Cranial ontogeny in the direct-developing frog,Eleutherodactylus coqui (anura: Leptodactylidae), analyzed using whole-mount immunohistochemistry

Natural History of Model Organisms: The big potential of the small frog Eleutherodactylus coqui

The Puerto Rican coquí frog Eleutherodactylus coqui is both a cultural icon and a species with an unusual natural history that has attracted attention from researchers in a number of different fields within biology. Unlike most frogs, the coquí frog skips the tadpole stage, which makes it of interest to developmental biologists. The frog is best known in Puerto Rico for its notoriously loud mating call, which has allowed researchers to study aspects of social behavior such as vocal communication and courtship, while the ability of coquí to colonize new habitats has been used to explore the biology of invasive species. This article reviews existing studies on the natural history of E. coqui and discusses opportunities for future research.

Increasing Hormonal Control of Skeletal Development: An Evolutionary Trend in Amphibians

The biphasic life history of amphibians includes metamorphosis, a complex developmental event that involves drastic changes in the morphology, physiology and biochemistry accompanying the transition from the larval to adult stage of development. Thyroid hormones (THs) are widely known to orchestrate this remodeling and, in particular, to mediate the development of the bony skeleton, which is a model system in evolutionary morphological studies of amphibians. Detailed experimental studies of the role of THs in the craniogenesis of diverse urodelan amphibians revealed that (i) these hormones affect both the timing and sequence of bone formation, (ii) TH involvement increases in parallel with the increase in divergence between larval and adult skull morphology, and (iii) among urodelans, TH-involvement in skull development changes from a minimum in basal salamanders (Hynobiidae) to the most pronounced in derived ones (Salamandridae and Plethodontidae). Given the increasing regulatory function of THs in urodelan evolution, we hypothesized a stronger involvement of THs in the control of skeletogenesis in anurans with their most complex and dramatic metamorphosis among all amphibians. Our experimental study of skeletal development in the hypo- and hyperthyroid yellow-bellied toad (Bombina variegata: Bombinatoridae) supports the greater involvement of THs in the mediation of all stages of anuran cranial and postcranial bones formation. Similar to urodelans, B. variegata displays enhancing TH involvement in the development of cranial bones that arise during larval ontogeny: while the hormonal impact on early larval ossifications is minimal, the skull bones forming during metamorphosis are strictly TH-inducible. However, in contrast to urodelans, all cranial bones, including the earliest to form, are TH-dependent in B. variegata; moreover, the development of all elements of the axial and limb skeleton is affected by THs. The more accentuated hormonal control of skeletogenesis in B. variegata demonstrates the advanced regulatory and inductive function of THs in the orchestration of anuran metamorphosis. Based on these findings, we discuss (i) changes in THs function in amphibian evolution and (ii) the role of THs in the evolution of life histories in amphibians.

Cranial skeletogenesis of one of the largest amphibians, Andrias japonicus, provides insight into ontogenetic adaptations for feeding in salamanders

Skeletal development in salamanders is greatly influenced by their complex life history. Although the relationship between skull development and life history has been investigated in a variety of salamander species, few studies have compared skull development among lineages. Here, we describe the cranial skeletogenesis of the Japanese giant salamander, Andrias japonicus (family Cryptobranchidae), for the first time. To conduct interspecific comparisons, we also describe the cranial skeletogenesis of Hynobius nebulosus (Hynobiidae), Pleurodeles waltl (Salamandridae) and Ambystoma mexicanum (Ambystomatidae). Our comparative analyses reveal interspecific differences in cranial skeletogenesis, particularly in the timing of the onset of ossification, the direction of ossification (e.g. from anterior to posterior) and the ossification pattern of the hyobranchial skeleton. The early onset of jaw ossification, posterior extension of the maxilla and posterolateral inclination of the squamosal in An. japonicus might be adaptations that allow An. japonicus to gulp feed, primarily using the suction force generated by opening the mouth wide, while also supplementing that force by depressing the hyobranchial skeleton. Multivariate regression of skull shape on log10-transformed centroid size revealed that the unique skull shape of adult and subadult An. japonicus forms through hypermorphosis.

Size, microhabitat, and loss of larval feeding drive cranial diversification in frogs

Habitat is one of the most important factors shaping organismal morphology, but it may vary across life history stages. Ontogenetic shifts in ecology may introduce antagonistic selection that constrains adult phenotype, particularly with ecologically distinct developmental phases such as the free-living, feeding larval stage of many frogs (Lissamphibia: Anura). We test the relative influences of developmental and ecological factors on the diversification of adult skull morphology with a detailed analysis of 15 individual cranial regions across 173 anuran species, representing every extant family. Skull size, adult microhabitat, larval feeding, and ossification timing are all significant factors shaping aspects of cranial evolution in frogs, with late-ossifying elements showing the greatest disparity and fastest evolutionary rates. Size and microhabitat show the strongest effects on cranial shape, and we identify a "large size-wide skull" pattern of anuran, and possibly amphibian, evolutionary allometry. Fossorial and aquatic microhabitats occupy distinct regions of morphospace and display fast evolution and high disparity. Taxa with and without feeding larvae do not notably differ in cranial morphology. However, loss of an actively feeding larval stage is associated with higher evolutionary rates and disparity, suggesting that functional pressures experienced earlier in ontogeny significantly impact adult morphological evolution.

Parallel evolution of direct development in frogs - Skin and thyroid gland development in African Squeaker Frogs (Anura: Arthroleptidae: Arthroleptis)

BACKGROUND

Cases of parallel evolution offer the possibility to identify adaptive traits and to uncover developmental constraints on the evolutionary trajectories of these traits. The independent evolution of direct development from the ancestral biphasic life history in frogs is such a case of parallel evolution. In frogs, aquatic larvae (tadpoles) differ profoundly from their adult forms and exhibit a stunning diversity regarding their habitats, morphology and feeding behaviors. The transition from the tadpole to the adult is a climactic, thyroid hormone (TH)-dependent process of profound and fast morphological rearrangement called metamorphosis. One of the organ systems that experiences the most comprehensive metamorphic rearrangements is the skin. Direct-developing frogs lack a free-swimming tadpole and hatch from terrestrial eggs as fully formed froglets. In the few species examined, development is characterized by the condensed and transient formation of some tadpole-specific features and the early formation of adult-specific features during a "cryptic" metamorphosis.

RESULTS

We show that skin in direct-developing African squeaker frogs (Arthroleptis) is also repatterned from a tadpole-like to an adult-like histology during a cryptic metamorphosis. This repatterning correlates with histological thyroid gland maturation. A comparison with data from the Puerto Rican coqui (Eleutherodactylus coqui) reveals that the evolution of direct development in these frogs is associated with a comparable heterochronic shift of thyroid gland maturation.

CONCLUSION

This suggests that the development of many adult features is still dependent on, and possibly constrained by, the ancestral dependency on thyroid hormone signaling.

Squamate egg tooth development revisited using three-dimensional reconstructions of brown anole (Anolis sagrei, Squamata, Dactyloidae) dentition

The egg tooth is a hatching adaptation, characteristic of all squamates. In brown anole embryos, the first tooth that starts differentiating is the egg tooth. It develops from a single tooth germ and, similar to the regular dentition of all the other vertebrates, the differentiating egg tooth of the brown anole passes through classic morphological and developmental stages named according to the shape of the dental epithelium: epithelial thickening, dental lamina, tooth bud, cap and bell stages. The differentiating egg tooth consists of three parts: the enamel organ, hard tissues and dental pulp. Shortly before hatching, the egg tooth connects with the premaxilla. Attachment tissue of the egg tooth does not undergo mineralization, which makes it different from the other teeth of most squamates. After hatching, odontoclasts are involved in resorption of the egg tooth's remains. This study shows that the brown anole egg tooth does not completely conform to previous reports describing iguanomorph egg teeth and reveals a need to investigate its development in the context of squamate phylogeny.

Evolutionary Conservation of Thyroid Hormone Receptor and Deiodinase Expression Dynamics in ovo in a Direct-Developing Frog, Eleutherodactylus coqui

Direct development is a reproductive mode in amphibians that has evolved independently from the ancestral biphasic life history in at least a dozen anuran lineages. Most direct-developing frogs, including the Puerto Rican coquí, Eleutherodactylus coqui, lack a free-living aquatic larva and instead hatch from terrestrial eggs as miniature adults. Their embryonic development includes the transient formation of many larval-specific features and the formation of adult-specific features that typically form postembryonically-during metamorphosis-in indirect-developing frogs. We found that pre-hatching developmental patterns of thyroid hormone receptors alpha (thra) and beta (thrb) and deiodinases type II (dio2) and type III (dio3) mRNAs in E. coqui limb and tail are conserved relative to those seen during metamorphosis in indirect-developing frogs. Additionally, thra, thrb, and dio2 mRNAs are expressed in the limb before formation of the embryonic thyroid gland. Liquid-chromatography mass-spectrometry revealed that maternally derived thyroid hormone is present throughout early embryogenesis, including stages of digit formation that occur prior to the increase in embryonically produced thyroid hormone. Eleutherodactylus coqui embryos take up much less 3,5,3'-triiodothyronine (T₃) from the environment compared with X. tropicalis tadpoles. However, E. coqui tissue explants mount robust and direct gene expression responses to exogenous T₃ similar to those seen in metamorphosing species. The presence of key components of the thyroid axis in the limb and the ability of limb tissue to respond to T₃ suggest that thyroid hormone-mediated limb development may begin prior to thyroid gland formation. Thyroid hormone-dependent limb development and tail resorption characteristic of metamorphosis in indirect-developing anurans are evolutionarily conserved, but they occur instead in ovo in E. coqui.

Chondrification and Character Identification in the Skull Exemplified for the Basicranial Anatomy of Early Squamate Embryos

The neurocranium of vertebrates is mainly derived from early cartilaginous anlagen, the so-called chondrocranium, the base of the future skull. Two initial bar-shaped and paired chondrifications flank the notochord, the rostral trabecles and the caudal parachordals. In most reptiles, there is an additional component, the transverse acrochordal, which is placed between trabecles and parachordals. All these elements compose the base of the future chondrocranium. There are several drastically different hypotheses concerning the development and interrelationship of these elements. We reexamined the basicranial development in four squamates and found that all species show very similar conditions of early chondrocranial development. The anterior part of the notochord is not embedded into the basal plate as it was previously reported. It remains free. The medial edges of the parachordals form the lateral walls of the basicranial fenestra. Only the posterior portions of the parachordals fuse and form the basal plate. The space in-between the parachordals is filled with a thin layer of cells, which never chondrify. The anterior tips of the parachordals later fuse with the posterior edge of the acrochordal, which ultimately delimitates, as crista sellaris, the basicranial fenestra anteriorly. We consider the observed processes a common development at least in lizards and review a variety of methodological approaches and differences in data interpretation as reasons for the anatomical differences reported in the literature. Moreover, based on our data we argue that the acrochordal is of mesodermal origin, which coincides with results of fate map experimental studies.

Heterochronies in the cranial development of Asian tree frogs (Amphibia: Anura: Rhacophoridae) with different life histories

The development of bony skull was studied in four species of Asian tree frogs (Rhacophoridae) with different life histories: biphasic development with free larval stage and direct development. In biphasic rhacophorids the sequence of the appearance of cranial bones generally followed the generalized pattern of craniogenesis, which was described for most studied anurans. In contrast, direct-developing species displayed some heterochronies in the formation of skull bones, namely, the accelerated formation of the anlagen of jaw and suspensorium bones. The obtained results support that the embryonization in amphibians is regularly accompanied by a heterochronic repatterning of craniogenesis, rather similar in different phyletic groups.

Comparative Postembryonic Skeletal Ontogeny in Two Sister Lineages of Old World Tree Frogs (Rhacophoridae: Taruga, Polypedates)

Rhacophoridae, a family of morphologically cryptic frogs, with many genetically distinct evolutionary lineages, is understudied with respect to skeletal morphology, life history traits and skeletal ontogeny. Here we analyze two species each from two sister lineages, Taruga and Polypedates, and compare their postembryonic skeletal ontogeny, larval chondrocrania and adult osteology in the context of a well-resolved phylogeny. We further compare these ontogenetic traits with the direct-developing Pseudophilautus silus. For each species, we differentially stained a nearly complete developmental series of tadpoles from early postembryonic stages through metamorphosis to determine the intraspecific and interspecific differences of cranial and postcranial bones. Chondrocrania of the four species differ in 1) size; 2) presence/absence of anterolateral and posterior process; and 3) shape of the suprarostral cartilages. Interspecific variation of ossification sequences is limited during early stages, but conspicuous during later development. Early cranial ossification is typical of other anuran larvae, where the frontoparietal, exoccipital and parasphenoid ossify first. The ossification sequences of the cranial bones vary considerably within the four species. Both species of Taruga show a faster cranial ossification rate than Polypedates. Seven cranial bones form when larvae near metamorphic climax. Ossification of all 18 cranial bones is initiated by larval Gosner stage 46 in T. eques. However, some cranial bone formation is not initiated until after metamorphosis in the other three species. Postcranial sequence does not vary significantly. The comparison of adult osteology highlights two characters, which have not been previously recorded: presence/absence of the parieto-squamosal plates and bifurcated base of the omosternum. This study will provide a starting point for comparative analyses of rhacophorid skeletal ontogeny and facilitate the study of the evolution of ontogenetic repatterning associated with the life history variation in the family.

Unearthing the Fossorial Tadpoles of the Indian Dancing Frog Family Micrixalidae

Tadpoles of the monotypic Indian dancing frog family Micrixalidae have remained obscure for over 125 years. Here we report the discovery of the elusive tadpoles of Micrixalus herrei from the sand beds of a forested stream in southern Western Ghats, and confirm their identity through DNA barcoding. These actively burrowing tadpoles lead an entirely fossorial life from eggs to late metamorphic stages. We describe their internal and external morphological characters while highlighting the following features: eel-like appearance, extensively muscularized body and tail, reduced tail fins, skin-covered eyes, delayed development of eye pigmentation in early pre-metamorphic stages (Gosner stages 25-29), prominent tubular sinistral spiracle, large transverse processes on vertebrae II and III, ankylosed ribs on transverse processes of vertebra II, notochord terminating before the atlantal cotyle-occipital condyle junction, absence of keratodonts, serrated well-formed jaw sheaths, and extensive calcified endolymphatic sacs reaching sacrum posteriorly. The tadpole gut contains mostly fine sediments and sand. We discuss the eel-like morphology and feeding habits of M. herrei in the context of convergence with other well-known fossorial tadpoles. This discovery builds the knowledge base for further comparative analyses and conservation of Micrixalus, an ancient and endemic lineage of Indian frogs.

Seasonal and ontogenetic variation of skin microbial communities and relationships to natural disease dynamics in declining amphibians

Recently, microbiologists have focused on characterizing the probiotic role of skin bacteria for amphibians threatened by the fungal disease chytridiomycosis. However, the specific characteristics of microbial diversity required to maintain health or trigger disease are still not well understood in natural populations. We hypothesized that seasonal and developmental transitions affecting susceptibility to chytridiomycosis could also alter the stability of microbial assemblages. To test our hypothesis, we examined patterns of skin bacterial diversity in two species of declining amphibians (Lithobates yavapaiensis and Eleutherodactylus coqui) affected by the pathogenic fungus Batrachochytrium dendrobatidis (Bd). We focused on two important transitions that affect Bd susceptibility: ontogenetic (from juvenile to adult) shifts in E. coqui and seasonal (from summer to winter) shifts in L. yavapaiensis. We used a combination of community-fingerprinting analyses and 16S rRNA amplicon sequencing to quantify changes in bacterial diversity and assemblage composition between seasons and developmental stages, and to investigate the relationship between bacterial diversity and pathogen load. We found that winter-sampled frogs and juveniles, two states associated with increased Bd susceptibility, exhibited higher diversity compared with summer-sampled frogs and adult individuals. Our findings also revealed that hosts harbouring higher bacterial diversity carried lower Bd infections, providing support for the protective role of bacterial communities. Ongoing work to understand skin microbiome resilience after pathogen disturbance has the potential to identify key taxa involved in disease resistance.

Development of neuroendocrine components of the thyroid axis in the direct-developing frog Eleutherodactylus coqui: formation of the median eminence and onset of pituitary TSH production

Direct-developing frogs lack, wholly or in part, a wide range of larval features found in metamorphosing species and form adult-specific features precociously, during embryogenesis. Most information on thyroid regulation of direct development relies on hormone manipulations; the ontogeny of many thyroid axis components has not been fully described. This analysis examines differentiation of the median eminence of the hypothalamus and production of thyroid-stimulating hormone (TSH) by the pituitary of the direct-developing frog Eleutherodactylus coqui. The median eminence is established two-thirds of the way through embryogenesis. Cells immunoreactive to human TSHβ antibodies are first detected during embryogenesis and quantitative changes in TSHβ-IR cells resemble those in metamorphosing amphibians. Formation of the median eminence of the hypothalamus and TSHβ production by the pituitary precede or coincide with morphological changes during embryogenesis that occur during metamorphosis in biphasic anurans. Thus, while the onset of neuroendocrine regulation has changed during the evolution of direct development, it is likely that these thyroid axis components still mediate the formation of adult features.

Cranial muscle development in frogs with different developmental modes: direct development versus biphasic development

Normal development in anurans includes a free swimming larva that goes through metamorphosis to develop into the adult frog. We have investigated cranial muscle development and adult cranial muscle morphology in three different anuran species. Xenopus laevis is obligate aquatic throughout lifetime, Rana(Lithobates) pipiens has an aquatic larvae and a terrestrial adult form, and Eleutherodactylus coqui has direct developing juveniles that hatch from eggs deposited on leaves (terrestrial). The adult morphology shows hardly any differences between the investigated species. Cranial muscle development of E. coqui shows many similarities and only few differences to the development of Rana (Lithobates) and Xenopus. The differences are missing muscles of the branchial arches (which disappear during metamorphosis of biphasic anurans) and a few heterochronic changes. The development of the mandibular arch (adductor mandibulae) and hyoid arch (depressor mandibulae) muscles is similar to that observed in Xenopus and Rana (Lithobates), although the first appearance of these muscles displays a midmetamorphic pattern in E. coqui. We show that the mix of characters observed in E. coqui indicates that the larval stage is not completely lost even without a free swimming larval stage. Cryptic metamorphosis is the process in which morphological changes in the larva/embryo take place that are not as obvious as in normal metamorphosing anurans with a clear biphasic lifestyle. During cryptic metamorphosis, a normal adult frog develops, indicating that the majority of developmental mechanisms towards the functional adult cranial muscles are preserved.

Metamorphosis in a frog that does not have a tadpole

The evolutionary removal of the tadpole from the frog life history is a very successful strategy, particularly in the tropics. These direct developers form limbs and a frog-like head early in embryogenesis, and they have reduced or lost tadpole-specific structures, like gills, a long, coiled intestine, and tadpole teeth and jaws. Despite the apparently continuous development to the frog morphology, the direct developer, Eleutherodactylus coqui, undergoes a cryptic metamorphosis requiring thyroid hormone. As in Xenopus laevis, there is a stimulation by corticotrophin-releasing factor (CRF) and an upregulation of thyroid hormone receptor β (thrb). In addition to changes in skin and muscle, thyroid hormone stimulates yolk utilization for froglet growth from a novel tissue, the nutritional endoderm. The activities of CRF and corticosterone (CORT) in metamorphosis may provide the basis for the multiple evolutionary origins of direct development in anuran amphibians. Potential roles for maternally supplied thyroid hormone and its receptor and for deiodinases in regulating tissue sensitivity to thyroid hormone should be the subjects of future investigations.

Expression of cyclin D1, cyclin D2, and N-myc in embryos of the direct developing frog Eleutherodactylus coqui, with a focus on limbs

Species of frogs that develop directly have removed the tadpole from their ontogeny and form adult structures precociously. To see whether cell cycle regulators could be involved in this altered embryogenesis, we examined the expression of ccnd1, ccnd2, and mycn in embryos of the direct developing frog, Eleutherodactylus coqui. Notable differences compared to embryos of Xenopus laevis, a species with a tadpole, included prominent expression of ccnd2 in the midbrain and ccnd1 in the mandibular neural crest. The former may contribute to the precocious appearance of the adult-type visual system and the latter to the adult-type jaw. Large domains of ccnd2 and mycn presage the early appearance of limb buds, and ccnd1 and mycn are implicated in digit development.

Developmental diversity of amphibians

The current model amphibian, Xenopus laevis, develops rapidly in water to a tadpole which metamorphoses into a frog. Many amphibians deviate from the X. laevis developmental pattern. Among other adaptations, their embryos develop in foam nests on land or in pouches on their mother's back or on a leaf guarded by a parent. The diversity of developmental patterns includes multinucleated oogenesis, lack of RNA localization, huge non-pigmented eggs, and asynchronous, irregular early cleavages. Variations in patterns of gastrulation highlight the modularity of this critical developmental period. Many species have eliminated the larva or tadpole and directly develop to the adult. The wealth of developmental diversity among amphibians coupled with the wealth of mechanistic information from X. laevis permit comparisons that provide deeper insights into developmental processes.

Ecological guild evolution and the discovery of the world's smallest vertebrate

Living vertebrates vary drastically in body size, yet few taxa reach the extremely minute size of some frogs and teleost fish. Here we describe two new species of diminutive terrestrial frogs from the megadiverse hotspot island of New Guinea, one of which represents the smallest known vertebrate species, attaining an average body size of only 7.7 mm. Both new species are members of the recently described genus Paedophryne, the four species of which are all among the ten smallest known frog species, making Paedophryne the most diminutive genus of anurans. This discovery highlights intriguing ecological similarities among the numerous independent origins of diminutive anurans, suggesting that minute frogs are not mere oddities, but represent a previously unrecognized ecological guild.

Early cranial patterning in the direct-developing frog Eleutherodactylus coqui revealed through gene expression

Genetic and developmental alterations associated with the evolution of amphibian direct development remain largely unexplored. Specifically, little is known of the underlying expression of skeletal regulatory genes, which may reveal early modifications to cranial ontogeny in direct-developing species. We describe expression patterns of three key skeletal regulators (runx2, sox9, and bmp4) along with the cartilage-dominant collagen 2alpha1 gene (col2a1) during cranial development in the direct-developing anuran, Eleutherodactylus coqui. Expression patterns of these regulators reveal transient skeletogenic anlagen that correspond to larval cartilages, but which never fully form in E. coqui. Suprarostral anlagen in the frontonasal processes are detected through runx2, sox9, and bmp4 expression. Previous studies have described these cartilages as missing from Eleutherodactylus cranial ontogeny. These transcriptionally active suprarostral anlagen fuse to the more posterior cranial trabeculae before they are detectable with col2a1 staining or with the staining techniques used in earlier studies. Additionally, expression of sox9 fails to reveal an early anterior connection between the palatoquadrate and the neurocranium, which is detectable through sox9 staining in Xenopus laevis embryos (a metamorphosing species). Absence of this connection validates an instance of developmental repatterning, where the larval quadratocranial commissure cartilage is lost in E. coqui. Expression of runx2 reveals dermal-bone precursors several developmental stages before their detection with alizarin red. This early expression of runx2 correlates with the accelerated embryonic onset of bone formation characteristic of E. coqui and other direct-developing anurans, but which differs from the postembryonic bone formation of most metamorphosing species. Together these results provide an earlier depiction of cranial patterning in E. coqui by using earlier markers of skeletogenic cell differentiation. These data both validate and modify previously reported instances of larval recapitulation and developmental repatterning associated with the evolution of anuran direct development.

Development of the retinotectal system in the direct-developing frog Eleutherodactylus coqui in comparison with other anurans

BACKGROUND

Frogs primitively have a biphasic life history with an aquatic larva (tadpole) and a usually terrestrial adult. However, direct developing frogs of the genus Eleutherodactylus have lost a free living larval stage. Many larval structures never form during development of Eleutherodactylus, while limbs, spinal cord, and an adult-like cranial musculoskeletal system develop precociously.

RESULTS

Here, I compare growth and differentiation of the retina and tectum and development of early axon tracts in the brain between Eleutherodactylus coqui and the biphasically developing frogs Discoglossus pictus, Physalaemus pustulosus, and Xenopus laevis using morphometry, immunohistochemical detection of proliferating cell nuclear antigen (PCNA) and acetylated tubulin, biocytin tracing, and in situ hybridization for NeuroD. Findings of the present study indicate that retinotectal development was greatly altered during evolution of Eleutherodactlyus mostly due to acceleration of cell proliferation and growth in retina and tectum. However, differentiation of retina, tectum, and fiber tracts in the embryonic brain proceed along a conserved slower schedule and remain temporally coordinated with each other in E. coqui.

CONCLUSION

These findings reveal a mosaic pattern of changes in the development of the central nervous system (CNS) during evolution of the direct developing genus Eleutherodactylus. Whereas differentiation events in directly interconnected parts of the CNS such as retina, tectum, and brain tracts remained coordinated presumably due to their interdependent development, they were dissociated from proliferation control and from differentiation events in other parts of the CNS such as the spinal cord. This suggests that mosaic evolutionary changes reflect the modular character of CNS development.

Cranial ontogeny inPhilautus silus (Anura: Ranidae: Rhacophorinae) reveals few similarities with other direct-developing anurans

Direct development has evolved in rhacophorine frogs independently from other anuran lineages, thereby offering an opportunity to assess features associated with this derived life history. Using a developmental series of the direct-developing Philautus silus (Ranidae: Rhacophorinae) from Sri Lanka, we examine features of cranial morphology that are part of a suite of adaptations that facilitate feeding in free-living tadpoles, but have been changed or lost in other direct-developing lineages. Larval-specific upper jaw cartilages, which are absent from many non-rhacophorine direct-developing species (such as Eleutherodactylus coqui), develop in embryos of P. silus. Similarly, lower jaw cartilages initially assume a larval morphology, which is subsequently remodeled into the adult jaw configuration before hatching. However, the cartilaginous jaw suspension and hyobranchial skeleton never assume a typical larval morphology. The palatoquadrate, which suspends the lower jaw, lacks the posterior connections to the braincase found in many metamorphosing species. Unlike in metamorphosing species, bone formation in P. silus begins before hatching. However, the sequence of bone formation resembles that of metamorphosing anurans more than that of other direct developers. In particular, P. silus does not exhibit precocious ossification of the lower jaw, which is characteristic of some frogs and caecilians that lack a free-living tadpole. These data reveal some similarities between Philautus and other direct-developing anurans. However, the departure of Philautus embryos from the generalized tadpole skeletal morphology is less pronounced than that observed in other direct-developing taxa.

Reassessing the Dlx code: the genetic regulation of branchial arch skeletal pattern and development

The branchial arches are meristic vertebrate structures, being metameric both between each other within the rostrocaudal series along the ventrocephalic surface of the embryonic head and within each individual arch: thus, just as each branchial arch must acquire a unique identity along the rostrocaudal axis, each structure within the proximodistal axis of an arch must also acquire a unique identity. It is believed that regional specification of metameric structures is controlled by the nested expression of related genes resulting in a regional code, a principal that is though to be demonstrated by the regulation of rostrocaudal axis development in animals exerted by the nested HOM-C/Hox homeobox genes. The nested expression pattern of the Dlx genes within the murine branchial arch ectomesenchyme has more recently led to the proposal of a Dlx code for the regional specification along the proximodistal axis of the branchial arches (i.e. it establishes intra-arch identity). This review re-examines this hypothesis, and presents new work on an allelic series of Dlx loss-of-function mouse mutants that includes various combinations of Dlx1, Dlx2, Dlx3, Dlx5 and Dlx6. Although we confirm fundamental aspects of the hypothesis, we further report a number of novel findings. First, contrary to initial reports, Dlx1, Dlx2 and Dlx1/2 heterozygotes exhibit alterations of branchial arch structures and Dlx2-/- and Dlx1/2-/- mutants have slight alterations of structures derived from the distal portions of their branchial arches. Second, we present evidence for a role for murine Dlx3 in the development of the branchial arches. Third, analysis of compound Dlx mutants reveals four grades of mandibular arch transformations and that the genetic interactions of cis first-order (e.g. Dlx5 and Dlx6), trans second-order (e.g. Dlx5 and Dlx2) and trans third-order paralogues (e.g. Dlx5 and Dlx1) result in significant and distinct morphological differences in mandibular arch development. We conclude by integrating functions of the Dlx genes within the context of a hypothesized general mechanism for the establishment of pattern and polarity in the first branchial arch of gnathostomes that includes regionally secreted growth factors such as Fgf8 and Bmp and other transcription factors such as Msx1, and is consistent both with the structure of the conserved gnathostome jaw bauplan and the elaboration of this bauplan to meet organismal end-point designs.

There's more than one frog in the pond: a survey of the Amphibia and their contributions to developmental biology

The study of developmental biology has benefited greatly from the insights gained using amphibians as experimental models. Although Xenopus is currently the predominant model, much of our embryological knowledge derives from research on other amphibians. I will review some of these discoveries, made through astute choice of model organism, and I will examine the reasons behind the adoption of Xenopus as the standard for amphibian research. Additionally, I will discuss the diversity in developmental and reproductive strategies that exists within the Amphibia, and consider some of the recent advances in our understanding of the mechanisms underlying this developmental diversity.

Serotonin metabolism in directly developing frog embryos during paternal care

Central serotonin (5-HT) metabolism during embryogenesis and a 3-day post-hatching period was analyzed using high performance liquid chromatography in the directly developing frog, Eleutherodactylus coqui. This anuran bypasses the free-swimming larval stage and embryos hatch as miniature frogs in the adult phenotype. During embryogenesis and for a short time immediately after hatching, male E. coqui provide paternal care by brooding and guarding eggs/embryos to prevent desiccation and predation. Serotonin and its catabolite, 5-HIAA, were measured from whole brain during embryogenesis and at 3 days post-hatch to identify critical periods in 5-HT development and to determine the relationship between 5-HT and life history events such as hatching and frog dispersal from the nest site. Serotonergic activity was highest during the early-mid embryonic stages as indicated by the ratio of 5-HIAA/5-HT, a general indicator of turnover and metabolism. There were significant increases in tissue concentrations of 5-HT during the latest or terminal embryonic stage, just prior to hatching, and also at 3 days post-hatch, shortly before neonates disperse into the rainforest. These two increases probably represent different functional requirements during development. The first may occur as a result of the surge of development in the 5-HT system during late embryogenesis that occurs in E. coqui and the second may be from the increase demand in sensory and motor neural development required before dispersal from the nest site.

Ontogeny of central serotonergic neurons in the directly developing frog, Eleutherodactylus coqui

Embryonic development of the central serotonergic neurons in the directly developing frog, Eleutherodactylus coqui, was determined by using immunocytochemistry. The majority of anuran amphibians (frogs) possess a larval stage (tadpole) that undergoes metamorphosis, a dramatic post-embryonic event, whereby the tadpole transforms into the adult phenotype. Directly developing frogs have evolved a derived life-history mode where the tadpole stage has been deleted and embryos develop directly into the adult bauplan. Embryonic development in E. coqui is classified into 15 stages (TS 1-15; 1 = oviposition/15 = hatching). Serotonergic immunoreactivity was initially detected at TS 6 in the raphe nuclei in the developing rhombencephalon. At TS 7, immunopositive perikarya were observed in the paraventricular organ in the hypothalamus and reticular nuclei in the hindbrain. Development of the serotonergic system was steady and gradual during mid-embryogenesis. However, starting at TS 13 there was a substantial increase in the number of serotonergic neurons in the paraventricular, raphe, and reticular nuclei, a large increase in the number of varicose fibers, and a differentiation of the reticular nuclei in the hindbrain. Consequentially, E. coqui displayed a well-developed central serotonergic system prior to hatching (TS 15). In comparison, the serotonergic system in metamorphic frogs typically starts to develop earlier but the surge of development that transpires in this system occurs post-embryonically, during metamorphosis, and not in the latter stages of embryogenesis, as it does in E. coqui. Overall, the serotonergic development in E. coqui is similar to the other vertebrates.

Development of the nasal chemosensory organs in two terrestrial anurans: the directly developing frog, Eleutherodactylus coqui (Anura: Leptodactylidae), and the metamorphosing toad, Bufo americanus (Anura: Bufonidae)

Nearly all vertebrates possess an olfactory organ but the vomeronasal organ is a synapomorphy for tetrapods. Nevertheless, it has been lost in several groups of tetrapods, including aquatic and marine animals. The present study examines the development of the olfactory and vomeronasal organs in two terrestrial anurans that exhibit different developmental modes. This study compares the development of the olfactory and vomeronasal organs in metamorphic anurans that exhibit an aquatic larva (Bufo americanus) and directly developing anurans that have eliminated the tadpole (Eleutherodactylus coqui). The olfactory epithelium in larval B. americanus is divided into dorsal and ventral branches in the rostral and mid-nasal regions. The larval olfactory pattern in E. coqui has been eliminated. Ontogeny of the olfactory system in E. coqui embryos starts to vary substantially from the larval pattern around the time of operculum development, the temporal period when the larval stage is hypothesized to have been eliminated. The nasal anatomy of the two frogs does not appear morphologically similar until the late stages of embryogenesis in E. coqui and the terminal portion of metamorphosis in B. americanus. Both species and their respective developing offspring, aquatic tadpoles and terrestrial egg/embryos, possess a vomeronasal organ. The vomeronasal organ develops at mid-embryogenesis in E. coqui and during the middle of the larval period in B. americanus, which is relatively late for neobatrachians. Development of the vomeronasal organ in both frogs is linked to the developmental pattern of the olfactory system. This study supports the hypothesis that the most recent common ancestor of tetrapods possessed a vomeronasal organ and was aquatic, and that the vomeronasal organ was retained in the Amphibia, but lost in some other groups of tetrapods, including aquatic and marine animals.

Mosaic evolution of neural development in anurans: acceleration of spinal cord development in the direct developing frog Eleutherodactylus coqui

Previous studies have shown that spinal cord development in direct developing frogs of the genus Eleutherodactylus, which have evolutionarily lost the tadpole stage, differs from that in biphasically developing anurans (with the larval and the adult stage separated by metamorphosis). The present study of spinal cord development in Eleutherodactylus coqui provides additional information about neurogenesis, neuronal differentiation and growth analyzed by immunostaining for proliferating cell nuclear antigen (PCNA), in situ hybridization for NeuroD, and morphometric measurements in various developmental stages. Furthermore, spinal cord development in the frogs Discoglossus pictus, Xenopus laevis, and Physalaemus pustulosus, which belong to different anuran families but all exhibit biphasic development, was similarly analyzed. This comparative analysis allows inference of the ancestral anuran pattern of spinal cord development and how it has been modified during the evolution of Eleutherodactylus. All biphasically developing frogs analyzed share a similar pattern of spinal cord development, suggesting that this is ancestral for anurans: after neural tube closure, levels of proliferation and neurogenesis in the spinal cord were low throughout embryogenesis until they were upregulated drastically at early larval stages followed by development of the lateral motor columns. In contrast, no such quiescent embryonic period exists in E. coqui, where rapid growth, high levels of proliferation and neurogenesis, and early formation of lateral motor columns occur shortly after neural tube closure, while other parts of the central nervous system develop more slowly. Thus, spinal cord development has been accelerated during the evolution of Eleutherodactylus relative to the development of other parts of the central nervous system, probably related to the precocious development of limbs in this lineage.

The evolution of development: two portraits of skull ossification in pipoid frogs

Development creates morphology, and the study of developmental processes has repeatedly shed light on patterns of morphological evolution. However, development itself evolves as well, often concomitantly with changes in life history or in morphology. In this paper, two approaches are used to examine the evolution of skull development in pipoid frogs. Pipoids have highly unusual morphologies and life histories compared to other frogs, and their development also proves to be remarkable. First, a phylogenetic examination of skull bone ossification sequences reveals that jaw ossification occurs significantly earlier in pipoids than in other frogs; this represents a reversal to the primitive vertebrate condition. Early jaw ossification in pipoids is hypothesized to result from the absence of certain larval specializations possessed by other frogs, combined with unusual larval feeding behaviors. Second, thin-plate spline morphometric studies of ontogenetic shape change reveal important differences between pipoid skull development and that of other frogs. In the course of frog evolution, there has been a shift away from salamander-like patterns of ontogenetic shape change. The pipoids represent the culmination of this trend, and their morphologies are highly derived in numerous respects. This study represents the first detailed examination of the evolution of skull development in a diverse vertebrate clade within a phylogenetic framework. It is also the first study to examine ossification sequences across vertebrates, and the first to use thin-plate spline morphometrics to quantitatively describe ontogenetic trajectories.

Comparative patterns of postcranial ontogeny in therian mammals: an analysis of relative timing of ossification events

Data on the relative sequence of ossification of postcranial elements for eight therian mammals (Myotis lucifugus, Homo sapiens, Rattus norvegicus, Mus musculus, Mesocricetus auratus, Cavia porcellus, Didelphis albiventris, and Sminthopsis macroura) and three outgroups (Chelydra serpentina, Alligator mississippiensis, and Lacerta vivipara) were taken from the literature. For each species, a matrix was constructed in which the relative timing of the onset of ossification in 24 elements was summarized. This resulted in 276 event pairs (characters) for each species. Thirty-three (33.3)% of the characters examined are uniform across all taxa, 16.3% are variable but uninformative in the phylogeny, and 50.4% potentially deliver diagnostic features for clades of two or more taxa. In all species examined, the clavicle is the first bone to appear. Placentalia is not unequivocally diagnosed by the state of any event pair, while Marsupialia has the largest amount of autapomorphies with 18. The acceleration in the timing of ossification of the scapula in relation to the hindlimb in marsupials is most probably causally correlated to movements after birth and during early phases of pre-weaning life. Marsupials are almost unique among amniotes in that the earliest onset of ossification of at least one element among carpals and among tarsals is simultaneous. Three parsimony analyses, each one with a different reptilian taxon as outgroup, were performed using event pairs as characters. In all cases, results were incongruent with the phylogeny of the studied taxa.

The effect of miniaturized body size on skeletal morphology in frogs

Miniaturization has evolved numerous times and reached impressive extremes in the Anura. I compared the skeletons of miniature frog species to those of closely related larger species to assess patterns of morphological change, sampling 129 species from 12 families. Two types of morphological data were examined: (1) qualitative data on bone presence and absence; and (2) thin-plate spline morphometric descriptions of skull structure and bone shape. Phylogenetic comparative methods were used to address the shared history of species. Miniature anurans were more likely to lose skull bones and phalangeal elements of the limbs. Their skulls also showed consistent differences compared to those of their larger relatives, including relatively larger braincases and sensory capsules, verticalization of lateral elements, rostral displacement of the jaw joint, and reduction of some skull elements. These features are explained by functional constraints and by paedomorphosis. Variation among lineages in the morphological response to miniaturization was also explored. Certain lineages appear to be unusually resistant to the morphological trends that characterize miniature frogs as a whole. This study represents the first large-scale examination of morphology and miniaturization across a major, diverse group of organisms conducted in a phylogenetic framework and with statistical rigor.