Fetal adaptations for viviparity in amphibians

Live‐bearing has evolved in all three orders of amphibians—frogs, salamanders, and caecilians. Developing young may be either yolk dependent, or maternal nutrients may be supplied after yolk is resorbed, depending on the species. Among frogs, embryos in two distantly related lineages develop in the skin of the maternal parents' backs; they are born either as advanced larvae or fully metamorphosed froglets, depending on the species. In other frogs, and in salamanders and caecilians, viviparity is intraoviductal; one lineage of salamanders includes species that are yolk dependent and born either as larvae or metamorphs, or that practice cannibalism and are born as metamorphs. Live‐bearing caecilians all, so far as is known, exhaust yolk before hatching and mothers provide nutrients during the rest of the relatively long gestation period. The developing young that have maternal nutrition have a number of heterochronic changes, such as precocious development of the feeding apparatus and the gut. Furthermore, several of the fetal adaptations, such as a specialized dentition and a prolonged metamorphosis, are homoplasious and present in members of two or all three of the amphibian orders. At the same time, we know little about the developmental and functional bases for fetal adaptations, and less about the factors that drive their evolution and facilitate their maintenance. J. Morphol. 276:941–960, 2015. © 2014 Wiley Periodicals, Inc.     

Mitochondrial evidence on the phylogenetic position of caecilians (Amphibia: Gymnophiona)

The complete nucleotide sequence (17,005 bp) of the mitochondrial genome of the caecilian Typhlonectes natans (Gymnophiona, Amphibia) was determined. This molecule is characterized by two distinctive genomic features: there are seven large 109-bp tandem repeats in the control region, and the sequence for the putative origin of replication of the L strand can potentially fold into two alternative secondary structures (one including part of the tRNA(Cys)). The new sequence data were used to assess the phylogenetic position of caecilians and to gain insights into the origin of living amphibians (frogs, salamanders, and caecilians). Phylogenetic analyses of two data sets-one combining protein-coding genes and the other combining tRNA genes-strongly supported a caecilian + frog clade and, hence, monophyly of modern amphibians. These two data sets could not further resolve relationships among the coelacanth, lungfishes, and tetrapods, but strongly supported diapsid affinities of turtles. Phylogenetic relationships among a larger set of species of frogs, salamanders, and caecilians were estimated with a mitochondrial rRNA data set. Maximum parsimony analysis of this latter data set also recovered monophyly of living amphibians and favored a frog + salamander (Batrachia) relationship. However, bootstrap support was only moderate at these nodes. This is likely due to an extensive among-site rate heterogeneity in the rRNA data set and the narrow window of time in which the three main groups of living amphibians were originated.     

Salamander-like development in a seymouriamorph revealed by palaeohistology

The amniotes generally lay eggs on land and are thereby differentiated from lissamphibians (salamanders, frogs and caecilians) by their developmental pattern. Although a number of 330-300-Myr old fossils are regarded as early tetrapods placed close to amniotes on the basis of anatomical data, we still do not know whether their developmental pattern was more similar to those of lissamphibians or amniotes. Here we report palaeohistological and skeletochronological evidence supporting a salamander-like development in the seymouriamorph Discosauriscus. Its long-bone growth pattern, slow diaphyseal growth rate and delayed sexual maturity (at more than 10 years old) are more comparable with growth features of extant salamanders rather than extant amniotes, even though they are mostly hypothesized to be phylogenetically closer to living amniotes than salamanders.     

大头蛙和脆皮大头蛙线粒体3个基因的测定及两栖类亲缘关系研究

现存的两栖类系统发生关系一直存在争议,特别是3个目间的亲缘关系。本文设计了5对引物,扩增和测定了大头蛙和脆皮大头蛙线粒体12S和16S rRNA基因和Cytb基因的全序列。在对所测序列进行分析的同时,基于3个基因全序列的相加数据,运用MEGA 3.1和PHYLIP 3.64软件中的NJ法、MP法和ML法,对两爬类17个物种,以鱼类非洲肺鱼为外群,重建出3个树形完全一致的分子系统树。研究结果显示:现存两栖类中无尾目和有尾目为姐妹群关系,并推断有尾目内小鲵科和隐鳃鲵科亲缘关系较近。此外,在研究两栖类系统发生关系方面,作者分析前人研究中产生两种不同观点的可能原因,同时总结了在此类研究中产生偏差的几种影响因素。     

Global patterns of diversification and species richness in amphibians

Geographic patterns of species richness ultimately arise through the processes of speciation, extinction, and dispersal, but relatively few studies consider evolutionary and biogeographic processes in explaining these diversity patterns. One explanation for high tropical species richness is that many species-rich clades originated in tropical regions and spread to temperate regions infrequently and more recently, leaving little time for species richness to accumulate there (assuming similar rates of diversification in temperate and tropical regions). However, the major clades of anurans (frogs) and salamanders may offer a compelling counterexample. Most salamander families are predominately temperate in distribution, but the one primarily tropical clade (Bolitoglossinae) contains nearly half of all salamander species. Similarly, most basal clades of anurans are predominately temperate, but one largely tropical clade (Neobatrachia) contains approximately 96% of anurans. In this article, I examine patterns of diversification in frogs and salamanders and their relationship to large-scale patterns of species richness in amphibians. I find that diversification rates in both frogs and salamanders increase significantly with decreasing latitude. These results may shed light on both the evolutionary causes of the latitudinal diversity gradient and the dramatic but poorly explained disparities in the diversity of living amphibian clades.     

Ontogenetic evidence for the Paleozoic ancestry of salamanders

The phylogenetic positions of frogs, salamanders, and caecilians have been difficult to establish. Data matrices based primarily on Paleozoic taxa support a monophyletic origin of all Lissamphibia but have resulted in widely divergent hypotheses of the nature of their common ancestor. Analysis that concentrates on the character states of the stem taxa of the extant orders, in contrast, suggests a polyphyletic origin from divergent Paleozoic clades. Comparison of patterns of larval development in Paleozoic and modern amphibians provides a means to test previous phylogenies based primarily on adult characteristics. This proves to be highly informative in the case of the origin of salamanders. Putative ancestors of salamanders are recognized from the Permo-Carboniferous boundary of Germany on the basis of ontogenetic changes observed in fossil remains of larval growth series. The entire developmental sequence from hatching to metamorphosis is revealed in an assemblage of over 600 specimens from a single locality, all belonging to the genus Apateon. Apateon forms the most speciose genus of the neotenic temnospondyl family Branchiosauridae. The sequence of ossification of individual bones and the changing configuration of the skull closely parallel those observed in the development of primitive living salamanders. These fossils provide a model of how derived features of the salamander skull may have evolved in the context of feeding specializations that appeared in early larval stages of members of the Branchiosauridae. Larvae of Apateon share many unique derived characters with salamanders of the families Hynobiidae, Salamandridae, and Ambystomatidae, which have not been recognized in any other group of Paleozoic amphibians.     

The skull and jaw musculature as guides to the ancestry of salamanders

The fossil record provides no evidence supporting a unique common ancestry for frogs, salamanders and apodans. The ancestors of the modern orders may have diverged from one another as recently as 250 million years ago, or as long ago as 400 million years according to current theories of various authors. In order to evaluate the evolutionary patterns of the modern orders it is necessary to determine whether their last common ancestor was a rhipidistian fish, a very primitive amphibian, a labyrimhodom or a ‘lissamphibian’. The broad cranial similarities of frogs and salamanders, especially the dominance of the braincase as a supporting element, can be associated with the small size of the skull in their immediate ancestors. Hynobiids show the most primitive cranial pattern known among the living salamander families and “provide a model for determining the nature of the ancestors of the entire order. Features expected in ancestral salamanders include: (1) Emargination of the cheek; (2) Movable suspensorium formed by the quadrate, squamosal and pterygoid; (3) Occipital condyle posterior to jaw articulation; (4) Distinct prootic and opisthotic; (5) Absence ol otic notch; (6) Stapes forming a structural link between braincase and cheek. In the otic region, cheek and jaw suspension, the primitive salamander pattern (resembles most closely the microsaurs among known Paleozoic amphibians, and shows no significant features in common with either ancestral frogs or the majority of labyrinth odonts. The basic pattern of the adductor jaw musculature is consistent within both frogs and salamanders, but major differences are evident between the two groups. The dominance of the adductor mandibulae externus in salamanders can be associated with the open cheek in all members of that order, and the small size of this muscle in frogs can be associated with the large otic notch. The spread of different muscles over the otic capsule, the longus head ol the adductor mandibulae posterior in frogs and the superficial head of the adductor mandibulae internus in salamanders, indicates that fenestration of the skull posterodorsal to the orbit occurred separately in the ancestors of the two groups. Reconstruction of the probable pattern of the jaw musculature in Paleozoic amphibians indicates that frogs and salamanders might have evolved from a condition hypothesized for primitive labyrinthodonts, but the presence of a large otic notch in dissorophids suggests specialization toward the anuran, not the urodele condition. The presence of either an einarginated cheek or an embayment of the lateral surface of the dentary and the absence of an otic notch in microsaurs indicate a salamander-like distribution of die adductor jaw muscles. The ancestors of frogs and salamanders probably diverged from one another in the early Carboniferous, Frogs later evolved from small labyrinthodonts and salamanders from microsaurs. Features considered typical of lissamphibians evolved separately in the two groups in the late Permian andTriassic.     

Variation in the trunk musculature of caecilians (Amphibia: Gymnophiona)

Variation in the trunk musculature of 28 species of caecilians. representing 24 of the 33 genera and all five families. is summarized. All forms examined have the same muscles in similar positions. Existing variation largely conforms to the current classification of the group. and some variation may be attributable to different modes of locomotion. such as burrowing versus swimming. Caecilian trunk musculature is more similar to that of salamanders than to that of frogs. but the similarity is probably syrnplesiomorphous. Trunk musculature so far has provided no clues to lissamphibian relationships.     

Development of the tectum in Gymnophiones,with comparison to other amphibians

Tectal development in a number of caecilian (Gymnophiona: Amphibia) species was examined and compared with that in frogs and salamanders. The caecilian optic tectum develops along the same rostrocaudal and lateromedial gradients as those of frogs and salamanders. However, differences exist in the time course of development. Our data suggest that, as in salamanders, simplification of morphological complexity in caecilians is due to a retardation or loss of late developmental stages. Differences in the time course of development (heterochrony) among different caecilian species are correlated with phylogenetic history as well as with variation in life histories. The most pronounced differences in development occur between the directly developing Hypogeophis rostratus and all other species examined. In this species, the increase in the degree of morphological complexity is greatly accelerated. J. Morphol. 236:233–246, 1998. © 1998 Wiley-Liss, Inc.     

Universal COI primers for DNA barcoding amphibians

DNA barcoding is a proven tool for the rapid and unambiguous identification of species, which is essential for many activities including the vouchering tissue samples in the genome 10K initiative, genealogical reconstructions, forensics and biodiversity surveys, among many other applications. A large‐scale effort is underway to barcode all amphibian species using the universally sequenced DNA region, a partial fragment of mitochondrial cytochrome oxidase subunit I COI. This fragment is desirable because it appears to be superior to 16S for barcoding, at least for some groups of salamanders. The barcoding of amphibians is essential in part because many species are now endangered. Unfortunately, existing primers for COI often fail to achieve this goal. Herein, we report two new pairs of primers (?, ?) that in combination serve to universally amplify and sequence all three orders of Chinese amphibians as represented by 36 genera. This taxonomic diversity, which includes caecilians, salamanders and frogs, suggests that the new primer pairs will universally amplify COI for the vast majority species of amphibians.     

Vertebral development and amphibian evolution

Amphibians provide an unparalleled opportunity to integrate studies of development and evolution through the investigation of the fossil record of larval stages. The pattern of vertebral development in modern frogs strongly resembles that of Paleozoic labyrinthodonts in the great delay in the ossification of the vertebrae, with the centra forming much later than the neural arches. Slow ossification of the trunk vertebrae in frogs and the absence of ossification in the tail facilitate the rapid loss of the tail during metamorphosis, and may reflect retention of the pattern in their specific Paleozoic ancestors. Salamanders and caecilians ossify their centra at a much earlier stage than frogs, which resembles the condition in Paleozoic lepospondyls. The clearly distinct patterns and rates of vertebral development may indicate phylogenetic separation between the ultimate ancestors of frogs and those of salamanders and caecilians within the early radiation of ancestral tetrapods. This divergence may date from the Lower Carboniferous. Comparison with the molecular regulation of vertebral development described in modern mammals and birds suggests that the rapid chondrification of the centra in salamanders relative to that of frogs may result from the earlier migration of sclerotomal cells expressing Pax1 to the area surrounding the notochord.     

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.     

Osseous pathologies in the lungless salamander Desmognathus fuscus (Plethodontidae)

The majority of reported pathologies in lissamphibians (salamanders, caecilians and frogs) include limb deformities such as missing limbs, multiple extra limbs and digits, or incomplete limb formation. However, comparatively little is known about congenital vertebral malformations or posttraumatic pathologies (e.g. injuries, infections) in the vertebral column of salamanders. In the present study, we describe eight vertebral deformities in three cleared and stained specimens of Desmognathus fuscus. Two specimens display developmental deformities which range from a potential non-segmented wedge vertebra to fully segmented hemivertebrae. The vertebral pathology in the third specimens possibly results from a parasitic infection. Apparently, these osseous deformities were not severe enough to prohibit survival of the specimens.     

Genetic drift and mutational hazard in the evolution of salamander genomic gigantism

Salamanders have the largest nuclear genomes among tetrapods and, excepting lungfishes, among vertebrates as a whole. Lynch and Conery (2003) have proposed the mutational‐hazard hypothesis to explain variation in genome size and complexity. Under this hypothesis, noncoding DNA imposes a selective cost by increasing the target for degenerative mutations (i.e., the mutational hazard). Expansion of noncoding DNA, and thus genome size, is driven by increased levels of genetic drift and/or decreased mutation rates; the former determines the efficiency with which purifying selection can remove excess DNA, whereas the latter determines the level of mutational hazard. Here, we test the hypothesis that salamanders have experienced stronger long‐term, persistent genetic drift than frogs, a related clade with more typically sized vertebrate genomes. To test this hypothesis, we compared dN/dS and Kr/Kc values of protein‐coding genes between these clades. Our results do not support this hypothesis; we find that salamanders have not experienced stronger genetic drift than frogs. Additionally, we find evidence consistent with a lower nucleotide substitution rate in salamanders. This result, along with previous work showing lower rates of small deletion and ectopic recombination in salamanders, suggests that a lower mutational hazard may contribute to genomic gigantism in this clade.     

Monophyly and affinities of albanerpetontid amphibians (Temnospondyli; Lissamphibia)

The Albanerpetontidae are Middle Jurassic-Miocene amphibians that have variously been regarded as caudates (salamanders), a clade distinct from caudates, or incertae sedis lissamphibians. Here I test for monophyly of the Albanerpetontidae and examine the affinities of the group, within the framework of a more inclusive Temnospondyli, by performing a cladistic analysis using 59 informative characters scored for four non-lissamphibian temnospondyl genera, stem- and crown-clade caudates, salientians (frogs), gymnophionans (caecilians), and the two recognized albanerpetontid genera Albanerpeton and Celtedens . Monophyly of the Albanerpetontidae is corroborated. I interpret synapomorphies of the marginal teeth (non-pedicellate; crowns chisel like, labiolingually compressed, with three mesiodistally aligned cuspules) in albanerpetontids as being associated with a shearing bite. Other synapomorphies evidently strengthened and increased the mobility of the skull, mandible, and cervical region for burrowing, feeding, or both. Nested sets of synapomorphies place the Albanerpetontidae within the Lissamphibia, as the sistertaxon of Caudata plus Salientia. None of the 17 characters previously advanced as albanerpetontid-caudate synapomorphies convincingly places the Albanerpetontidae within the Caudata or allies the two groups as sistertaxa. Albanerpetontids are better interpreted not as aberrant caudates, but as a distinct clade of lissamphibians in which numerous apomorphies are superimposed upon an otherwise primitive lissamphibian body plan.     

The phylogeny of amphibian metamorphosis

Frogs have one of the most extreme metamorphoses among vertebrates. How did this metamorphosis evolve? By combining the methods previously proposed by Mabee and Humphries (1993) and Velhagen (1997), I develop a phylogenetic method suited for rigorous analysis of this question. In a preliminary analysis using 12 transformation sequence characters and 36 associated event sequence characters, all drawn from the osteology of the skull, the evolution of metamorphosis is traced on an assumed phylogeny. This phylogeny has lissamphibians (frogs, salamanders, and caecilians) monophyletic, with frogs the sister group of salamanders. Successive outgroups used are temnospondyls and discosauriscids, both of which are fossil groups for which ontogenetic data are available. In the reconstruction of character evolution, an unambiguous change (synapomorphy) along the branch leading to lissamphibians is a delay in the lengthening of the maxilla until metamorphosis, in accordance with my previous suggestion (Reiss, 1996). However, widening of the interpterygoid vacuity does not appear as a synapomophy of lissamphibians, due to variation in the character states in the outgroups. From a more theoretical perspective, the reconstructed evolution of amphibian metamorphosis involves examples of heterochrony, through the shift of ancestral premetamorphic events to the metamorphic period, caenogenesis, through the origin of new larval features, and terminal addition, through the origin of new adult features. Other changes don't readily fit these categories. This preliminary study provides evidence that metamorphic changes in frogs arose as further modifications of changes unique to lissamphibians, as well as a new method by which such questions can be examined.     

Development and evolution of the tetrapod skull–neck boundary

The origin and evolution of the vertebrate skull have been topics of intense study for more than two centuries. Whereas early theories of skull origin, such as the influential vertebral theory, have been largely refuted with respect to the anterior (pre‐otic) region of the skull, the posterior (post‐otic) region is known to be derived from the anteriormost paraxial segments, i.e. the somites. Here we review the morphology and development of the occiput in both living and extinct tetrapods, taking into account revised knowledge of skull development by augmenting historical accounts with recent data. When occipital composition is evaluated relative to its position along the neural axis, and specifically to the hypoglossal nerve complex, much of the apparent interspecific variation in the location of the skull–neck boundary stabilizes in a phylogenetically informative way. Based on this criterion, three distinct conditions are identified in (i) frogs, (ii) salamanders and caecilians, and (iii) amniotes. The position of the posteriormost occipital segment relative to the hypoglossal nerve is key to understanding the evolution of the posterior limit of the skull. By using cranial foramina as osteological proxies of the hypoglossal nerve, a survey of fossil taxa reveals the amniote condition to be present at the base of Tetrapoda. This result challenges traditional theories of cranial evolution, which posit translocation of the occiput to a more posterior location in amniotes relative to lissamphibians (frogs, salamanders, caecilians), and instead supports the largely overlooked hypothesis that the reduced occiput in lissamphibians is secondarily derived. Recent advances in our understanding of the genetic basis of axial patterning and its regulation in amniotes support the hypothesis that the lissamphibian occipital form may have arisen as the product of a homeotic shift in segment fate from an amniote‐like condition.     

Complete nucleotide sequence and gene arrangement of the mitochondrial genome of the crab-eating frog Fejervarya cancrivora and evolutionary implications

The complete nucleotide sequence of the mitochondrial (mt) genome of the crab-eating frog, Fejervarya cancrivora Gravenhorst (Amphibia: Anura: Ranidae), was determined. The mt genome is 17,843 bp long and contains 13 protein-coding (ATP6, ATP8, COI-III, ND1-6 and 4L, and Cyt b) and two ribosomal RNA (12S and 16SrRNA) genes. Although metazoan mt genomes typically encode 22 transfer RNA genes (tRNAs), the F. cancrivora mtDNA contains 23 tRNAs due to the presence of an extra copy of tRNA(Met). A major noncoding region and a prominent intergenic spacer corresponding to the control region and light-strand replication origin were also found. To confirm the phylogenetic position of F. cancrivora, we compared the gene arrangement with that of other anurans and performed phylogenetic analyses based on mt genomic data. The genome organization of F. cancrivora mtDNA differs from that of typical vertebrates and neobatrachian frogs but is identical with that of F. limnocharis, suggesting that the unique gene arrangement occurred in the common ancestor of the genus. Phylogenetic analyses supported the monophyly of the Fejervarya species used here as well as the dicroglossini clade. Although the family Ranidae as previously recognized (= Ranidae, Discoglossidae, and some other natatanuran families; sensu Frost et al., 2006) is shown as a clade in the maximum parsimony analysis, the maximum likelihood and the Bayesian analyses suggest the paraphyly of the Ranidae with respect to the families, Mantellidae and Rhacophoridae. Three-tandem duplications of gene regions followed by subsequent deletions of supernumerary genes were proposed to explain the evolution of the extra tRNA(Met) and translocation of ND5 from the original neobatrachian gene order.