Evolution of pipoid frogs: intergeneric relationships of the aquatic frog family Pipidae (Anura)

Morphological, acoustic and genetic identification of a reproducing population of the invasive African clawed frog Xenopuslaevis (Anura, Pipidae) recently discovered in Belgium

Using external morphology of adults and tadpoles, osteology from high-resolution microcomputed tomography, vocalization analysis, and DNA sequence data, the identity of a reproducing Belgian population of invasive Xenopus at the current northernmost edge of the distribution of the genus in Europe was assessed. All data concur to an identification as Xenopus (Xenopus) laevis (Daudin, 1802). Genetically it is most closely related to populations of the Cape region in South Africa. No studies on the natural history of the Belgian Xenopus population and its impact on the local environment have been made to date.

Effects of Development on Bone Mineral Density and Mechanical Properties in the Aquatic Frog, Xenopus Laevis, and a Terrestrial Frog, Lithobates Catesbianus

The limb bones of vertebrates have a critical role in supporting the weight of the body and transmitting forces that power locomotion. The loads that limb bones experience can vary in association with a range of factors, including locomotor environment or developmental stage. Limbed vertebrates that are habitually found in environments with low locomotor loads (e.g., water) might be predicted to also exhibit limb bones with less elevated mechanical properties, such as yield stiffness and yield stress. Frogs provide a distinctive case, in which these ideas can be tested as they experience changes in both locomotor style and habitat as they develop. However, while many frog taxa shift from aquatic to terrestrial habitats as they metamorphose, some lineages, such as pipids, maintain an aquatic lifestyle even after metamorphosis, providing a comparative framework for the effects of habitat shifts on developing limbs in vertebrates. This study compares the material composition and mechanical properties of the femur between frog species that are aquatic specialists (Xenopus laevis) vs generalists that spend considerable time both on land and in water (Lithobates catesbeianus) as they transition from metamorphic tadpoles to fully grown adults. MicroCT scanning was used to determine changes in bone density related to developmental stage and hindlimb use during swimming. Microindentation was then used to collect hardness values from the cortical bone of each femur, which was used to evaluate bone material properties. We found that aquatic frogs had less overall bone mineral density (BMD) than terrestrial frogs and that BMD was more elevated in the cortical region of the diaphysis than trabeculae and distal and proximal epiphyses. Despite its less elevated BMD, bone mechanical properties were not significantly different in aquatic specialist X. laevis than in more terrestrial L. catesbeianus. Our results suggest that the limb bones of aquatic frogs may experience compensatory effects through development to offset their lower BMD. Furthermore, changes in bone density and material properties across development may help to explain some of the differences in locomotor performance found between aquatic and terrestrial metamorphic frogs, providing insight into how environmental factors might correlate with bone ossification.

XROMM Analysis of Feeding Mechanics in Toads: Interactions of the Tongue, Hyoid, and Pectoral Girdle

During feeding in many terrestrial vertebrates, the tongue acts in concert with the hyoid and pectoral girdle. In frogs, these three elements are interconnected by musculature. While the feeding mechanics of the anuran tongue are well-studied, little is known of how the motions of the tongue relate to the movements of the skeleton or how buccal structures move following closure of the mouth. Although features such as the pectoral girdle and hyoid are not externally visible in frogs, their motions can be tracked in X-ray video. We used XROMM (X-ray Reconstruction of Moving Morphology) techniques to track the 3D movements of the tongue, hyoid apparatus, pectoral girdle, skull, and jaw during the feeding cycle of the cane toad, Rhinella marina . We show how the movements of these elements are integrated during tongue protrusion and prey capture, as well as during prey transport, swallowing, and recovery. Our findings suggest that the hyoid apparatus is important both for prey manipulation and swallowing. The tongue consistently stretches posterior to the skull during swallowing, often more than it stretches during protrusion to reach the prey. Feeding kinematics are similar between individuals, and the kinematics of unsuccessful strikes generally resemble those of successful strikes. Our data also provide a new perspective on the potential role of the pectoral girdle, an element with a predominant locomotor function, during feeding events. This work raises new questions about the evolution of feeding in frogs, as well as how the diversity of pectoral and buccal anatomy observed across anurans may influence feeding kinematics.

Evolutionary rates and shape variation along the anuran vertebral column with attention to phylogeny, body size, and ecology

The vertebral column is critical to a vertebrate species' flexibility and skeletal support, making vertebrae a clear target for selection. Anurans (frogs and toads) have a unique, truncated vertebral column that appears constrained to provide axial rigidity for efficient jumping. However, no study has examined how presacral vertebrae shape varies among anuran species at the macroevolutionary scale nor how intrinsic (developmental and phylogenetic) and extrinsic (ecological) factors may have influenced vertebrae shape evolution. We used microCT scans and phylogenetic comparative methods to examine the vertebrae of hundreds of anuran species that vary in body size as well as adult and larval ecology. We found variation in shape and evolutionary rates among anuran vertebrae, dispelling any notion that trunk vertebrae evolve uniformly. We discovered the highest evolutionary rates in the cervical vertebrae and in the more caudal trunk vertebrae. We found little evidence for selection pressures related to adult or larval ecology affecting vertebrae evolution, but we did find body size was highly associated with vertebrae shape and microhabitat (mainly burrowing) affected those allometric relationships. Our results provide an interesting comparison to vertebrae evolution in other clades and a jumping-off point for studies of anuran vertebrae evolution and development.

Ontogeny as a way to understand morphology of nasal capsule structures in Pipidae, with focus on Pipa arrabali (Lissamphibia: Anura)

After more than one-hundred years of studies, there is still no consensus regarding the names for the nasal elements in Pipidae and their correspondence to that of other frogs. Names vary depending on authors’ preferences for an explanatory process associated with the observed state of the structure. Some of the names indicate absence, while others indicate fusion. As naming morphological structures relates to some sort of consensus regarding systematics’ knowledge, one would expect results of the two activities to go hand-in-hand. Within the monophyletic Pipidae, one would expect that the names of structures would be all settled. However, the situation is the contrary. Our efforts to pursue such a research and properly name the involved structures resulted in a deeper understanding of the evolution within the taxa involved. Herein we present the results of the study of embryos and juveniles of P. arrabali; based on which we offer additional evidence for the proper naming of the involved elements of the nasal region. Furthermore, using key structures as topographical markers, and by comparing our evaluation to those presented by previous researchers, we have sorted out the compound nature of some key structures and present a proper naming for the morphology investigated.

Chromosome spreading of the (TTAGGG)n repeats in the Pipa carvalhoi Miranda-Ribeiro, 1937 (Pipidae, Anura) karyotype

Pipidae is a clade of Anura that diverged relatively early from other frogs in the phylogeny of the group. Pipids have a unique combination of morphological features, some of which appear to represent a mix of adaptations to aquatic life and plesiomorphic characters of Anura. The present study describes the karyotype of Pipa carvalhoi Miranda-Ribeiro, 1937, including morphology, heterochromatin distribution, and location of the NOR site. The diploid number of P. carvalhoi is 2n=20, including three metacentric pairs (1, 4, 8), two submetacentric (2 and 7), three subtelocentric (3, 5, 6), and two telocentric pairs (9 and 10). C-banding detected centromeric blocks of heterochromatin in all chromosome pairs and the NOR detected in chromosome pair 9, as confirmed by FISH using the rDNA 28S probe. The telomeric probes indicated the presence of interstitial telomeric sequences (ITSs), primarily in the centromeric region of the chromosomes, frequently associated with heterochromatin, suggesting that these repeats are a significant component of this region. The findings of the present study provide important insights for the understanding of the mechanisms of chromosomal evolution in the genus Pipa, and the diversification of the Pipidae as a whole.

Sesamoids in tetrapods: the origin of new skeletal morphologies

Along with supernumerary bones, sesamoids, defined as any organized intratendinous/intraligamentous structure, including those composed of fibrocartilage, adjacent to an articulation or joint, have been frequently considered as enigmatic structures associated with the joints of the skeletal system of vertebrates. This review allows us to propose a dynamic model to account for part of skeletal phenotypic diversity: during evolution, sesamoids can become displaced, attaching to and detaching from the long bone epiphyses and diaphysis. Epiphyses, apophyses and detached sesamoids are able to transform into each other, contributing to the phenotypic variability of the tetrapod skeleton. This dynamic model is a new paradigm to delineate the contribution of sesamoids to skeletal diversity. Herein, we first present a historical approach to the study of sesamoids, discussing the genetic versus epigenetic theories of their genesis and growth. Second, we construct a dynamic model. Third, we present a summary of literature on sesamoids of the main groups of tetrapods, including veterinary and human clinical contributions, which are the best-studied aspects of sesamoids in recent decades. Finally, we discuss the identity of certain structures that have been labelled as sesamoids despite insufficient formal testing of homology. We also propose a new definition to help the identification of sesamoids in general. This review is particularly timely, given the recent increasing interest and research activity into the developmental biology and mechanics of sesamoids. With this updated and integrative discussion, we hope to pave the way to improve the understanding of sesamoid biology and evolution.

Comparative description and ossification patterns of Dendropsophus labialis (Peters, 1863) and Scinax ruber (Laurenti, 1758) (Anura: Hylidae)

Although comparative studies of anuran ontogeny have provided new data on heterochrony in the life cycles of frogs, most of them have not included ossification sequences. Using differential staining techniques, we observe and describe differences and similarities of cranial and postcranial development in two hylid species, Scinax ruber (Scinaxinae) and Dendropsophus labialis (Hylinae), providing new data of ontogenetic studies in these Colombian species. We examined tadpoles raining from Gosner Stages 25 to 45. We found differences between species in the infrarostral and suprarostral cartilages, optic foramen, planum ethmoidale, and gill apparatus. In both species, the first elements to ossify were the atlas and transverse processes of the vertebral column and the parasphenoid. Both species exhibited suprascapular processes as described in other hylids. Although the hylids comprise a large group (over 700 species), postcranial ossification sequence is only known for 15 species. Therefore, the descriptions of the skeletal development and ossification sequences provided herein will be useful for future analyses of heterochrony in the group.

Peptidomic analysis of skin secretions of the Mexican burrowing toad Rhinophrynus dorsalis (Rhinophrynidae): Insight into the origin of host-defense peptides within the Pipidae and characterization of a proline-arginine-rich peptide

The Mexican burrowing toad Rhinophrynus dorsalis is the sole extant representative of the Rhinophrynidae. United in the superfamily Pipoidea, the Rhinophrynidae is considered to be the sister-group to the extant Pipidae which comprises Hymenochirus, Pipa, Pseudhymenochirus and Xenopus. Cationic, α-helical host-defense peptides of the type found in Hymenochirus, Pseudhymenochirus, and Xenopus species (hymenochirins, pseudhymenochirins, magainins, and peptides related to PGLa, XPF, and CPF) were not detected in norepinephrine-stimulated skin secretions of R. dorsalis. Skin secretions of representatives of the genus Pipa also do not contain cationic α-helical host-defense peptides which suggest, as the most parsimonious hypothesis, that the ability to produce such peptides by frogs within the Pipidae family arose in the common ancestor of (Hymenochirus+Pseudhymenochirus)+Xenopus after divergence from the line of evolution leading to extant Pipa species. Peptidomic analysis of the R. dorsalis secretions led to the isolation of rhinophrynin-27, a proline-arginine-rich peptide with the primary structure ELRLPEIARPVPEVLPARLPLPALPRN, together with rhinophrynin-33 containing the C-terminal extension KMAKNQ. Rhinophrynin-27 shows limited structural similarity to the porcine multifunctional peptide PR-39 but it lacks antimicrobial and cytotoxic activities. Like PR-39, the peptide adopts a poly-l-proline helix but some changes in the circular dichroism spectrum were observed in the presence of anionic sodium dodecylsulfate micelles consistent with the stabilization of turn structures.

Digital dissection of the model organism Xenopus laevis using contrast-enhanced computed tomography

The African clawed frog, Xenopus laevis, is one of the most widely used model organisms in biological research. However, the most recent anatomical description of X. laevis was produced nearly a century ago. Compared with other anurans, pipid frogs - including X. laevis - exhibit numerous unusual morphological features; thus, anatomical descriptions of more 'typical' frogs do not detail many aspects of X. laevis skeletal and soft-tissue morphology. The relatively new method of using iodine-based agents to stain soft tissues prior to high-resolution X-ray imaging has several advantages over gross dissection, such as enabling dissection of very small and fragile specimens, and preserving the three-dimensional topology of anatomical structures. Here, we use contrast-enhanced computed tomography to produce a high-resolution three-dimensional digital dissection of a post-metamorphic X. laevis to successfully visualize: skeletal and muscular anatomy; the nervous, respiratory, digestive, excretory and reproductive systems; and the major sense organs. Our digital dissection updates and supplements previous anatomical descriptions of this key model organism, and we present the three-dimensional data as interactive portable document format (PDF) files that are easily accessible and freely available for research and educational purposes. The data presented here hold enormous potential for applications beyond descriptive purposes, particularly for biological researchers using this taxon as a model organism, comparative anatomy and biomechanical modelling.

The suction mechanism of the pipid frog, Pipa pipa (Linnaeus, 1758)

Most suction-feeding, aquatic vertebrates create suction by rapidly enlarging the oral cavity and pharynx. Forceful enlargement of the pharynx is powered by longitudinal muscles that retract skeletal elements of the hyoid, more caudal branchial arches, and, in many fish, the pectoral girdle. This arrangement was thought to characterize all suction-feeding vertebrates. However, it does not exist in the permanently aquatic, tongueless Pipa pipa, an Amazonian frog that can catch fish. Correlating high-speed (250 and 500 fps) video records with anatomical analysis and functional tests shows that fundamental features of tetrapod body design are altered to allow P. pipa to suction-feed. In P. pipa, the hyoid apparatus is not connected to the skull and is enclosed by the pectoral girdle. The major retractor of the hyoid apparatus arises not from the pectoral girdle but from the femur, which lies largely within the soft tissue boundaries of the trunk. Retraction of the hyoid is coupled with expansion of the anterior trunk, which occurs when the hypertrophied ventral pectoral elements are depressed and the urostyle and sacral vertebra are protracted and slide forward on the pelvic girdle, thereby elongating the entire trunk. We suggest that a single, robust pair of muscles adduct the cleithra to depress the ventral pectoral elements with force, while modified tail muscles slide the axial skeleton cranially on the pelvic girdle. Combined hyoid retraction, axial protraction, and pectoral depression expand the buccopharyngeal cavity to a volume potentially equal to that of the entire resting body of the frog. Pipa may be the only tetrapod vertebrate clade that enlarges its entire trunk during suction-feeding.

Genetics, Morphology, Advertisement Calls, and Historical Records Distinguish Six New Polyploid Species of African Clawed Frog (Xenopus, Pipidae) from West and Central Africa

African clawed frogs, genus Xenopus, are extraordinary among vertebrates in the diversity of their polyploid species and the high number of independent polyploidization events that occurred during their diversification. Here we update current understanding of the evolutionary history of this group and describe six new species from west and central sub-Saharan Africa, including four tetraploids and two dodecaploids. We provide information on molecular variation, morphology, karyotypes, vocalizations, and estimated geographic ranges, which support the distinctiveness of these new species. We resurrect Xenopus calcaratus from synonymy of Xenopus tropicalis and refer populations from Bioko Island and coastal Cameroon (near Mt. Cameroon) to this species. To facilitate comparisons to the new species, we also provide comments on the type specimens, morphology, and distributions of X. epitropicalis, X. tropicalis, and X. fraseri. This includes significantly restricted application of the names X. fraseri and X. epitropicalis, the first of which we argue is known definitively only from type specimens and possibly one other specimen. Inferring the evolutionary histories of these new species allows refinement of species groups within Xenopus and leads to our recognition of two subgenera (Xenopus and Silurana) and three species groups within the subgenus Xenopus (amieti, laevis, and muelleri species groups).

Independent Evolution of Suction Feeding in Neobatrachia: Feeding Mechanisms in Two Species of Telmatobius (Anura:Telmatobiidae)

The most common feeding mechanism among aquatic vertebrates as fishes, turtles, and salamanders is inertial suction. However, among the more than 6,400 species of anurans, suction feeding occurs only in pipids. Pipidae is a small basal lineage relative to Neobatrachia, an enormous clade that contains about 96% of extant anurans. The Andean neobatrachian frogs of the genus Telmatobius include strictly aquatic and semiaquatic species. Diet analyses indicate that some species of Telmatobius feed on strictly aquatic prey, but until now their feeding mechanisms have been unknown. Herein, the feeding mechanisms in two species of Telmatobius, that represent the two predominant modes of life in the genus, are explored. The semiaquatic T. oxycephalus and the fully aquatic T. rubigo are studied using high-speed cinematography and standard anatomical techniques to provide a qualitative approach to feeding behavior and a detailed morphological description of the mouth, tongue, hyoid and related muscles. T. oxycephalus uses similar mechanisms of aquatic prey capture as do the vast majority of anurans that are capable of forage in water, whereas the fully aquatic T. rubigo is an inertial suction feeder. This is the first report of an objective record of this unique feeding behavior in a Neobatrachian. Several morphological characters seem to be related with this function and are convergent with those of pipids.

Xenopus in Space and Time: Fossils, Node Calibrations, Tip-Dating, and Paleobiogeography

Published data from DNA sequences, morphology of 11 extant and 15 extinct frog taxa, and stratigraphic ranges of fossils were integrated to open a window into the deep-time evolution of Xenopus. The ages and morphological characters of fossils were used as independent datasets to calibrate a chronogram. We found that DNA sequences, either alone or in combination with morphological data and fossils, tended to support a close relationship between Xenopus and Hymenochirus, although in some analyses this topology was not significantly better than the Pipa + Hymenochirus topology. Analyses that excluded DNA data found strong support for the Pipa + Hymenochirus tree. The criterion for selecting the maximum age of the calibration prior influenced the age estimates, and our age estimates of early divergences in the tree of frogs are substantially younger than those of published studies. Node-dating and tip-dating calibrations, either alone or in combination, yielded older dates for nodes than did a root calibration alone. Our estimates of divergence times indicate that overwater dispersal, rather than vicariance due to the splitting of Africa and South America, may explain the presence of Xenopus in Africa and its closest fossil relatives in South America.

Characterization of the host-defense peptides from skin secretions of Merlin's clawed frog Pseudhymenochirus merlini: insights into phylogenetic relationships among the Pipidae

The family Pipidae comprises the genera Hymenochirus, Pipa, Pseudhymenochirus, Silurana, and Xenopus but phylogenetic relationships within the family are unclear. Peptidomic analysis of norepinephrine-stimulated skin secretions from Pseudhymenochirus merlini Chabanaud, 1920, the single species within the genus Pseudhymenochirus, led to identification of 13 host-defense peptides with antimicrobial activity. Two peptides (hymenochirin-1Pa and -1Pb) show structural similarity to hymenochirin-1B from Hymenochirus boettgeri and eight peptides (hymenochirin-5Pa, -5Pb, -5Pc, -5Pd, -5Pe, -5Pf, 5Pg and -5Ph) are structurally similar to each other and to hymenochirin-5B from H. boettgeri. Two peptides differing by a single amino acid (IKIPSFFRNILKKVGKEAVSLM/I AGALKQS), termed pseudhymenochirin-1Pa and -1Pb, and pseudhymenochirin-2Pa (GIFPIFAKLLGKVIKVASSLISKGRTE) do not resemble host-defense peptides previously isolated from pipid frogs. Hymenochirin-5Pe was the most abundant peptide in the secretions and hymenochirin-1Pa the most potent against Staphylococcus aureus (MIC=2.5μM) and Escherichia coli (MIC=10μM). The data support a close phylogenetic relationship between Hymenochirus and Pseudhymenochirus that is distinct from the Xenopodinae (Xenopus+Silurana) clade with Pipa sister-group to all other extant pipids.

The hymenochirins: a family of host-defense peptides from the Congo dwarf clawed frog Hymenochirus boettgeri (Pipidae)

Skin secretions of frogs from the subfamily Xenopodinae (Xenopus+Silurana) within the family Pipidae are a rich source of antimicrobial peptides with therapeutic potential but species from the sister taxon Hymenochirus in the subfamily Pipinae (Hymenochirus+Pseudhymenochirus+Pipa) have not been investigated. Peptidomic analysis of norepinephrine-stimulated skin secretions from two distinct populations of the Congo dwarf clawed frog Hymenochirus boettgeri (Tornier, 1896) has led to identification of five structurally related peptides with broad-spectrum antimicrobial activity. Hymenochirin-1B (IKLSPETKDNLKKVLKGAIKGAIAVAKMV.NH(2)) is C-terminally α-amidated whereas hymenochirins-2B-5B have the general structure XKIPX(2)VKDTLKKVAKGX(2)SX(2)AGAX(3).COOH. Hymenochirin-3B (IKIPAVVKDTLKKVAKGVLSAVAGALTQ) was the most abundant peptide in the secretions. The hymenochirins show very low structural similarity with the antimicrobial peptides isolated from skin secretions of Silurana tropicalis and Xenopus laevis consistent with the proposed ancient divergence of the Pipinae and Xenopodinae. Synthetic replicates of hymenochirin-1B-4B inhibit the growth of Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, and Staphylococcus aureus (MIC in the range 10-40 μM) and Candida albicans (MIC=80 μM). The peptides display relatively weak hemolytic activity against human erythrocytes (LC(50) in the range 160 to >300 μM).

Olfactory metamorphosis in the coastal tailed frog Ascaphus truei (Amphibia, Anura, Leiopelmatidae)

The structure of the olfactory organ in larvae and adults of the basal anuran Ascaphus truei was examined using light micrography, electron micrography, and resin casts of the nasal cavity. The larval olfactory organ consists of nonsensory anterior and posterior nasal tubes connected to a large, main olfactory cavity containing olfactory epithelium; the vomeronasal organ is a ventrolateral diverticulum of this cavity. A small patch of olfactory epithelium (the "epithelial band") also is present in the preoral buccal cavity, anterolateral to the choana. The main olfactory epithelium and epithelial band have both microvillar and ciliated receptor cells, and both microvillar and ciliated supporting cells. The epithelial band also contains secretory ciliated supporting cells. The vomeronasal epithelium contains only microvillar receptor cells. After metamorphosis, the adult olfactory organ is divided into the three typical anuran olfactory chambers: the principal, middle, and inferior cavities. The anterior part of the principal cavity contains a "larval type" epithelium that has both microvillar and ciliated receptor cells and both microvillar and ciliated supporting cells, whereas the posterior part is lined with an "adult-type" epithelium that has only ciliated receptor cells and microvillar supporting cells. The middle cavity is nonsensory. The vomeronasal epithelium of the inferior cavity resembles that of larvae but is distinguished by a novel type of microvillar cell. The presence of two distinct types of olfactory epithelium in the principal cavity of adult A. truei is unique among previously described anuran olfactory organs. A comparative review suggests that the anterior olfactory epithelium is homologous with the "recessus olfactorius" of other anurans and with the accessory nasal cavity of pipids and functions to detect water-borne odorants.

Reversal to air-driven sound production revealed by a molecular phylogeny of tongueless frogs, family Pipidae

Background

Evolutionary novelties often appear by conferring completely new functions to pre-existing structures or by innovating the mechanism through which a particular function is performed. Sound production plays a central role in the behavior of frogs, which use their calls to delimit territories and attract mates. Therefore, frogs have evolved complex vocal structures capable of producing a wide variety of advertising sounds. It is generally acknowledged that most frogs call by moving an air column from the lungs through the glottis with the remarkable exception of the family Pipidae, whose members share a highly specialized sound production mechanism independent of air movement.

Results

Here, we performed behavioral observations in the poorly known African pipid genus Pseudhymenochirus and document that the sound production in this aquatic frog is almost certainly air-driven. However, morphological comparisons revealed an indisputable pipid nature of Pseudhymenochirus larynx. To place this paradoxical pattern into an evolutionary framework, we reconstructed robust molecular phylogenies of pipids based on complete mitochondrial genomes and nine nuclear protein-coding genes that coincided in placing Pseudhymenochirus nested among other pipids.

Conclusions

We conclude that although Pseudhymenochirus probably has evolved a reversal to the ancestral non-pipid condition of air-driven sound production, the mechanism through which it occurs is an evolutionary innovation based on the derived larynx of pipids. This strengthens the idea that evolutionary solutions to functional problems often emerge based on previous structures, and for this reason, innovations largely depend on possibilities and constraints predefined by the particular history of each lineage.

Xenopus tropicalis: an ideal experimental animal in amphibia

Studies using amphibians have contributed to the progress of life science including developmental biology and cell biology for more than one hundred years. Since the 1950s Xenopus laevis in particular has been used by scientists in many fields for experiments, resulting in the development of various techniques such as microsurgery on early embryos, biosynthesis of gene-encoded protein in oocytes by mRNA injection, misexpression experiments by mRNA injection into embryos, gene knockdown studies by injection of morpholino anti-sense oligonucleotide into fertilized eggs, transgenesis by the I-SceI meganuclease method, and so on. In this paper we will introduce Xenopus tropicalis as an alternative experimental animal. It has a shorter generation time and smaller diploid genome, together with whole-genome sequence data. The procedures available for Xenopus laevis can work well with Xenopus tropicalis, and embryos of both species develop at similar rates according to the developmental staging system of Nieuwkoop and Faber. Experimental systems of Xenopus tropicalis will pave the way for a new era of vertebrate genomics and genetics.

Aquatic feeding in pipid frogs: the use of suction for prey capture

Inertial suction feeding is the most common method of prey capture among aquatic vertebrates. However, it had been unclear whether the aquatic frogs in the family Pipidae also used inertial suction for prey capture. In this study, we examined feeding behavior in four species of pipids, Pipa pipa, Xenopus laevis, Hymenochirus boettgeri and Pseudhymenochirus merlini. Pressure in the buccopharyngeal cavity was measured during prey capture. These pressure measurements were coupled with high-speed recordings of feeding behavior. For each species, the internal buccopharyngeal pressure was found to drop significantly below ambient pressure, and changes in pressure corresponded with the onset of mouth opening. Kinematic analysis revealed that all species of pipids generated subambient pressure during prey capture; H. boettgeri and P. merlini relied solely on inertial suction feeding. Pipa pipa and X. laevis additionally employed forelimb scooping during prey capture but both of these species demonstrated the ability to capture prey with inertial suction alone. Based on buccopharyngeal pressure measurements as well as kinematic analyses, we conclude that inertial suction feeding is used during prey capture in these four species of pipids.

Orthologs of magainin, PGLa, procaerulein-derived, and proxenopsin-derived peptides from skin secretions of the octoploid frog Xenopus amieti (Pipidae)

The Volcano clawed frog Xenopus amieti Kobel, du Pasquier, Fischberg, and Gloor, 1980, with a chromosome number of 2n=72, is believed to have undergone two rounds of genome duplication since evolving from a diploid ancestor. Nine peptides with differential antimicrobial activity against Escherichia coli and Staphylococcus aureus were isolated from norepinephrine-stimulated skin secretions of X. amieti that showed structural similarity to peptides previously isolated from the tetraploid frog X. laevis (2n=36) and the diploid frog Silurana (formerly Xenopus) tropicalis (2n=20). Two peptides (magainin-AM1 and -AM2) are othologous to the magainins, two peptides (PGLa-AM1 and -AM2) orthologous to peptide glycine-leucine-amide, four peptides (CPF-AM1, -AM2, -AM3, -AM4) orthologous to caerulein-precursor fragments, and one peptide (XPF-AM1) structurally similar to xenopsin-precursor fragments were characterized. CFP-AM1 (GLGSVLGKALKIGANLL.NH(2)) was the most potent peptide present in the secretions and magainin-AM2 (GVSKILHSAGKFGKAFLGEIMKS) was the most abundant. The data indicate that nonfunctionalization has been the most common fate of duplicated antimicrobial peptide genes following the polyploidization events in the X. amieti lineage. However, the very low antimicrobial activity of the magainin-AM1 and PGLa-AM2 paralogs suggests the possibility that certain peptides may have evolved toward a new, as yet undetermined, function (neofunctionalization).

Fadrozole and finasteride exposures modulate sex steroid- and thyroid hormone-related gene expression in Silurana (Xenopus) tropicalis early larval development

Steroidogenic enzymes and their steroid products play critical roles during gonadal differentiation in amphibians; however their roles during embryogenesis remain unclear. The objective of this study was to investigate the expression and activity of aromatase (cyp19; estrogen synthase) and 5 beta-reductase (srd5 beta; 5 beta-dihydrotestosterone synthase) during amphibian embryogenesis. Expression and activity profiles of cyp19 and srd5 beta were first established during Silurana (Xenopus) tropicalis embryogenesis from Nieuwkoop-Faber (NF) stage 2 (2-cell stage; 1h post-fertilization) to NF stage 46 (beginning of feeding; 72 h post-fertilization). Exposures to fadrozole (an aromatase inhibitor; 0.5, 1.0 and 2.0 microM) and finasteride (a putative 5-reductase inhibitor; 25, 50 and 100 microM) were designed to assess the consequences of inhibiting these enzymes on gene expression in early amphibian larval development. Exposed embryos showed changes in both enzyme activities and sex steroid- and thyroid hormone-related gene expression. Fadrozole treatment inhibited cyp19 activity and increased androgen receptor and thyroid hormone receptor (alpha and beta) mRNAs. Finasteride treatment inhibited srd5 beta (activity and mRNA), decreased cyp19 mRNA and activity levels and increased estrogen receptor alpha mRNA. Both treatments altered the expression of deiodinases (thyroid hormone metabolizing enzymes). We conclude that cyp19 and srd5 beta are active in early embryogenesis and larval development in Silurana tropicalis and their inhibition affected transcription of genes associated with the thyroid and reproductive axes.

Expression and T3 regulation of thyroid hormone- and sex steroid-related genes during Silurana (Xenopus) tropicalis early development

In amphibians, thyroid hormones (THs) are the primary regulators of metamorphosis; however, their physiological role during embryogenesis remains unclear. First, we established complete developmental profiles for TH receptors (tr alpha and tr beta), deiodinases (dio; types 1, 2, 3), estrogen receptors (er alpha and er beta) and androgen receptor (ar) mRNA levels during embryogenesis and early larval stages in Silurana (Xenopus) tropicalis (from Nieuwkoop and Faber (NF) stage 2 until NF 46). Real-time RT-PCR analyses in whole embryos and larvae revealed that all transcripts except tr alpha were detected throughout development; tr alpha only appears after gastrulation. The first significant increase in the expression of tralpha and tr beta was observed before hatching, between NF 21 and NF 27 (2.5- and 11-fold, respectively). In order to test if these genes could be regulated by THs during early larval development, embryos were exposed to triiodothyronine (T3; 0.5, 5.0, 50 nM) from NF 27 to NF 46. T3 exposure caused a dose-dependent increase relative to control in the expression of tr alpha, tr beta, dio (types 2 and 3), ar, and 5 alpha-reductase type 1 in whole larvae. These results indicate that in S. tropicalis, tr and dio can be induced by T3 as early as NF 46, a response that had only been characterized later during frog metamorphosis. In addition, T3 also affected androgen-related gene expression, supporting our hypothesis that THs are involved in male development in frogs.

Skeletal morphogenesis of the vertebral column of the miniature hylid frog Acris crepitans, with comments on anomalies

Although the vertebral columns of anurans have received much study in the last 150 years, few detailed descriptions exist of the skeletal morphogenesis of this anatomical unit. Herein, the ontogeny of the vertebral skeleton of the hylid frog Acris crepitans is described based on cleared and double-stained specimens, radiographs, and 3D reconstructions generated from synchrotron microCT scans. The adult axial formula is 1-7-1-1, and the vertebral centra are epichordal and procoelous. The neural arches are nonimbricate, and there is a medial articulation between the laminae of Presacrals I and II. Free ribs are absent. The sacral diapophyses are uniform in width or slightly expanded distally. The urostyle is slender, round in cross section, and about equal in length to the presacral region. Presacral vertebrae are the first to form, developing in a cephalic-to-caudal sequence. However, development and growth are decoupled and growth is fastest initially in the posterior presacrals and sacrum. In addition, there is a time lag between the formation of the presacral/sacral region and the postsacral region. More than 8.5% of the specimens examined have vertebral anomalies, and about 50% display small variants from the typical vertebral column morphology. However, these malformations do not seem to have been so severe as to have affected survival.

Use of paleontological and molecular data in supertrees for comparative studies: the example of lissamphibian femoral microanatomy

A new method to assemble time-calibrated supertrees is able to incorporate paleontological and molecular dates. This method, along with new branch length transformations, is implemented in the Stratigraphic Tools for Mesquite. It was used here to analyse a dataset on bone microanatomy, body size and habitat of 46 species of lissamphibians through a variety of methods (Felsenstein independent contrasts, variance partition with phylogenetic eigenvector regression, discriminant analyses and simple regressions). Our analyses showed that the new methods can produce adequate standardization for several characters on a tree whose branch lengths can represent evolutionary time. The analyses confirmed previous conclusions about the presence of an ecological signal in bone microanatomical data.

Control of breathing in anuran amphibians

The primary role of the respiratory system is to ensure adequate tissue oxygenation, eliminate carbon dioxide and help to regulate acid-base status. To maintain this homeostasis, amphibians possess an array of receptors located at peripheral and central chemoreceptive sites that sense respiration-related variables in both internal and external environments. As in mammals, input from these receptors is integrated at central rhythmogenic and pattern-forming elements in the medulla in a manner that meets the demands determined by the environment within the constraints of the behavior and breathing pattern of the animal. Also as in mammals, while outputs from areas in the midbrain may modulate respiration directly, they do not play a significant role in the production of the normal respiratory rhythm. However, despite these similarities, the breathing patterns of the two classes are different: mammals maintain homeostasis of arterial blood gases through rhythmic and continuous breathing, whereas amphibians display an intermittent pattern of aerial respiration. While the latter is also often rhythmic, it allows a degree of fluctuation in key respiratory variables that has led some to suggest that control is not as tight in these animals. In this review we will focus specifically on recent advances in studies of the control of ventilation in anuran amphibians. This is the group of amphibians that has attracted the most recent attention from respiratory physiologists.

Distribution of orexin/hypocretin immunoreactivity in the nervous system of the green Treefrog, Hyla cinerea

We examined the distribution of orexin/hypocretin immunoreactive neurons and projections throughout the brain of the green treefrog (Hyla cinerea). Orexin A and B neurons were located in a single population centered on the suprachiasmatic nucleus. Orexin A and B fibers were visible across the brain, with the highest density within the preoptic area and hypothalamus. Our data suggest different distributions of orexin neurons but not projections between families of amphibians.

A mitochondrial DNA phylogeny of African clawed frogs: phylogeography and implications for polyploid evolution

The African clawed frogs (Silurana and Xenopus), model organisms for scientific inquiry, are unusual in that allopolyploidization has occurred on multiple occasions, giving rise to tetraploid, octoploid, and dodecaploid species. To better understand their evolution, here we estimate a mitochondrial DNA phylogeny from all described and some undescribed species. We examine the timing and location of diversification, and test hypotheses concerning the frequency of polyploid speciation and taxonomy. Using a relaxed molecular clock, we estimate that extant clawed frog lineages originated well after the breakup of Gondwana, about 63.7 million years ago, with a 95% confidence interval from 50.4 to 81.3 million years ago. Silurana and two major lineages of Xenopus have overlapping distributions in sub-Saharan Africa, and dispersal-vicariance analysis suggests that clawed frogs originated in central and/or eastern equatorial Africa. Most or all extant species originated before the Pleistocene; recent rainforest refugia probably acted as "lifeboats" that preserved existing species, rather than "species pumps" where many new successful lineages originated. We estimate that polyploidization occurred at least six times in clawed frogs.