Thyroid hormone responsive QTL and the evolution of paedomorphic salamanders

The transformation of ancestral phenotypes into novel traits is poorly understood for many examples of evolutionary novelty. Ancestrally, salamanders have a biphasic life cycle with an aquatic larval stage, a brief and pronounced metamorphosis, followed by a terrestrial adult stage. Repeatedly during evolution, metamorphic timing has been delayed to exploit growth-permissive environments, resulting in paedomorphic salamanders that retain larval traits as adults. We used thyroid hormone (TH) to rescue metamorphic phenotypes in paedomorphic salamanders and then identified quantitative trait loci (QTL) for life history traits that are associated with amphibian life cycle evolution: metamorphic timing and adult body size. We demonstrate that paedomorphic tiger salamanders (Ambystoma tigrinum complex) carry alleles at three moderate effect QTL (met1–3) that vary in responsiveness to TH and additively affect metamorphic timing. Salamanders that delay metamorphosis attain significantly larger body sizes as adults and met2 explains a significant portion of this variation. Thus, substitution of alleles at TH-responsive loci suggests an adaptive pleiotropic basis for two key life-history traits in amphibians: body size and metamorphic timing. Our study demonstrates a likely pathway for the evolution of novel paedomorphic species from metamorphic ancestors via selection of TH-response alleles that delay metamorphic timing and increase adult body size.     

Taste organ growth and development in the direct developing frog Eleutherodactylus coqui (Lissamphibia: Eleutherodactylidae)

Previous research on amphibian taste organs concerned amphibians with a biphasic life history, that is, with larval period and metamorphosis. Direct developing frog species, such as Eleutherodactylus coqui, undergo a cryptic metamorphosis before hatching, and many larval‐specific features are vestigial or have been lost entirely from their ontogeny. Taste buds are present in larval stages of biphasically developing anurans and are replaced by taste discs during metamorphosis. One goal of this study was to characterize the ontogeny of taste buds and/or discs in E. coqui. The other goal was to examine correlations between body size and taste organ density and size in different regions of oral epithelium. The research reveals the presence of only one type of taste organ, characteristic of metamorphs of biphasic amphibians, namely taste disc. In addition, taste disc density and the area of the taste disc sensory zone change dramatically during growth.     

Risk of extinction increases towards higher elevations across the world's amphibians

Aim

Life in mountains is associated with multiple features that increase the risk of demographic collapses in populations – small geographic ranges, short breeding seasons, specialization to harsh climates – leading to the hypothesis that extinction risk is exacerbated in species inhabiting higher elevations. Here, we implement the first test of this hypothesis across the amphibian tree of life – the tetrapods with the largest proportion of montane species, and nature's most threatened animals.

Location

Global.

Time Period

Present.

Major Taxa Studied

Class Amphibia.

Methods

We collated a dataset spanning 8042 species from across all three amphibian orders (Anura, Caudata and Gymnophiona). We preformed phylogenetic logistic regressions to test the predictions that extinction risk increases with elevation, and whether this effect is caused by factors previously hypothesised to drive high-elevation declines, including restrictions on species' geographic ranges, variation in their life histories and the presence of infectious disease.

Results

Globally, extinction risk increases towards higher elevations. At order-level, this relationship holds for frogs and salamanders. Even when controlling for geographic range size, life histories and infectious disease, extinction risk increases with elevation for amphibians combined and frogs globally, and in the Americas. In contrast, whereas extinction risk is greater among high-elevation Eurasian amphibians, this relationship is explained by larger body sizes and lower fecundity.

Main Conclusions

Our analyses indicate that after considering factors previously thought to explain the increase in extinction risk towards higher elevations (e.g., geographic range size, disease), elevation remains a significant predictor of amphibian extinction risk. Given that the only available tests of this hypothesis in other tetrapods (birds and reptiles) conflict with our findings, we suggest that physiological or life-history features of amphibians may explain this observed phenomenon.     

A predictive framework and review of the ecological impacts of exotic plant invasions on reptiles and amphibians

The invasive spread of exotic plants in native vegetation can pose serious threats to native faunal assemblages. This is of particular concern for reptiles and amphibians because they form a significant component of the world's vertebrate fauna, play a pivotal role in ecosystem functioning and are often neglected in biodiversity research. A framework to predict how exotic plant invasion will affect reptile and amphibian assemblages is imperative for conservation, management and the identification of research priorities. Here, we present a new predictive framework that integrates three mechanistic models. These models are based on exotic plant invasion altering: (1) habitat structure; (2) herbivory and predator‐prey interactions; (3) the reproductive success of reptile and amphibian species and assemblages. We present a series of testable predictions from these models that arise from the interplay over time among three exotic plant traits (growth form, area of coverage, taxonomic distinctiveness) and six traits of reptiles and amphibians (body size, lifespan, home range size, habitat specialisation, diet, reproductive strategy). A literature review provided robust empirical evidence of exotic plant impacts on reptiles and amphibians from each of the three model mechanisms. Evidence relating to the role of body size and diet was less clear‐cut, indicating the need for further research. The literature provided limited empirical support for many of the other model predictions. This was not, however, because findings contradicted our model predictions but because research in this area is sparse. In particular, the small number of studies specifically examining the effects of exotic plants on amphibians highlights the pressing need for quantitative research in this area. There is enormous scope for detailed empirical investigation of interactions between exotic plants and reptile and amphibian species and assemblages. The framework presented here and further testing of predictions will provide a basis for informing and prioritising environmental management and exotic plant control efforts.     

Spatial variation in abiotic and biotic factors in a floodplain determine anuran body size and growth rate at metamorphosis

Body size at metamorphosis is a critical trait in the life history of amphibians. Despite the wide-spread use of amphibians as experimental model organisms, there is a limited understanding of how multiple abiotic and biotic factors affect the variation in metamorphic traits under natural conditions. The aim of our study was to quantify the effects of abiotic and biotic factors on spatial variation in the body size of tadpoles and size at metamorphosis of the European common toad (Bufo b. spinosus). Our study population was distributed over the riverbed (active tract) and the fringing riparian forest of a natural floodplain. The riverbed had warm ponds with variable hydroperiod and few predators, whereas the forest had ponds with the opposite characteristics. Spatial variation in body size at metamorphosis was governed by the interactive effects of abiotic and biotic factors. The particular form of the interaction between water temperature and intraspecific tadpole density suggests that abiotic factors laid the foundation for biotic factors: intraspecific density decreased growth only at high temperature. Predation and intraspecific density jointly reduced metamorphic size. Interspecific density had a negligible affect on body size at metamorphosis, suggesting weak inter-anuran interactions in the larval stage. Population density at metamorphosis was about one to two orders of magnitudes higher in the riverbed ponds than in the forest ponds, mainly because of lower tadpole mortality. Based on our results, we conclude that ponds in the riverbed appear to play a pivotal role for the population because tadpole growth and survival is best in this habitat.     

The hyal and ventral branchial muscles in caecilian and salamander larvae: Homologies and evolution

Amphibians (Lissamphibia) are characterized by a bi‐phasic life‐cycle that comprises an aquatic larval stage and metamorphosis to the adult. The ancestral aquatic feeding behavior of amphibian larvae is suction feeding. The negative pressure that is needed for ingestion of prey is created by depression of the hyobranchial apparatus as a result of hyobranchial muscle action. Understanding the homologies of hyobranchial muscles in amphibian larvae is a crucial step in understanding the evolution of this important character complex. However, the literature mostly focuses on the adult musculature and terms used for hyal and ventral branchial muscles in different amphibians often do not reflect homologies across lissamphibian orders. Here we describe the hyal and ventral branchial musculature in larvae of caecilians (Gymnophiona) and salamanders (Caudata), including juveniles of two permanently aquatic salamander species. Based on previous alternative terminology schemes, we propose a terminology for the hyal and ventral branchial muscles that reflects the homologies of muscles and that is suited for studies on hyobranchial muscle evolution in amphibians. We present a discussion of the hyal and ventral branchial muscles in larvae of the most recent common ancestor of amphibians (i.e. the ground plan of Lissamphibia). Based on our terminology, the hyal and ventral branchial musculature of caecilians and salamanders comprises the following muscles: m. depressor mandibulae, m. depressor mandibulae posterior, m. hyomandibularis, m. branchiohyoideus externus, m. interhyoideus, m. interhyoideus posterior, m. subarcualis rectus I, m. subarcualis obliquus II, m. subarcualis obliquus III, m. subarcualis rectus II‐IV, and m. transversus ventralis IV. Except for the m. branchiohyoideus externus, all muscles considered herein can be assigned to the ground plan of the Lissamphibia with certainty. The m. branchiohyoideus externus is either apomorphic for the Batrachia (frogs + salamanders) or salamander larvae depending on whether or not a homologous muscle is present in frog tadpoles. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

Breeding site characteristics regulating life history traits of the brown tree frog, Litoria ewingii

Past research has determined the habitat requirements of amphibian species predominantly from presence/absence studies. This study tested the hypothesis that relationships between breeding site habitat components, life history traits and fitness may provide a higher resolution of biological data relating to the habitat requirements of amphibian species. We tested this novel approach by using Litoria ewingii as our model species. We correlated larval and metamorph life history traits with habitat variables at 28 small to medium sized ponds within a commercially logged forest in southern Tasmania, Australia. To avoid larval mortality due to pond desiccation, L. ewingii laid eggs and metamorphosed earlier in smaller ponds. Snout vent length at metamorphosis increased with elevation and metamorphosis was earlier in less shaded ponds. Breeding ponds that maximised the fitness of L. ewingii were higher elevation ponds with reduced shading, steeper bank slopes and reduced pond isolation. The findings of the study equip land managers with a greater ecological understanding of ecosystem function in relation to specific species. The methodological approach has broad application to conservation biology where an awareness of the specific habitat requirements of amphibians is critical to successful ecosystem management.     

Mapping amphibian contact zones and phylogeographical break hotspots across the United States

Identifying congruence in the geographical position of lineage breaks and species range limits across multiple taxa is a focus of the field of comparative phylogeography. These regions are biogeographical hotspots for investigations into the processes driving divergence at multiple phylogenetic levels. We used spatially explicit statistical methods to identify these regions for amphibians across the United States. Significant clustering occurred in the Appalachian Mountains and in the general area of Alabama - a region underappreciated as an important amphibian hotspot. When the orders Caudata and Anura were examined separately, spatial clustering was still found in Alabama for both. However, in Caudata the Appalachians and California were also important, and for Anura, the Great Lakes region was highlighted. When species richness was statistically controlled, cluster hotspots shifted out of Alabama and the Appalachians and moved to broader swaths of the Great Lakes region, southwestern United States and California. The exact location of particular suture zones is probably a result of complex interactions between historical and ecological factors including physiography, climate and distance from glacial refugia. These contact zone and phylogeographical break hotspots are ideal arenas in which to test alternative speciation hypotheses and examine the extent of reproductive isolation using novel, integrative approaches combining modern methods in statistical phylogeography, ecological niche modelling and genomics.     

中国两栖类生活史和生态学特征数据集

截至2021年底, 中国共记载591种两栖类(不包括外来入侵种), 是世界上两栖类物种数最多的国家之一。物种特征影响和决定了物种在环境中的生存能力, 在进化生物学、生态学和保护生物学研究中具有重要作用。目前, 中国两栖类在线数据库(https://www.amphibiachina.org)已经整理了我国两栖类的形态特征、生物学信息和地理分布等部分物种特征, 但仍缺乏一个完整的、可自由检索的数据集。通过系统查阅文献和数据检索, 本文收集整理了中国全部591种本土两栖类的22个特征数据, 包括物种发现时间(最早发现和描述新物种文章的发表时间)、模式产地、中国受威胁等级、全球受威胁等级、致危因子、是否中国特有种、是否岛屿特有种、雄性体长、雌性体长、窝卵数、卵径、产卵地点、幼体主要生境、成体微生境、活动模式、生殖周期、繁殖时间、是否有亲本照顾、受精方式、海拔范围、分布地区、分布省份数。除了物种发现时间、是否岛屿特有种、分布地区、分布省份数外, 其余特征数据都有不同程度的缺失, 数据完整度为27.92%-100%, 平均完整度为71.72%, 标准差为24%。本数据集是目前我国两栖类最新和最全的物种特征数据库, 希望可以为我国两栖类生态学、保护生物学、生物地理学等研究提供数据支持。     

Initial diversification of living amphibians predated the breakup of Pangaea

The origin and divergence of the three living orders of amphibians (Anura, Caudata, Gymnophiona) and their main lineages are one of the most hotly debated topics in vertebrate evolution. Here, we present a robust molecular phylogeny based on the nuclear RAG1 gene as well as results from a variety of alternative independent molecular clock calibrations. Our analyses suggest that the origin and early divergence of the three living amphibian orders dates back to the Palaeozoic or early Mesozoic, before the breakup of Pangaea, and soon after the divergence from lobe-finned fishes. The resulting new biogeographic scenario, age estimate, and the inferred rapid divergence of the three lissamphibian orders may account for the lack of fossils that represent plausible ancestors or immediate sister taxa of all three orders and the heretofore paradoxical distribution of some amphibian fossil taxa. Furthermore, the ancient and rapid radiation of the three lissamphibian orders likely explains why branch lengths connecting their early nodes are particularly short, thus rendering phylogenetic inference of implicated relationships especially difficult.     

Epinephrine, norepinephrine and dopamine concentrations in amphibian brain

The whole brain concentrations of epinephrine, norepinephrine and dopamine in North American amphibians, orders Caudata and Salientia, are reported. Epinephrine is the major catecholamine in the Salientia while norepinephrine and epinephrine concentrations are roughly equivalent in suborders of Caudata. Relative regional concentrations are similar to other classes (mammals, reptiles and birds) although the absolute concentration of epinephrine is considerably higher in amphibians than in most other species.     

甘肃两栖动物的保护研究

调查表明甘肃有两栖类动物24种,其中有尾目3科3属3种,无尾目6科21种.属于古北界的有5种和亚种,属东洋界的有14种,其余5种为两界兼有种.对引起甘肃省两栖类动物种群衰退、变异和灭绝的主要因素进行了初步分析,提出了若干针对甘肃两栖类动物的保护对策.     

贵州大沙河自然保护区两栖爬行动物多样性研究

对贵州道真县大沙河保护区的两栖爬行动物进行了3次调查研究。共采获两栖动物2目8科15属27种（亚种）,占贵州省两栖动物总种数62种（亚种）的43．5％;爬行动物3目10科（亚科）28属34种（亚种）,占贵州省爬行动物总种数104种的32．7％。从物种组成和区系分析,本保护区的两栖动物以无尾目的蛙科为主,共12种,占保护区内总种数27种的44．4％;主要区系成分为东洋界的华中区种,共11种,占保护区总种数的40．74％;其物种的组成与中国及贵州的两栖动物区系特点一致。爬行动物以游蛇科为其主要组成部分,共有17种,占该保护区爬行动物种数的50．0％;这一特征与我国以及贵州爬行动物的组成特征是基本一致的。但是大沙河地区爬行动物组成中,蝮亚科有2属5种,居第2位,与我国以及贵州的爬行动物组成特点不同,故蝮亚科为构成该区爬行动物区系的主要特征成分之一。本文还运用G-F指数对大沙河自然保护区的两栖爬行动物进行科属间的物种多样性分析。结果表明爬行动物的科内和科间的多样性较高,属内和属间的多样性较低,G-F指数为正值;两栖动物的科内和科间的多样性较低,属内和属间的多样性较高,G—F指数为负值。     

Insular shifts and trade-offs in life-history traits in pond frogs in the Zhoushan Archipelago, China

Island and mainland populations of animal species often differ strikingly in life-history traits such as clutch size, egg size, total reproductive effort and body size. However, despite widespread recognition of insular shifts in these life-history traits in birds, mammals and reptiles, there have been no reports of such life-history shifts in amphibians. Furthermore, most studies have focused on one specific life-history trait without explicit consideration of coordinated evolution among these intimately linked life-history traits, and thus the relationships among these traits are poorly studied. Here we provide the first evidence of insular shifts and trade-offs in a coordinated suite of life-history traits for an amphibian species, the pond frog Rana nigromaculata . Life-history data were collected from eight islands in the Zhoushan Archipelago and neighboring mainland China. We found consistent, significant shifts in all life-history traits between mainland and island populations. Island populations had smaller clutch sizes, larger egg sizes, larger female body size and invested less in total reproductive effort than mainland populations. Significant negative relationships were found between egg size and clutch size and between egg size and total reproductive effort among frog populations after controlling for the effects of body size. Therefore, decreased reproductive effort and clutch size, larger egg size and body size in pond frogs on islands were selected through trade-offs as an overall life-history strategy. Our findings contribute to the formation of a broad, repeatable ecological generality for insular shifts in life-history traits across a range of terrestrial vertebrate taxa.     

Regulation by thyroid hormones of terminal differentiation in the skeletal dorsal muscle. II. Urodelan amphibians

In the urodelan amphibian Pleurodeles waltlii, spontaneous anatomical metamorphosis was correlated with an increase in the serum level of thyroxine (T4). It was also accompanied by a change in the myofibrillar ATPase profile of the dorsal skeletal muscle; fibers of larval type were gradually replaced by the adult fiber types I, II A, and II B. Likewise, a myosin isoenzymic transition was observed in dorsal muscle, larval isomyosins were replaced by adult isoforms. In a related species, Ambystoma mexicanum, in which no spontaneous external metamorphosis occurs under standard conditions, the serum T4 level was shown to remain low. During further development, the myofibrillar ATPase profile acquired the adult fiber types, but a high percentage of immature fibers of type II C persisted. Myosin isoenzymic transition was also incomplete; larval isoforms were still distinguished in the neotenic adults. In experimental hypothyroidian P. waltlii, no external metamorphosis occurred; the myofibrillar ATPase profile was of the immature type, and the larval isomyosins persisted. Triiodothyronine induced experimental anatomical metamorphosis in A. mexicanum; only limited changes in the myofibrillar ATPase profile resulted from the treatment, but a complete myosin isoenzymic transition was observed. These results tend to indicate that a moderate increase in the level of thyroid hormone is sufficient to induce the differentiation of adult fiber types, together with the production of adult myosin isoforms in the skeletal dorsal muscle of amphibians, while a pronounced increase would be necessary for repressing the initial larval features.     

Life history tradeoffs influence mortality associated with the amphibian pathogen Batrachochytrium dendrobatidis

Fatal amphibian chytridiomycosis has typically been associated with the direct costs of infection. However the relationship between exposure to the pathogen, infection and mortality may not be so straightforward. Using results from both field work and experiments we report how exposure of common toads to Batrachochytrium dendrobatidis influences development and survival and how developmental stage influences host responses. Our results show that costs are accrued in a dose dependent manner during the larval stage and are expressed at or soon after metamorphosis. Exposure to B. dendrobatidis always incurs a growth cost for tadpoles and can lead to larval mortality before or soon after metamorphosis even when individuals do not exhibit infection at time of death. In contrast, exposure after metamorphosis almost always results in infection, but body size dictates survival to a greater extent than does dose. These data show that amphibian survival in the face of challenge by an infectious agent is dependent on host condition as well as life history stage. Under current models of climate change, many species of amphibia are predicted to increasingly occur outside their environmental optima. In this case, condition-dependent traits such as we have demonstrated may weigh heavily on species survival.     

探讨两栖纲三个目之间的系统发生关系

现存两栖纲分为3个目,然而它们的系统发生关系仍存在较大的争议.选择Genbank上25种两栖动物线粒体全序列,结合已测定的福建大头蛙(Lim nonectes fujianensis)线粒体基因组全序列,以天鹅和眼镜凯门鳄为外群,用线粒体tRNA基因的合并数据重建系统树.采用P AU P(version 4.0b10)软件构建MP和NJ树,tree-puzzle5.2构建ML树.结果显示,NJ树和ML树以较高的自引导值支持有尾目和蚓螈目为姐妹群,而MP树则支持无尾目和有尾目为姐妹群.     

Density dependence in the terrestrial life history stage of two anurans

Populations of species with complex life cycles have the potential to be regulated at multiple life history stages. However, research tends to focus on single stage density-dependence, which can lead to inaccurate conclusions about population regulation and subsequently hinder conservation efforts. In amphibians, many studies have demonstrated strong effects of larval density and have often assumed that populations are regulated at this life history stage. However, studies examining density regulation in the terrestrial stages are rare, and the functional relationships between terrestrial density and vital rates in amphibians are unknown. We determined the effects of population density on survival, growth and reproductive development in the terrestrial stage of two amphibians by raising juvenile wood frogs (Rana sylvatica) and American toads (Bufo americanus) at six densities in terrestrial enclosures. Density had strong negative effects on survival, growth and reproductive development in both species. We fitted a priori recruitment functions to describe the relationship between initial density and the density of survivors after one year, and determined the functional relationship between initial density and mass after one year. Animals raised at the lowest densities experienced growth and survival rates that were over twice as great as those raised at the highest density. All female wood frogs in the lowest density treatment showed signs of reproductive development, compared to only 6% in the highest density treatment. Female American toads reached minimum reproductive size only at low densities, and male wood frogs and American toads reached maturity only in the three lowest density treatments. Our results demonstrate that in the complex life cycle of amphibians, density in the terrestrial stage can reduce growth, survival and reproductive development and may play an important role in amphibian population regulation. We discuss the implications of these results for population regulation in complex life cycles and for amphibian conservation.     

Larval Environment Alters Amphibian Immune Defenses Differentially across Life Stages and Populations

Recent global declines, extirpations and extinctions of wildlife caused by newly emergent diseases highlight the need to improve our knowledge of common environmental factors that affect the strength of immune defense traits. To achieve this goal, we examined the influence of acidification and shading of the larval environment on amphibian skin-associated innate immune defense traits, pre and post-metamorphosis, across two populations of American Bullfrogs (Rana catesbeiana), a species known for its wide-ranging environmental tolerance and introduced global distribution. We assessed treatment effects on 1) skin-associated microbial communities and 2) post-metamorphic antimicrobial peptide (AMP) production and 3) AMP bioactivity against the fungal pathogen Batrachochytrium dendrobatidis (Bd). While habitat acidification did not affect survival, time to metamorphosis or juvenile mass, we found that a change in average pH from 7 to 6 caused a significant shift in the larval skin microbial community, an effect which disappeared after metamorphosis. Additionally, we found shifts in skin-associated microbial communities across life stages suggesting they are affected by the physiological or ecological changes associated with amphibian metamorphosis. Moreover, we found that post-metamorphic AMP production and bioactivity were significantly affected by the interactions between pH and shade treatments and interactive effects differed across populations. In contrast, there were no significant interactions between treatments on post-metamorphic microbial community structure suggesting that variation in AMPs did not affect microbial community structure within our study. Our findings indicate that commonly encountered variation in the larval environment (i.e. pond pH and degree of shading) can have both immediate and long-term effects on the amphibian innate immune defense traits. Our work suggests that the susceptibility of amphibians to emerging diseases could be related to variability in the larval environment and calls for research into the relative influence of potentially less benign anthropogenic environmental changes on innate immune defense traits.     

Parasites lost? An overlooked hypothesis for the evolution of alternative reproductive strategies in amphibians

Amphibians exhibit the greatest diversity of reproductive strategies of all tetrapod vertebrates. While authors have traditionally attributed the evolution of these strategies to factors such as complex topography, unpredictable larval environments, and predation on larvae and eggs, support for any of these hypotheses has been limited. Importantly, most authors have ignored parasites, including unicellular pathogens and multicellular parasites, as selective agents capable of influencing amphibian evolution. Insights in disease transmission, amphibian immunity, and their interaction with various life histories require that we consider parasites to be selective pressures in our exploration of the evolution of amphibian reproductive strategies. I review recent findings and describe how these principles converge to form a novel conceptual hypothesis for the evolution of alternative reproductive strategies in amphibians. I offer some specific predictions and recommend that parasites be considered with other selective pressures when constructing formal, falsifiable hypotheses during evaluative studies of amphibian reproductive behavior.