleles↗
PubMed1999

Small effective population size in the long-toed salamander.

The effective population sizes (Ne) of six populations of the long-toed salamander (Ambystoma macrodactylum) from Montana and Idaho, USA were estimated from allozyme data from samples collected in 1978, 1996 and 1997 using the temporal allele frequency method. Five of the six estimates ranged from 23 to 207 (mean = 123 +/- 79); one estimate was indistinguishable from infinity. In order to infer the actual Ne of salamander populations, we compared the frequency distribution of our observed Ne estimates with distributions obtained from simulated populations of known Ne. Our observed Ne estimate distribution was consistent with distributions from simulated populations with Ne values of 10, 25, and 50, suggesting an actual Ne for each of the six salamander populations of less than 100. This Ne estimate agrees with most other Ne estimates for amphibians. We conclude by discussing the conservation implications of small Ne values in amphibians in the context of increasing isolation of populations due to habitat fragmentation.

Alleles↗
PubMed2003

Testing Pleistocene refugia theory: phylogeographical analysis of Desmognathus wrighti, a high-elevation salamander in the southern Appalachians.

During the colder climates of the Pleistocene, the ranges of high-elevation species in unglaciated areas may have expanded, leading to increased gene flow among previously isolated populations. The phylogeography of the pygmy salamander, Desmognathus wrighti, an endemic species restricted to the highest mountain peaks of the southern Appalachians, was examined to test the hypothesis that the range of D. wrighti expanded along with other codistributed taxa during the Pleistocene. Analyses of genetic variation at 14 allozymic loci and of the 12S rRNA gene in the mtDNA genome was conducted on individuals sampled from 14 population isolates throughout the range of D. wrighti. In contrast to the genetic patterns of many other high-elevation animals and plants, genetic distances derived from both molecular markers showed significant isolation by distance and genetic structuring of populations, suggesting long-term isolation of populations. Phylogeographical analyses revealed four genetically distinct population clusters that probably remained fragmented during the Pleistocene, although there was also evidence supporting recent gene flow among some population groups. Support for isolation by distance is rare among high-elevation species in unglaciated areas of North and Middle America, although not uncommon among Plethodontid Salamanders, and this pattern suggests that populations of D. wrighti did not expand entirely into suitable habitat during the Pleistocene. We propose that intrinsic barriers to dispersal, such as species interactions with other southern Appalachian plethodontid salamanders, persisted during the Pleistocene to maintain the fragmented distribution of D. wrighti and allow for significant genetic divergence of populations by restricting gene flow.

Animals↗
PubMed2001

Extreme morphological and ecological homoplasy in tropical salamanders.

Fossorial salamanders typically have elongate and attenuated heads and bodies, diminutive limbs, hands and feet, and extremely elongate tails. Batrachoseps from California, Lineatriton from eastern México, and Oedipina from southern México to Ecuador, all members of the family Plethodontidae, tribe Bolitoglossini, resemble one another in external morphology, which has evolved independently. Whereas Oedipina and Batrachoseps are elongate because there are more trunk vertebrae, a widespread homoplasy (parallelism) in salamanders, the genus Lineatriton is unique in having evolved convergently by an alternate "giraffe-neck" developmental program. Lineatriton has the same number of trunk vertebrae as related, nonelongated taxa, but individual trunk vertebrae are elongated. A robust phylogenetic hypothesis, based on sequences of three mtDNA genes, finds Lineatriton to be deeply nested within a clade characterized by generalized ecology and morphology. Lineatriton lineolus, the only currently recognized taxon in the genus, shows unanticipated genetic diversity. Surprisingly, geographically separated populations of L. lineolus are not monophyletic, but are sister taxa of different species of the morphologically generalized genus Pseudoeurycea. Lineatriton, long thought to be a unique monospecific lineage, is polyphyletic. Accordingly, the specialized morphology of Lineatriton displays homoplasy at two hierarchical levels: (i) with respect to other elongate lineages in the family (convergence), and (ii) within what is currently recognized as a single taxon (parallelism). These evolutionary events are of adaptive significance because to invade the lowland tropics salamanders must be either arboreal or fossorial; the repeated evolution of elongation and attenuation has led to multiple lowland invasions.

Animals↗
PubMed2005

Resolving deep phylogenetic relationships in salamanders: analyses of mitochondrial and nuclear genomic data.

Phylogenetic relationships among salamander families illustrate analytical challenges inherent to inferring phylogenies in which terminal branches are temporally very long relative to internal branches. We present new mitochondrial DNA sequences, approximately 2,100 base pairs from the genes encoding ND1, ND2, COI, and the intervening tRNA genes for 34 species representing all 10 salamander families, to examine these relationships. Parsimony analysis of these mtDNA sequences supports monophyly of all families except Proteidae, but yields a tree largely unresolved with respect to interfamilial relationships and the phylogenetic positions of the proteid genera Necturus and Proteus. In contrast, Bayesian and maximum-likelihood analyses of the mtDNA data produce a topology concordant with phylogenetic results from nuclear-encoded rRNA sequences, and they statistically reject monophyly of the internally fertilizing salamanders, suborder Salamandroidea. Phylogenetic simulations based on our mitochondrial DNA sequences reveal that Bayesian analyses outperform parsimony in reconstructing short branches located deep in the phylogenetic history of a taxon. However, phylogenetic conflicts between our results and a recent analysis of nuclear RAG-1 gene sequences suggest that statistical rejection of a monophyletic Salamandroidea by Bayesian analyses of our mitochondrial genomic data is probably erroneous. Bayesian and likelihood-based analyses may overestimate phylogenetic precision when estimating short branches located deep in a phylogeny from data showing substitutional saturation; an analysis of nucleotide substitutions indicates that these methods may be overly sensitive to a relatively small number of sites that show substitutions judged uncommon by the favored evolutionary model.

Animals↗
PubMed2006

Salamander limb regeneration involves the activation of a multipotent skeletal muscle satellite cell population.

In contrast to mammals, salamanders can regenerate complex structures after injury, including entire limbs. A central question is whether the generation of progenitor cells during limb regeneration and mammalian tissue repair occur via separate or overlapping mechanisms. Limb regeneration depends on the formation of a blastema, from which the new appendage develops. Dedifferentiation of stump tissues, such as skeletal muscle, precedes blastema formation, but it was not known whether dedifferentiation involves stem cell activation. We describe a multipotent Pax7+ satellite cell population located within the skeletal muscle of the salamander limb. We demonstrate that skeletal muscle dedifferentiation involves satellite cell activation and that these cells can contribute to new limb tissues. Activation of salamander satellite cells occurs in an analogous manner to how the mammalian myofiber mobilizes stem cells during skeletal muscle tissue repair. Thus, limb regeneration and mammalian tissue repair share common cellular and molecular programs. Our findings also identify satellite cells as potential targets in promoting mammalian blastema formation.

Adipocytes↗
PubMed1996

Multiple visual pigments in a photoreceptor of the salamander retina.

Although a given retina typically contains several visual pigments, each formed from a retinal chromophore bound to a specific opsin protein, single photoreceptor cells have been thought to express only one type of opsin. This design maximizes a cell's sensitivity to a particular wavelength band and facilitates wavelength discrimination in retinas that process color. We report electrophysiological evidence that the ultraviolet-sensitive cone of salamander violates this rule. This cell contains three different functional opsins. The three opsins could combine with the two different chromophores present in salamander retina to form six visual pigments. Whereas rods and other cones of salamander use both chromophores, they appear to express only one type of opsin per cell. In visual pigment absorption spectra, the bandwidth at half-maximal sensitivity increases as the pigment's wavelength maximum decreases. However, the bandwidth of the UV-absorbing pigment deviates from this trend; it is narrow like that of a red-absorbing pigment. In addition, the UV-absorbing pigment has a high apparent photosensitivity when compared with that of red- and blue-absorbing pigments and rhodopsin. These properties suggest that the mechanisms responsible for spectrally tuning visual pigments separate two absorption bands as the wavelength of maximal sensitivity shifts from UV to long wavelengths.

Animals↗
PubMed2006

Specific dynamic action of ambystomatid salamanders and the effects of meal size, meal type, and body temperature.

The past decade has witnessed a dramatic increase in studies of amphibian and reptile specific dynamic action (SDA). These studies have demonstrated that SDA, the summed energy expended on meal digestion and assimilation, is affected significantly by meal size, meal type, and body size and to some extent by body temperature. While much of this attention has been directed at anuran and reptile SDA, we investigated the effects of meal size, meal type, and body temperature on the postprandial metabolic responses and the SDA of the tiger salamander (Ambystoma tigrinum tigrinum). We also compared the SDA responses among six species of Ambystoma salamanders representing the breadth of Ambystoma phylogeny. Postprandial peaks in VO(2) and VO(2), duration of elevated metabolism, and SDA of tiger salamanders increased with the size of cricket meals (2.5%-12.5% of body mass). For A. tigrinum, as for other ectotherms, a doubling of meal size results in an approximate doubling of SDA, a function of equal increases in peak VO(2) and duration. For nine meal types of equivalent size (5% of body mass), the digestion of hard-bodied prey (crickets, superworms, mealworms, beetles) generated larger SDA responses than the digestion of soft-bodied prey (redworms, beetle larvae). Body temperature affected the profile of postprandial metabolism, increasing the peak and shortening the duration of the profile as body temperature increased. SDA was equivalent among three body temperatures (20 degrees, 25 degrees, and 30 degrees C) but decreased significantly at 15 degrees C. Comparatively, the postprandial metabolic responses and SDA of Ambystoma jeffersonianum, Ambystoma maculatum, Ambystoma opacum, Ambystoma talpoideum, Ambystoma texanum, and the conspecific Ambystoma tigrinum mavortium digesting cricket meals that were 5% of their body mass were similar (independent of body mass) to those of A. t. tigrinum. Among the six species, standard metabolic rate, peak postprandial VO(2), and SDA scaled with body mass with mass exponents of 0.72, 0.78, and 1.05, respectively.

Animals↗
PubMed2001

Amiloride blocks salt taste transduction of the glossopharyngeal nerve in metamorphosed salamanders.

Studies in the last two decades have shown that amiloride-sensitive Na(+) channels play a role in NaCl transduction in rat taste receptors. However, this role is not readily generalized for salt taste transduction in vertebrates, because functional expression of these channels varies across species and also in development in a species. Glossopharyngeal nerve responses to sodium and potassium salts were recorded in larval and metamorphosed salamanders and compared before and after the oral floor was exposed to amiloride, a blocker of Na(+) channels known to be responsible for epithelial ion transport. Pre-exposure to amiloride (100 microM) did not affect salt taste responses in both axolotls (Ambystoma mexicanum) and larval Ezo salamanders (Hynobius retardatus). In contrast, in metamorphosed Ezo salamanders the nerve responses to NaCl were significantly reduced by amiloride. In amphibians amiloride-sensitive components in salt taste transduction seem to develop during metamorphosis.

Ambystoma↗