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Direct development: An alternative way to make a frog
... Direct development in frogs is a derived life history mode found in one or more species of at least 10 different families or family level taxa of frogs and is believed to have evolved independently in these families (Duellman and Trueb, 1986;Hanken, 1999). Direct developing frogs undergo most of their development inside eggs, and development is completely terrestrial with miniature adults hatching from the eggs (Callery et al., 2001;Elinson, 2001). This mode of development is typified by a variable degree of loss or re-patterning of larval features of the ancestral biphasic developmental mode, including vestigial or absent lateral line organs, cement glands, larval mouthparts and a coiled gut (Callery and Elinson, 2000a;Callery et al., 2001;Elinson, 2001). ...
... Direct developing frogs undergo most of their development inside eggs, and development is completely terrestrial with miniature adults hatching from the eggs (Callery et al., 2001;Elinson, 2001). This mode of development is typified by a variable degree of loss or re-patterning of larval features of the ancestral biphasic developmental mode, including vestigial or absent lateral line organs, cement glands, larval mouthparts and a coiled gut (Callery and Elinson, 2000a;Callery et al., 2001;Elinson, 2001). The tail is the only obvious remnant structure but it often serves a respiratory rather than locomotory role (Elinson, 2001). ...
... This mode of development is typified by a variable degree of loss or re-patterning of larval features of the ancestral biphasic developmental mode, including vestigial or absent lateral line organs, cement glands, larval mouthparts and a coiled gut (Callery and Elinson, 2000a;Callery et al., 2001;Elinson, 2001). The tail is the only obvious remnant structure but it often serves a respiratory rather than locomotory role (Elinson, 2001). Direct development evolved from species with biphasic development, where a postembryonic larval phase is separated from an adult phase by metamorphosis constituting the developmental transition from aquatic to terrestrial life (Shi, 2000). ...
Direct developing frogs lack a free-living larval phase, such that miniature adults hatch directly from the eggs. Even under such extreme reorganization of the ancestral biphasic developmental pattern, direct developers still undergo thyroid hormone (TH)-dependent post-embryonic development. Hypothalamic regulation of TH synthesis and release plays a central role in controlling the timing of metamorphosis in biphasic developers. In particular, the neuropeptide corticotropin-releasing factor (CRF) regulates TH in tadpoles, but in adults, both thyrotropin-releasing hormone (TRH) and CRF regulate TH. Because direct developers lack a tadpole stage, it was not clear whether hypothalamic regulation of TH would be tadpole-like or adult-like prior to hatching. To test this, we injected pre-hatching Eleutherodactylus coqui daily with CRF, TRH or astressin (a CRF receptor blocker). CRF but not TRH significantly accelerated the developmental rate compared to controls. Astressin-treated animals showed a near complete developmental arrest, which confirmed that development requires CRF. To support the idea that CRF acts to regulate development in E. coqui via thyroid physiology, we showed the TH-direct response gene TRβ is up-regulated 24 and 48 h after CRF injection. In addition, treatment with 50 nM T3 (triiodothyronine, the active form of TH) increased the developmental rate similar to CRF injections. Our results extend the evidence for a cryptic metamorphosis in direct developers by showing that neuroendocrine signaling is conserved between biphasic and direct developers. Furthermore, the conserved neuroendocrine regulation implies that changes at the peripheral level of hormone action underlie the evolution of the radically divergent development in direct developers.
... The direct developing Puerto Rican tree frog (E. coqui) has also accelerated the development of the adult morphology and has lost many of the larval structures (Elinson 2001). ...
... However, there might be other factors than food driving the evolution of direct development, such as predation, interspecific competition, variation in environmental conditions, or hostile environments. Amphibians with direct development, for example, do no longer rely on water for reproduction (e.g., Elinson 2001), which can be a huge advantage in dry regions. Mortality rates often differ among habitats, which could strongly affect the loss of metamorphosis. ...
Although metamorphosis is widespread in the animal kingdom, several species have evolved life cycle modifications to avoid complete metamorphosis. Some species, e.g., many salamanders and newts, have deleted the adult stage via a process called paedomorphosis. Others, e.g., some frog species and marine invertebrates, no longer have a distinct larval stage and reach maturation via direct development. Here we study which ecological conditions can lead to the loss of metamorphosis via the evolution of direct development. To do so, we use size‐structured consumer‐resource models in conjunction with the adaptive‐dynamics approach. In case the larval habitat deteriorates, individuals will produce larger offspring and in concert accelerate metamorphosis. While this leads to the evolutionary transition from metamorphosis to direct development when the adult habitat is highly favourable, the population will go extinct in case the adult habitat does not provide sufficient food in order to escape metamorphosis. With a phylogenetic approach we furthermore show that among amphibians the transition of metamorphosis to direct development is indeed, in line with model predictions, conditional on and preceded by the evolution of larger egg sizes.
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... jgoldberg@conicet.gov.ar or pharyngeal arches development; and 3) new structures can arise such as the egg tooth (e.g., Hanken et al., 1997a, b;Callery et al., 2001;Elinson, 2001). Among anurans, the genus Eleutherodactylus, and particulary E. coqui, is regarded as the classical example of direct development and it is stated to exhibit the most pronounced ontogenetic changes relative to the ancestral, metamorphic ontogeny (Townsend and Stewart, 1985;Elinson, 1990;Hanken et al., 1997a, b;Hanken, 1999;Callery et al., 2001;Elinson, 2001;Kerney et al., 2010;Singamsetty and Elinson, 2010). ...
... jgoldberg@conicet.gov.ar or pharyngeal arches development; and 3) new structures can arise such as the egg tooth (e.g., Hanken et al., 1997a, b;Callery et al., 2001;Elinson, 2001). Among anurans, the genus Eleutherodactylus, and particulary E. coqui, is regarded as the classical example of direct development and it is stated to exhibit the most pronounced ontogenetic changes relative to the ancestral, metamorphic ontogeny (Townsend and Stewart, 1985;Elinson, 1990;Hanken et al., 1997a, b;Hanken, 1999;Callery et al., 2001;Elinson, 2001;Kerney et al., 2010;Singamsetty and Elinson, 2010). Recent studies have partitioned this genus and proposed the unranked taxon Terrarana with four or five families and 940 species that probably share direct development (Hedges et al., 2008;Heinicke et al., 2009;Frost, 2011;Pyron and Wiens, 2011). ...
Within Anura, direct development involves ontogenetic changes of the biphasic ancestral pattern. The recent
partitioning of the genus Eleutherodactylus, along with the proposition of the unranked taxon Terrarana, has renewed an interest to the morphological and ecological diversity among direct-developing frogs. The morphological changes during embryonic development of Oreobates barituensis is similar to those of other Neotropical direct-developing species, including the reduction or absence of several larval and embryonic characters (e.g., external gills and adhesive glands), heterochronic changes (e.g., early developing limbs and late persistence of ciliated epidermal cells), and the appearance of new structures (e.g., egg tooth). The tail achieves an extraordinary peramorphic development (encloses the entire embryo), and the location of its expanded part is interpreted as a heterotopic change resulting in a novel trait. An enveloping tail with apparently non-heterotopic fins, combined with the absence of gills, has been only reported for a species of the related genus Craugastor, and these morphologies suggest an informative perspective for the study of evolution of direct development in terraranans
... Two no exclusive mechanisms have been evoked to explain the evolution of endotrophy: (1) the loss of some specific tadpole characters and (2) the acceleration in the development of some adult characters (Elinson 2001). Empirical evidence seems to support these ideas; data suggest that endotrophy is associated with the remodeling and/ or loss of cartilaginous elements typical of tadpoles, as the suprarostral cartilages and palatoquadrate, and with changes in the onset of some skeletal elements (Hanken et al. 1992;Yeh 2002;Kerney et al. 2007). ...
Frostius pernambucensis is a phytotelm-breeding frog with endotrophic larvae. Although the larvae were formally described, no aspect of its internal morphology is known. In this paper, we re-describe the tadpole based on a large sample, describe its internal anatomy (buccopharyngeal cavity and musculo-skeletal system), provide data on natural history, and discuss the evolution of endotrophy and phytotelma colonization. The tadpoles of F. pernambucensis are highly modified, with depressed bodies, reduced mouthparts, and long tails. Many character-states described for these tadpoles can be related to its endotrophic development. Consequence of this highly modified phenotype, we propose several novel putative synapomorphies for the genus: (1) labial tooth row formula 1/1; (2) absence of pustulation in the buccal roof and (3) floor; (4) absence of median ridge; (5) absence of lateral ridge papillae; (6) absence of secretory ridges and pores; (7) absence of filter plates; (7) m. subarcualis rectus II–IV originating on ceratobranchial III; (8) m. subarcualis rectus II–IV inserting on ceratobranchial I; (8) ventral slip of the m. subarcualis rectus I inserting on the ceratobranchial III; (9) suprarostral corpora fused to the cornua trabeculae; (10) commissura quadratoorbitalis absent; (11) cerabranchial II attached to the planum hypobranchiale; and (12) ceratobranchial III attached to the planum hypobranchiale. Finally, we also propose that the presence of a single pair of infralabial papilla could represent a synapomorphy of bufonids. The colonization of phytotelma seem to have created a selective pression on the development of F. pernambucenis, favoring the evolution of endotrophy.
... Particularly noticeable, the tail is used for gas exchanges instead of swimming. The presence of rudimentary tadpole structures indicates that E. coqui direct development is a character derived from an ancestor with tadpoles and metamorphosis (Elinson 2001;Ziermann and Diogo 2014). The modifications of E. coqui early development include changes in the cleavage pattern, the blastocoel roof, and germ layer formation and the development of a special structure, the nutritional endoderm (Elinson and Fang 1998;Ninomiya et al. 2001;Elinson and Beckham 2002;Chatterjee and Elinson 2013;Karadge and Elinson 2013). ...
The developmental adaptations of the marsupial frogs Gastrotheca riobambae and Flectonotus pygmaeus (Hemiphractidae) are described and compared with frogs belonging to seven additional families. Incubation of embryos by the mother in marsupial frogs is associated with changes in the anatomy and physiology of the female, modifications of oogenesis, and extraordinary changes in embryonic development. The comparison of early development reveals that gene expression is highly conserved. However, the timing of gene expression varies between frog species. There are two modes of gastrulation according to the onset of convergent extension. In gastrulation mode 1, convergent extension is an intrinsic mechanism of gastrulation. This gastrulation mode occurs in frogs with aquatic reproduction, such as Xenopus laevis. In gastrulation mode 2, convergent extension occurs after the completion of gastrulation movements. Gastrulation mode 2 occurs in frogs with terrestrial reproduction, such as the marsupial frog, G. riobambae. The two modes of frog gastrulation resemble the two transitions toward meroblastic cleavage of ray-finned fishes (Actinopterygii). The comparison indicates that a major event in the evolution of frog terrestrial development is the separation of convergent extension from gastrulation.
... Amphibians vary greatly, more so than any other chordate, in their reproductive strategies, and many of them lay relatively small numbers of very large eggs (up to about 1 cm), with large stores of yolk, and the adults hide, brood, and guard them in a myriad of different, exotic ways (Duellman and Trueb, 1994). Some of these develop directly to the adult stage without a larval stage (direct development) (Elinson, 1994;Elinson, 2001). Eggs with large amounts of yolk present a challenge for the machinery of gastrulation. ...
... In order to go through metamorphosis inside of the brood chambers formed by the dorsal skin of females, the embryos have a small space to develop and grow, and do not feed or swim. It is possible that some of the steps of development may have been deleted and/or there is acceleration in the development relative to other frogs without free-swimming tadpoles (e.g., Callery et al., 2001;Downie et al., 2004;Elinson, 2001;Lynn, 1942). ...
Two different perspectives guide ontogenetic studies. On one hand, there is a concern with objective processes and the understanding of the underlying mechanisms that governed the appearance of morphological differences between taxa; on the other hand, there are similarities between ontogenetic patterns that are used as phylogenetic characters. One focus of ontogenetic studies is to understand how morphology and timing of development differ among species. Such studies also aim to serve as a guide in proposing hypotheses of homology, especially when extremely modified structures are under scrutiny, such as the hyobranchial apparatus of amphibians. Here, we describe the morphology and development of the hyobranchial apparatus of Pipa arrabali (based on the examination of 51 embryos and six newly hatched specimens). Its morphology is compared to that described in the literature for other species. In P. arrabali, Copula I is absent, Copula II is present, and the ceratobranchials are simple, without spines or cartilaginous rays. In general, the ontogeny of the hyobranchial apparatus of P. arrabali seems to be accelerated when compared to the hyobranchia of other frogs, with some stages being skipped and the absence of some tadpole-specific structures observed in other species of pipids and non-pipids.
... These frogs have abandoned metamorphosis! Direct development is a derived strategy; all direct developers evolved from predecessors that used metamorphosis (e.g., Elinson, 2001). 298 ANIMAL BEHAVIOR Figure 9.1. ...
Our goal is for this chapter to alter the way you look at life. More specifically, we think we can open your eyes to new, interesting, and important ways to see, to understand, and to appreciate development. Recognition that development is an important part of the scientific study of animal behavior has a very respectable history (Chapter 2). Niko Tinbergen, one of three ethologists recognized with a Nobel Prize in 1973, said that in order to understand fully an animal’s behavior, it was essential to understand how it develops (Tinbergen, 1963). Tinbergen saw development as one of the four basic aims of a science of ethology or animal behavior (Box 9.1). We agree with Tinbergen that development is an essential component in a biological understanding of behavior. In fact, we think that understanding development not only addresses many important aspects of behavior but links the other components of a complete analysis of behavior.
Much of the field of animal behavior involves discovering and studying the marvelous and diverse modes by which animals “earn their living” in the natural world. For example, we can learn about kinds of navigation and migration(Chapter 12), hunting and feeding strategies (Volume 2, Chapter 1), courtship and reproductive patterns (Volume 2, Chapter 6), and many other such phenomena. When we learn about such stunning examples and then think about the study of development, it is natural to conclude that the study of development, or ontogeny, is the study of how an infant becomes the adult form that is capable of such stunning feats of behavioral life. John Tyler Bonner (1958, p.1) bluntly expressed this viewpoint when he stated, “the goal of development is the final form and function of the adult.” This is a view that emphasizes development as a process of “becoming.” There is a focus on an endpoint reproduction, territorial defense, nest building—and development is the process that prepares the offspring to achieve the endpoint. Viewed this way, the developmental process includes growth—with increasing strength, expanded and improved sensory function, and acquisition of motor patterns including complex behavioral displays and signaling. Special body features and coloration also develop, often as part of sexual maturation or with the attainment of dominance status, and these physical features are often used in behavioral displays. The view of development as the process of becoming is popular, and it is very likely that this is basically the way that you look at development
... A second noticeable feature of embryos of E. coqui and other direct developing frogs is the early formation and rapid development of the limbs. The initiation and growth of limbs in E. coqui look more like that in amniotes, such as birds and mammals, than in frog tadpoles (Elinson, 1990(Elinson, , 2001. In tadpoles, small limb buds form around the time that feeding begins, and they grow slowly until metamorphosis. ...
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.
... The evolution of anuran ontogeny and concomitant morphological change has been widely researched (e.g. Emerson 1986;Alberch 1989;Hanken et al. 1997;Elinson and Fang 1998;Elinson 2001;Chipman 2002). Furthermore, anurans that are easily bred have been used in embryological research (Gurdon and Hopwood 2000;Beck and Slack 2001;Chipman 2002;Callery 2006). ...
Most anurans have a biphasic life cycle, which includes metamorphosis from a tadpole stage to an adult frog. This process involves extensive transformations of the cranial skeleton, which have been of long-standing interest with respect to anuran skeletal evolution and taxonomy. In this study, large-scale patterns of anuran skeletal ossification are assessed by collecting the most comprehensive data set on anuran cranial ossification to date from the literature, including data for 45 anuran and one caudate outgroup species. Ossification sequences were translated into event-pair matrices for explorative phylogenetic analysis and phylogenetically informed parsimony search for heterochrony using the Parsimov algorithm. Rank variability of single bones across species was also analysed. Little phylogenetic signal was retrieved from a parsimony-based phylogenetic analysis of event-pairs, and only a few species that are generally agreed to be closely related are placed close to each other (e. g. some Pipidae and Costata). Parsimov analysis revealed some clade-specific heterochrony in anuran clades of varying inclusiveness. Our results show that relating heterochronic changes in anuran cranial ontogeny to parameters such as direct development or miniaturization is problematic because of the high evolvability of cranial ossification sequences. Rank variation analysis suggests that anuran cranial bones are highly variable in their sequence positioning, possibly because tadpole and adult cranial morphology do not co-evolve. Elements which are lost in some species ossify at the end of the sequence, providing evidence for the notion that failure of anuran cranial elements to ossify is due to processes of paedomorphosis.
... This toad has an aquatic, larval stage that undergoes metamorphosis into a terrestrial anuran. The Puerto Rican Coquí, Eleutherodactylus coqui, is a directly developing anuran that bypasses the aquatic tadpole stage and hatches directly as a terrestrial anuran in the adult phenotype (for reviews see Hanken, 1999;Callery et al., 2001;Elinson, 2001). This frog exhibits many developmental modifications from the metamorphic anuran pattern, including an altered sequence of cranial ontogeny (Hanken et al., 1992), modified peripheral nerve development (Schlosser and Roth, 1997), early activation of the thyroid axis (Jennings and Hanken, 1998), a thyroid hormone-dependent period during embryogenesis (Callery and Elinson, 2000a), and modified operculum development and ontogenetic reorganization (Callery and Elinson, 2000b). ...
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. J Morphol. 261:225–248, 2004. © 2004 Wiley-Liss, Inc.
... Larvae have been deleted from the life history of all three orders of amphibians, producing the pattern known as direct development. 15,20,27,40,41,43,[108][109][110] The differences in morphology between the larva and the adult are much greater in frogs than in either urodeles or caecilians, so the appearance of anu- ran direct developers is particularly striking. 16,[111][112][113][114][115][116] Although there have been multiple origins of direct developing anurans, their embryos look similar indi- cating convergent evolution. ...
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. WIREs Dev Biol 2012, 1:345–369. doi: 10.1002/wdev.23
This article is categorized under: Early Embryonic Development > Development to the Basic Body Plan
Comparative Development and Evolution > Model Systems
Comparative Development and Evolution > Evolutionary Novelties
... Many species, however, exhibit variations on this common theme. The most dramatic departure from biphasic development is direct development , which in the neotropical frog Eleutherodactylus coqui is characterized by the deposition of terrestrial eggs that hatch as miniature froglets instead of aquatic tadpoles (Orton 1951; Hanken 2003; Sampson 1900; Elinson 2001). Whereas limb bud formation in metamorphosing anurans typically begins during the tadpole stage, well after embryogenesis, limb formation in E. coqui begins in the embryo before complete closure of the neural tube (Elinson 1994; Richardson et al. 1998). ...
The growing field of skeletal developmental biology provides new molecular markers for the cellular precursors of cartilage and bone. These markers enable resolution of early features of skeletal development that are otherwise undetectable through conventional staining techniques. This study investigates mRNA distributions of skeletal regulators runx2 and sox9 along with the cartilage-dominant collagen 2(alpha)1 (col2a1) in embryonic limbs of the direct-developing anuran, Eleutherodactylus coqui. To date, distributions of these genes in the limb have only been examined in studies of the two primary amniote models, mouse and chicken. In E. coqui, expression of transcription factors runx2 and sox9 precedes that of col2a1 by 0.5-1 developmental stage (approximately 12-24 h). Limb buds of E. coqui contain unique distal populations of both runx2- and sox9-expressing cells, which appear before formation of the primary limb axis and do not express col2a1. The subsequent distribution of col2a1 reveals a primary limb axis similar to that described for Xenopus laevis. Precocious expression of both runx2 and sox9 in the distal limb bud represents a departure from the conserved pattern of proximodistal formation of the limb skeleton that is central to prevailing models of vertebrate limb morphogenesis. Additionally, runx2 is expressed in the early joint capsule perichondria of the autopod and in the perichondria of long bones well before periosteum formation. The respective distributions of sox9 and col2a1 do not reveal the joint perichondria but instead are expressed in the fibrocartilage that fills each presumptive joint capsule. These distinct patterns of runx2- and sox9-expressing cells reveal precursors of chondrocyte and osteoblast lineages well before the appearance of mature cartilage and bone.
... Unlike anurans with tadpoles, anurans that develop directly without a tadpole form limb buds early, as in amniotes (Townsend and Stewart, '85;Elinson, 2001). We recently analyzed an unusual lethal syndrome in embryos of the direct developing frog, Eleutherodactylus coqui, in which excessive fluid retention, edema, was coupled with failure of forelimb formation (Lee and Elinson, 2008). ...
Embryos of the direct developing frog, Eleutherodactylus coqui, provide opportunities to examine frog early limb development that are not available in species with tadpoles. We cloned two retinaldehyde dehydrogenase genes, EcRaldh1 and EcRaldh2, to see which enzyme likely supplies retinoic acid for limb development. EcRaldh1 is expressed in the dorsal retina, otic vesicle, pronephros, and pronephric duct, but not in the limb. EcRaldh2 is expressed early at the blastoporal lip and then in the mesoderm in the neurula, so this expression could function in forelimb initiation. Later EcRaldh2 is expressed in the mesoderm at the base of the limbs and in the ventral spinal cord where motor neurons innervating the limbs emerge. These observations on a frog support the functional conservation of EcRaldh2 in forelimb initiation in Osteichthyans and in limb patterning and motor neuron specification in tetrapods.
... Most species of frogs have two successive posthatching life-history stages, a herbivorous, aquatic larva and a carnivorous , terrestrial adult, which are separated by a discrete metamorphosis. Direct-developing species, however, bypass the free-living larval stage and develop directly into adults (Hanken, '92, '99; Elinson, 2001; Fig. 1). Many adult features that form only after hatching in metamorphic anurans, such as the limbs, instead form during embryogenesis in direct developers (Elinson, '90, '94). ...
Mechanisms that mediate limb development are regarded as highly conserved among vertebrates, especially tetrapods. Yet, this assumption is based on the study of relatively few species, and virtually none of those that display any of a large number of specialized life-history or reproductive modes, which might be expected to affect developmental pattern or process. Direct development is an alternative life history found in many anuran amphibians. Many adult features that form after hatching in metamorphic frogs, such as limbs, appear during embryogenesis in direct-developing species. Limb development in the direct-developing frog Eleutherodactylus coqui presents a mosaic of apparently conserved and novel features. The former include the basic sequence and pattern of limb chondrogenesis, which are typical of anurans generally and appear largely unaffected by the gross shift in developmental timing; expression of Distal-less protein (Dlx) in the distal ectoderm; expression of the gene Sonic hedgehog (Shh) in the zone of polarizing activity (ZPA); and the ability of the ZPA to induce supernumerary digits when transplanted to the anterior region of an early host limb bud. Novel features include the absence of a morphologically distinct apical ectodermal ridge, the ability of the limb to continue distal outgrowth and differentiation following removal of the distal ectoderm, and earlier cessation of the inductive ability of the ZPA. Attempts to represent tetrapod limb development as a developmental "module" must allow for this kind of evolutionary variation among species.
... The Puerto Rican Coquı´frog, E. coqui, is a directly developing anuran that has no aquatic tadpole stage and hatches directly as a terrestrial anuran. This frog exhibits many developmental modifications from the metamorphic anuran pattern including: repatterned skull development (Hanken et al. 1992), modified operculcum development and ontogenetic reorganization (Callery and Elinson 2000b), absence of the larval olfactory organs (Jermakowicz et al. 2004), early thyroid gland development (Jennings and Hanken 1998), and a T 3dependent developmental period, similar to what occurs during metamorphosis (Callery and Elinson 2000a; for reviews see Hanken 1999;Callery et al. 2001;Elinson 2001). ...
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.
... In zebrafish, pectoral fin morphogenesis, chondrogenesis, and intestine development are TH dependent (Brown, 1997; Liu and Chan, 2002), though skin development apparently is not. A striking example of changes in tissue dependence on TH within frogs is found in direct developers, where there is no free-living tadpole stage and froglets hatch out of the egg (Fig. 6D) (Elinson, 2001; Jennings and Hanken, 1998). As in Xenopus, adult skin formation, MeckelÕs cartilage proliferation, and other events are TH-regulated in Eleutherodactylus (Callery and Elinson, 2000). ...
The current review focuses on the molecular mechanisms and developmental roles of thyroid hormone receptors (TRs) in gene regulation and metamorphosis in Xenopus laevis and discusses implications for TR function in vertebrate development and diversity. Questions addressed are: (1) what are the molecular mechanisms of gene regulation by TR, (2) what are the developmental roles of TR in mediating the thyroid hormone (TH) signal, (3) what are the roles of the different TR isoforms, and (4) how do changes in these molecular and developmental mechanisms affect evolution? Even though detailed knowledge of molecular mechanisms of TR-mediated gene regulation is available from in vitro studies, relatively little is known about how TR functions in development in vivo. Studies on TR function during frog metamorphosis are leading the way toward bridging the gap between in vitro and in vivo studies. In particular, a dual function model for the role of TR in metamorphosis has been proposed and investigated. In this model, TRs repress genes allowing tadpole growth in the absence of TH during premetamorphosis and activate genes important for metamorphosis when TH is present. Despite the lack of metamorphosis in most other vertebrates, TR has important functions in development across vertebrates. The underlying molecular mechanisms of TR in gene regulation are conserved through evolution, so other mechanisms involving TH-target genes and TH tissue-sensitivity and dependence underlie differences in role of TR across vertebrates. Continued analysis of molecular and developmental roles of TR in X. laevis will provide the basis for understanding how TR functions in gene regulation in vivo across vertebrates and how TR is involved in the generation of evolutionary diversity.
... E. coqui, on the other hand, develops without a tadpole directly to a frog from a large egg laid on land (Fig. 1). The loss of the tadpole and the increase in egg size are derived from the more typical anuran development, and this direct development has originated at least ten times among the anurans (Heyer, 1975;Duellman and Trueb, 1986;Elinson, 1990;Callery et al., 2001;Elinson, 2001). There are several indications that early development is different between E. coqui and anurans such as Xenopus laevis, due to the amount of yolk. ...
The origin of the amniote egg is one of the most significant events in the evolution of terrestrial vertebrates. This innovation was probably driven by increased egg size, and to find potential parallels, we can examine the derived development of extant amphibians with large eggs. The embryo of the Puerto Rican tree frog, Eleutherodactylus coqui, exhibits an alteration of its fate map and a secondary coverage of its yolky cells, reflecting the large 3.5 mm egg. Comparable changes may have occurred with the derivation of an amniote pattern of development. Future investigations should focus on the molecular organization of the egg. In the model amphibian for development, Xenopus laevis, information for embryonic germ layers, the dorsal axis, and germ cells is stored mainly as localized RNAs at the vegetal pole of the egg. These localizations would likely be changed with increased egg size. A review of the orthologues of the key X. laevis genes raises the possibility that their activities are not conserved in other vertebrates.
... Unfortunately, the implications of life history variations on embryonic development have only been studied in two other species. One is the marsupial frog Gastrotheca riobambae (del Pino and Elinson, 1983;Elinson and del Pino, 1985;del Pino and Loor-Vela, 1990) and the other is the direct developing Eleutherodactylus coqui (Townsend and Stewart, 1985;Elinson, 1987b;Hanken et al., 1992;Elinson, 2001). Only in G. riobambae were early developmental events studied in detail. ...
Although anuran development is generally thought to be relatively conservative, a great deal of variation is evident when different species are compared. This report summarizes the results of comparative analyses of different aspects of anuran development. These include differences in sequence and timing of developmental events, the effects of genome size, and the effects of different life history strategies on anuran embryogenesis. The results show that anuran development is plastic at the evolutionary level, and many changes can occur in the developmental processes of anurans throughout their evolution. Changes are apparently rapid, and are as common as cladogenic events. This evolutionary plasticity can be attributed to the modular nature of anuran development. Different modules can shift relative to one another in time or in space, creating variations in the observed developmental patterns. However, shifts in modules can occur even without having a significant effect on the ultimate outcome of the process. I discuss the implications of the modular nature of development on the evolution of anuran development, and of the group in general.
... We study a species of direct-developing frog, Eleutherodactylus coqui, whose pattern of development is drastically different to that seen in most anurans. Many adult characters that form after hatching in species with a biphasic life history, e.g., limbs, instead form during embryogenesis in E. coqui, which at the same time lacks many typical larval features (Elinson, 1990Elinson, , 1994Elinson, , 2001 Elinson, 1996, 1999; Richardson et al., 1998; Schlosser et al., 1999; Callery and Elinson, 2000; Callery et al., 2001; Hanken et al., 2001; Ninomiya et al., 2001 ). Consequences of the evolution of direct development are especially conspicuous in the head; derived features include embryonic development of bone (which does not form until well after hatching in metamorphic species), and altered patterning of cranial cartilages and musculature (Hanken et al., 1992; 1997b). ...
Direct development is a specialized reproductive mode that has evolved repeatedly in many different lineages of amphibians, especially anurans. A fully formed, albeit miniature adult hatches directly from the egg; there is no free-living larva. In many groups, the evolution of direct development has had profound consequences for cranial development and morphology, including many components that are derived from the embryonic neural crest. Yet, the developmental bases of these effects remain poorly known. In order to more fully characterize these changes, we used three molecular markers to analyze cranial neural crest-cell emergence and migration in the direct-developing frog, Eleutherodactylus coqui: HNK-1 immunoreactivity, Dlx protein expression, and cholinesterase activity. Our study validates and extends earlier results showing that the comprehensive changes in embryonic cranial patterning, differentiation, and developmental timing that are associated with direct development in Eleutherodactylus have not affected gross features of cranial neural crest biology: the relative timing of crest emergence and the number, configuration and identity of the principal migratory streams closely resemble those seen in metamorphic anurans. The three markers are variably expressed within and among neural crest-cell populations. This variation suggests that determination of cranial neural crest-cells may already have begun at or soon after the onset of migration, when the cells emerge from the neural tube. It is not known how or even if this variation correlates with differential cell lineage or fate. Finally, although HNK-1 expression is widely used to study neural crest migration in teleost fishes and amniotes, E. coqui is the only amphibian known in which it effectively labels migrating neural crest-cells. There are not enough comparative data to determine whether this feature is functionally associated with direct development or is instead unrelated to reproductive mode.
Heterochrony, defined as a change in the timing of developmental events altering the course of evolution, was first recognized by Ernst Haeckel in 1866. Haeckel's original definition was meant to explain the observed parallels between ontogeny and phylogeny, but the interpretation of his work became a source of controversy over time. Heterochrony took its modern meaning following the now classical work in the 1970–80s by Steven J. Gould, Pere Alberch, and co-workers. Predicted and described heterochronic scenarios emphasize the many ways in which developmental changes can influence evolution. However, while important examples of heterochrony detected with comparative morphological methods have multiplied, the more mechanistic understanding of this phenomenon lagged conspicuously behind. Considering the rapid progress in imaging and molecular tools available now for developmental biologists, this review aims to stress the need to take heterochrony research to the next level. It is time to synchronize the different levels of heterochrony research into a single analysis flow: from studies on organismal-level morphology to cells to molecules and genes, using complementary techniques. To illustrate how to achieve a more comprehensive understanding of phyletic morphological diversification associated with heterochrony, we discuss several recent case studies at various phylogenetic scales that combine morphological, cellular, and molecular analyses. Such a synergistic approach offers to more fully integrate phylogenetic and ontogenetic dimensions of the fascinating evolutionary phenomenon of heterochrony.
Highlights
• Heterochrony research focuses on comparative morphology but lags on mechanistic understanding.
• We propose integrating the different levels of study into a single analysis flow: from morphology to cells to molecules and genes, using modern techniques.
Direct development has evolved independently several times in anurans and direct-developing species are
characterized by large-scale developmental repatterning and a complete, or near complete, absence of most tadpole specific structures. Earlier studies stressed the similarities among different direct-developing species, but more recent studies have indicated differences in the reduction of tadpole-specific structures among different taxa. Here, we describe egg deposition, clutch characteristics and embryonic development of the direct-developing squeaker frogs of the genus Arthroleptis, providing the first detailed description of
direct development in Arthroleptidae. Embryonic development in Arthroleptis is characterized by the presence of an opercular fold that still encloses the developing forelimbs, the absence of external gills and an only moderately extended tail. A comparison with published information on other direct developing anurans reveals broad dissimilarities in the formation of an opercular fold and very different tail morphology among different taxa. An egg tooth, often considered characteristic of direct-developing anurans, seems to be restricted to
NewWorld Terrarana. The embryonic diversity seen in direct developing anuran taxa argues against simplistic assumptions about the evolution of direct development.
Embryonic development in fifteen Australopapuan microhylid frogs of the genera Austrochaperina, Cophixalus and Oreophryne is described. These frogs have direct development during which the embryo develops to a minute froglet within the jelly capsule. Development of the operculum, presence of external gills, tail structure, gut development and timing of forelimb emergence are described and compared with the direct-developing eleutherodactylid Eleutherodactylus coqui from Puerto Rico and three Australian myobatrachid genera with direct development (Arenophryne, Metacrinia and Myobatrachus). We comment on those differences that likely reflect examples of convergent and divergent evolution and heterochrony.
The New World direct-developing frogs (Brachycephaloidea = Terrarana) comprise nearly a thousand species that share direct development among other putative synapomorphies, yet embryonic development in this group has been thoroughly described in only about 20 species. Here we describe the early ontogeny of the craugastorid Haddadus binotatus, making special emphasis on tail structure and development, and its differences and similarities with that of other terraranans. The morphological changes during embryonic development of H. binotatus and those of other Neotropical direct-developing species are alike, with some variation including the absence of external gills, timing of limb differentiation, and tail configuration. The tail with a rotated core axis and lateral and asymmetric fins that cover the posterior half of the embryo represents an outstanding case of developmental repatterning. We present some interpretations of the evolution of the tail and its three major aspects, the rotation of the core axis, and the origin and extensions of the fins, and pinpoint that those mechanisms underlying fin development should be fairly plastic, allowing the ontogenetic and evolutionary variation within the Brachycephaloidea clade.
"The Chromosomes of Terraranan Frogs" is the most comprehensive, original and comparative cytogenetic study of vertebrates ever performed, presenting results obtained over a 36-year period of more than 70 expeditions to the Neotropics (Central and South America and the Caribbean) and from several laboratories (Canada, Costa Rica, Cuba, Germany, USA, and Venezuela). The karyotypes and genomes of 227 recognized species and 2,548 individual specimens of Terrarana were examined using a variety of staining techniques, molecular probe in situ hybridization, and genome size measurements. Furthermore, all previously published data on the cytogenetics of Terrarana have been included. The karyotype characters were mapped onto the molecular phylogeny obtained by analyses of several mitochondrial and nuclear genes. This allowed reconstruction of the various karyophylogenies as well as calculation of the chromosomal mutation rates. The results were always interpreted in context with the cytogenetic data published for other amphibian taxa or vertebrates. The general properties of eukaryote chromosomes, the rules governing structural and numerical chromosome repatterning as well as the principles of chromosome evolution are explained. An extensive 'Material and Methods' section was compiled with the intention of providing the necessary guidance for everyone interested in studying amphibian chromosomes. This monograph provides detailed current and archival information on terraranan karyotypes and genomes to colleagues who are already recognized specialists in amphibian or vertebrate cytogenetics. It is also valuable reading for herpetologists involved in the evolution of neotropical amphibians and provides comprehensive insights into vertebrate cytogenetics for students.
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.
The nocturnal, terrestrial frog Eleutherodactylus coqui, known as the Coqui, is endemic to Puerto Rico and was accidentally introduced to Hawai‘i via nursery plants in the late 1980s. Over the past two decades E. coqui has spread to the four main Hawaiian Islands, and a major campaign was launched to eliminate and control it. One of the primary reasons this frog has received attention is its loud mating call (85–90 dB at 0.5 m). Many homeowners do not want the frogs on their property, and their presence has influenced housing prices. In addition, E. coqui has indirectly impacted the floriculture industry because customers are reticent to purchase products potentially infested with frogs. Eleutherodactylus coqui attains extremely high densities in Hawai‘i, up to 91,000 frogs ha-1, and can reproduce year-round, once every 1–2 months, and become reproductive around 8–9 months. Although the Coqui has been hypothesized to potentially compete with native insectivores, the most obvious potential ecological impact of the invasion is predation on invertebrate populations and disruption of associated ecosystem processes. Multiple forms of control have been attempted in Hawai‘i with varying success. The most successful control available at this time is citric acid. Currently, the frog is established throughout the island of Hawai‘i but may soon be eliminated on the other Hawaiian Islands via control efforts. Eradication is deemed no longer possible on the island of Hawai‘i.
Vera Candioti, M.F., Nuñez, J.J. and Úbeda, C. 2011. Development of the nidicolous tadpoles of Eupsophus emiliopugini (Anura: Cycloramphidae) until metamorphosis, with comments on systematic relationships of the species and its endotrophic developmental mode. —Acta Zoologica (Stockholm) 92: 27–45.
Species of Eupsophus are unique within Alsodinae in having nidicolous tadpoles. They are characterized by traits typical of generalized exotrophic (e.g., oral disc and spiracular tube) and endotrophic larvae (e.g., scant pigmentation and large hind limbs). The larval morphology and development of E. emiliopugini, including external, buccal, and musculoskeletal features, is described herein. Like the larvae of other alsodines, these larvae have four lingual and four infralabial papillae, quadratoethmoid process, and an m. rectus cervicis with a double insertion. Among the traits exclusive to the genus are: the absence of the pseudopterygoid process and quadrato-orbital commissure; presence of the m. subarcualis rectus I with two slips; and presence of the m. subarcualis rectus II–IV inserting on Ceratobranchial II. The development and metamorphosis of Eupsophus include some characters that develop later (e.g., degeneration of mouthparts and chondrocranium with minimum calcification), characters that develop earlier (e.g., hind-limb appearance and jaw and suspensorium ossification), and characters that develop at the same time (e.g., most external features and cranial muscles) than in most exotrophic species. Some distinctive characters (third lower labial ridge absent, general configuration of the hyobranchial skeleton, skeletal development with retention of larval traits) resemble those of other endotrophic species, and the precocious ossification of jaws and suspensorium is shared with several direct-developing species among recent amphibians.
In wing polyphenisms that produced alternative wing morphs depending on environmental conditions, the developmental regulations to balance between flight and reproductive abilities should be important. Many species of aphids exhibit wing polyphenisms, and the development of wing and flight muscles is thought to incur costs of reproductive ability. To evaluate the relationship between flight and reproduction, the fecundity and the wing- and ovarian development in the parthenogenetic generations were compared between winged and wingless aphids in the vetch aphid Megoura crassicauda. Although no differences in offspring number and size were detected, the onset of larviposition after imaginal molt was delayed in winged adults. The comparison of growth in flight apparatus revealed that, after the second-instar nymphs, the flight-apparatus primordia of presumptive wingless aphids were degenerated while those of winged nymphs rapidly developed. In the ovaries of winged line, the embryo size was smaller and the embryonic stages were delayed from third to fifth instars, although these differences had disappeared by the time of larviposition. It is therefore likely that the delay in larviposition in winged aphids is due to the slower embryonic development. The correlation between embryo size and developmental stage suggests that the embryos of winged aphids are better developed than similarly sized embryos in wingless aphids. These heterochronic shifts would facilitate the rapid onset of larviposition after the dispersal flight. This developmental regulation of embryogenesis in the aphid wing polyphenism is suggested to be an adaptation that compensates the delay of reproduction caused by the wing development.
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.
This study investigates ovulation and egg deposition behaviors in the anuran Eleutherodactylus coqui from Puerto Rico in response to stimulation with gonadotropin and gonadotropin releasing hormones. Five hormones were tested by injection over a range of doses, including mammalian LHRH, avian LHRH, fish LHRH, D-Ala6, des-Gly10 ethylamide LHRH and hCG. We report a low level of ovulation and egg deposition in response to all hormones, with the most complete and consistent results from the non-natural D-Ala6, des-Gly10 ethylamide LHRH derivative. To confirm the viability of eggs produced in this manner we performed in vitro fertilization experiments that resulted in the development of normal frogs. Reproductive behaviors in E. coqui are apparently not controlled by a mammalian form of LHRH as reported in other common laboratory anuran species. D-Ala6, des-Gly10 ethylamide LHRH induces ovulation and deposition of mature and fertilizable eggs in E. coqui.
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.
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.
The use of Wnt ligands for signaling between cells is a conserved feature of metazoan development. Activation of Wnt signal transduction pathways upon ligand binding can regulate diverse processes including cell proliferation, migration, polarity, differentiation and axon outgrowth. A 'canonical' Wnt signaling pathway has been elucidated in vertebrate and invertebrate model systems. In the canonical pathway, Wnt binding leads to the stabilization of the transcription factor beta-catenin, which enters the nucleus to regulate Wnt pathway target genes. However, Wnt binding also acts through beta-catenin-independent, noncanonical pathways, such as the planar cell polarity (PCP) pathway and a pathway involving Ca2+ signaling. This chapter examines our current understanding of Wnt signaling and Wnt-mediated processes in the nematode C. elegans. Like other species, the C. elegans genome encodes multiple genes for Wnt ligands (five) and Wnt receptors (four frizzleds, one Ryk/Derailed). Unlike vertebrates or Drosophila, the C. elegans genome encodes three beta-catenin genes, which appear to have distinct functions in Wnt signaling and cell adhesion. Canonical Wnt signaling clearly exists in C. elegans, utilizing the beta-catenin BAR-1. However, a noncanonical pathway utilizing the beta-catenin WRM-1 also exists, and to date a similar pathway has not been described in other species. Evidence for beta-catenin independent noncanonical Wnt signaling is currently limited. The role of Wnt signaling in over a dozen C. elegans developmental processes, including the regulation of cell fate, polarity and migration, is described.
This chapter discusses the hormonal control of larval development and evolution in amphibians. In amphibians, almost all aspects of larval development, including the culminating transformation into adult form (commonly known as metamorphosis), are under the direct and primary control of the thyroid hormone (TH). This combination of a recurring tendency to exploit ontogenetic variation for evolutionary divergence and a singular reliance upon hormones for developmental regulation offers a unique opportunity to investigate the developmental basis of morphological evolution. Much of the amphibian postembryonic development is controlled by the interaction of two parameters: hormone activity, defined here as the profile of hormone plasma concentration over larval and metamorphic stages; and tissue sensitivity, which is a measure of a tissue's responsiveness to a hormone and is the minimum concentration required for its activation. The primary hormones involved (that is, those that directly mediate tissue responses) are the following: the two forms of TH, thyroxine or tetraiodothyronine (T4) and triiodothyronine (T3); the major corticosteroid in larval amphibians, corticosterone (CORT); and prolactin (PRL). Additional hormones appear to modulate the activity of these hormones, as well as regulate larval growth and metabolic activities related to metamorphosis.
SYNOPSIS. A diversity of issues related to metamorphosis are discussed in this paper, ranging from biomechanical constraints on behavior in premetamorphic anurans, to environmental factors that regulate metamorphosis. I begin by reviewing key behavioral features of tadpoles that have implications to their form and locomotion.
Tadpoles lack all visceral behaviors, such as emesis, coughing, and egg-laying, that require major elevations of pressure in the pleuroperitoneal cavity. I suggest that the inability to forcefully elevate pleuroperitoneal pressure is a consequence of the tadpoles' short and inflexible spine. Although postmetamorphic frogs have a similarly short vertebral column, they can compress viscera by movement at sacroiliac joints, which are absent in tadpoles. I argue that the short torso of tadpoles is a necessary consequence of the need to innervate the frog's limbs early in development so that they can be fully functional by the end of metamorphosis.
The short torso is a good mechanical design for the saltatory frog, but afflicts the tadpoles with a distinctly globous shape that intuitively appears inefficient for aquatic locomotion. Computational fluid dynamics (CFD) models, however, reveal that tadpoles are, in fact, efficient undulatory swimmers compared to subcarangiform fishes and that their swimming kinematics work in concert with their shape to help generate thrust. Furthermore the CFD models show that the crease in the dorsal profile of tadpoles, where the body abruptly meets the tail, produces a “dead water” zone immediately behind the body of the tadpole. As a result, tadpoles can grow hindlimbs in this region without those limbs seriously impeding flow. Thus the shape of tadpoles nicely matches their need to transform into limbed tetrapods.
In the latter half of this paper I review ideas on how environmental factors could influence metamorphosis in anurans. One speculation is that tadpoles may secrete a metamorphic inhibitor in their oral mucus, which they then ingest with their food, with the result that metamorphosis is delayed when food is abundant. An epidermal growth factor-like peptide has been suggested as a possible metamorphic inhibitor along the pathway just proposed. Strengths and weaknesses of this hypothesized agent and pathway are reviewed. An experiment to test the hypothesis that orally ingested EGF inhibits metamorphosis yielded inconclusive results. Orally secreted EGF-like peptide may not inhibit metamorphosis of the tadpole gut, but other yet-to-be-identified factors in tadpole oral mucus could still play a role in regulating metamorphosis of the alimentary tract in anurans.
The essay ends with some broad speculations on the role that peptides in frogs' skin may play in the metamorphic process as well as several unanswered questions about how other organisms and the physical environment influence tadpole metamorphosis.
Thyroid hormones and their receptors (TRs) have critical functions in development. Here we show that a chicken TR beta cDNA clone encodes a receptor with a novel, short N-terminal domain. In vitro-expressed TR beta protein bound thyroid hormone with similar affinity as the chicken TR alpha. Comparison of expression of TR alpha and TR beta mRNAs throughout chicken development until 3 weeks post-hatching revealed ubiquitous expression of TR alpha mRNAs (in 14 different tissues) with some variations in levels, from early embryonic stages. In contast, expression of TR beta mRNA was restricted, occurring notably in brain, eye, lung, yolk sac and kidney, and was subject to striking developmental control, especially in brain where levels increased 30-fold upon hatching. Levels also sharply increased in late embryonic lung, but were relatively high earlier in embryonic eye and yolk sac. RNase protection analyses detected no obvious mRNAs for alpha and beta TRs with variant C-termini as demonstrated previously for the rat TR alpha gene. The data suggest a general role for TR alpha and specific developmental functions for TR beta, and that thyroid-dependent development involves temporal and tissue-specific expression of the TR beta gene.
Exogenous thyroid hormone (TH) induces premature differentiation of the zebrafish pectoral fins, which are analogous to the forelimbs of tetrapods. It accelerates the growth of the pelvic fins but not precociously. Goitrogens, which are chemical inhibitors of TH synthesis by the thyroid gland, inhibit the transition from larva to juvenile fish including the formation of scales, and pigment pattern; they stunt the growth of both pectoral and pelvic paired fins. Inhibition by goitrogens is rescued by the simultaneous addition of thyroxine. The effect of adding TH to the rearing water of the postembryonic Mexican axolotl was reinvestigated under conditions that permit continued growth and development. In addition to morphological changes that have been described, TH greatly stimulates axolotl limb growth causing the resulting larva to be proportioned as an adult in about two months. This study extends the known evolutionary relatedness of tetrapod limbs and fish fins to include the TH stimulation of salamander limb and zebrafish fin growth, and suggests that TH is required to complete the life cycle of a typical bony fish and a salamander at the same developmental stage that it controls anuran and flounder metamorphosis.
The biological activities of thyroid hormones are thought to be mediated by receptors generated by the TRalpha and TRbeta loci. The existence of several receptor isoforms suggests that different functions are mediated by specific isoforms and raises the possibility of functional redundancies. We have inactivated both TRalpha and TRbeta genes by homologous recombination in the mouse and compared the phenotypes of wild-type, and single and double mutant mice. We show by this method that the TRbeta receptors are the most potent regulators of the production of thyroid stimulating hormone (TSH). However, in the absence of TRbeta, the products of the TRalpha gene can fulfill this function as, in the absence of any receptors, TSH and thyroid hormone concentrations reach very high levels. We also show that TRbeta, in contrast to TRalpha, is dispensable for the normal development of bone and intestine. In bone, the disruption of both TRalpha and TRbeta genes does not modify the maturation delay observed in TRalpha -/- mice. In the ileum, the absence of any receptor results in a much more severe impairment than that observed in TRalpha -/- animals. We conclude that each of the two families of proteins mediate specific functions of triiodothyronin (T3), and that redundancy is only partial and concerns a limited number of functions.
One of the genes that is up-regulated by thyroid hormone (TH) during Xenopus laevis metamorphosis encodes a type III deiodinase (D3) that inactivates TH. Transgenic X. laevis tadpoles overexpressing a GFP-D3 fusion protein were produced. These transgenic tadpoles had high levels of deiodinase activity and were resistant to exogenous TH added 1 week after fertilization. They developed normally throughout embryogenesis and premetamorphic stages but became retarded in their development late in prometamorphosis when endogenous TH reaches its highest level. Gill and tail resorption were delayed and most of the animals arrested and died. One tadpole completed its metamorphosis without resorbing its tail. These results demonstrate that D3 can modulate the action of TH in vivo, and document the value of the new transgenic method for functional analysis of genes involved in metamorphosis.
Thyroid hormone (T3) has widespread functions in development and homeostasis, although the receptor pathways by which this diversity arises are unclear. Deletion of the T3 receptors TRalpha1 or TRbeta individually reveals only a small proportion of the phenotypes that arise in hypothyroidism, implying that additional pathways must exist. Here, we demonstrate that mice lacking both TRalpha1 and TRbeta (TRalpha1(-/-)beta-/-) display a novel array of phenotypes not found in single receptor-deficient mice, including an extremely hyperactive pituitary-thyroid axis, poor female fertility and retarded growth and bone maturation. These results establish that major T3 actions are mediated by common pathways in which TRalpha1 and TRbeta cooperate with or substitute for each other. Thus, varying the balance of use of TRalpha1 and TRbeta individually or in combination facilitates control of an extended spectrum of T3 actions. There was no evidence for any previously unidentified T3 receptors in TRalpha1(-/-)beta-/- mouse tissues. Compared to the debilitating symptoms of severe hypothyroidism, the milder overall phenotype of TRalpha1(-/-)beta-/- mice, lacking all known T3 receptors, indicates divergent consequences for hormone versus receptor deficiency. These distinctions suggest that T3-independent actions of T3 receptors, demonstrated previously in vitro, may be a significant function in vivo.
The direct developing anuran, Eleutherodactylus coqui, lacks a tadpole, hatching as a tiny frog. We investigated the role of the metamorphic trigger, thyroid hormone (TH), in this unusual ontogeny. Expression patterns of the thyroid hormone receptors, TRalpha and TRbeta, were similar to those of indirect developers. TRbeta mRNA levels increased dramatically around the time of thyroid maturation, when remodeling events reminiscent of metamorphosis occur. Treatment with the goitrogen methimazole inhibited this remodeling, which was reinitiated on cotreatment with TH. Despite their radically altered ontogeny, direct developers still undergo a TH-dependent metamorphosis, which occurs before hatching. We propose a new model for the evolution of anuran direct development.
Patients with mutations in the thyroid hormone receptor beta (TRbeta) gene manifest resistance to thyroid hormone (RTH), resulting in a constellation of variable phenotypic abnormalities. To understand the molecular basis underlying the action of mutant TRbeta in vivo, we generated mice with a targeted mutation in the TRbeta gene (TRbetaPV; PV, mutant thyroid hormone receptor kindred PV) by using homologous recombination and the Cre/loxP system. Mice expressing a single PV allele showed the typical abnormalities of thyroid function found in heterozygous humans with RTH. Homozygous PV mice exhibit severe dysfunction of the pituitary-thyroid axis, impaired weight gains, and abnormal bone development. This phenotype is distinct from that seen in mice with a null mutation in the TRbeta gene. Importantly, we identified abnormal expression patterns of several genes in tissues of TRbetaPV mice, demonstrating the interference of the mutant TR with the gene regulatory functions of the wild-type TR in vivo. These results show that the actions of mutant and wild-type TRbeta in vivo are distinct. This model allows further study of the molecular action of mutant TR in vivo, which could lead to better treatment for RTH patients.
Direct development in amphibians is an evolutionarily derived life‐history mode that involves the loss of the free‐living, aquatic larval stage. We examined embryos of the direct‐developing anuran Eleutherodactylus coqui (Leptodactylidae) to evaluate how the biphasic pattern of cranial ontogeny of metamorphosing species has been modified in the evolution of direct development in this lineage. We employed whole‐mount immunohistochemistry using a monoclonal antibody against the extracellular matrix component Type II collagen, which allows visualization of the morphology of cartilages earlier and more effectively than traditional histological procedures; these latter procedures were also used where appropriate. This represents the first time that initial chondrogenic stages of cranial development of any vertebrate have been depicted in whole‐mounts.
Many cranial cartilages typical of larval anurans, e.g., suprarostrals, cornua trabeculae, never form in Eleutherodactylus coqui . Consequently, many regions of the skull assume an adult, or postmetamorphic, morphology from the inception of their development. Other components, e.g., the lower jaw, jaw suspensorium, and the hyobranchial skeleton, initially assume a mid‐metamorphic configuration, which is subsequently remodeled before hatching. Thirteen of the adult complement of 17 bones form in the embryo, beginning with two bones of the jaw and jaw suspensorium, the angulosplenial and squamosal. Precocious ossification of these and other jaw elements is an evolutionarily derived feature not found in metamorphosing anurans, but shared with some direct‐developing caecilians. Thus, in Eleutherodactylus cranial development involves both recapitulation and repatterning of the ancestral metamorphic ontogeny. These modifications, however, are not associated with any fundamental change in adult morphology and cannot at this time be causally linked to the evolutionary success of this extraordinarily speciose genus.
Thyroid-hormone-dependent development of the neuroretina has principally been described in amphibia. Here, we show by in situ hybridisation that mRNAs coding for three distinct thyroid hormone receptors (TRs), TR alpha and two TR beta variants, are differentially expressed during chick retinal development. We isolated a cDNA for a novel N-terminal variant of chick TR beta (cTR beta 2) that is predominantly expressed in retinal development. Interestingly, in its N-terminal A/B domain cTR beta 2 is 70% homologous to the rat pituitary-specific TR beta 2. Expression of cTR beta 2 mRNA was high at embryonic day 6 (E6) in the retinal outer nuclear layer (ONL) and decreased to low levels at hatching. mRNA for the previously described chick beta receptor, cTR beta 0, was expressed at low levels in both the ONL and the inner nuclear layer (INL) after E10. In contrast, cTR alpha expression occurred in the ONL, INL and ganglion cell layer at intermediate and later stages. Finally, cTR beta 2 confers a stronger trans-activation of reporter gene transcription than cTR beta 0. The distinctive kinetics and localisation of TR alpha and beta gene expression suggest cell- and stage-specific functions for TRs, both individually and in combinations, in chick neuroretinal development.
Metamorphosis of cranial cartilages in anuran amphibians constitutes one of the most dramatic and extensive ontogenetic transformations in vertebrates. We quantitatively examined the role of thyroid hormone (3,3',5-triiodo-L-thyronine; T3) in mediating gross aspects of this morphological repatterning in the skull of the Oriental fire-bellied toad, Bombina orientalis. T3 was administered via plastic (Elvax) micropellets in three treatment dosages (2.5, 0.25, and 0.025 microgram) and one control dosage (0 microgram) to tadpoles of three Gosner developmental stages--28/29, 30/31, and 32/33; tadpoles were recoved up to 8 d (treatment and control dosages) or 14 d (control dosage) later. Response of larval cartilages to exogenous T3 was dosage dependent but not implant-stage dependent; chondrogenic tissues that participate in metamorphic transformation are competent to respond to T3 well before they normally do. Metamorphic effects of T3 were visible within 2 d; in most treatment groups, the normal metamorphic sequence was two-thirds complete after 8 d. While T3 also induced precocious ossification, the normal temporal relation between bone formation and cartilage transformation was dissociated in experimental groups. Morphological integration between cartilage and bone during cranial metamorphosis is at least partly the result of each tissue responding independently to endocrine factors.
The role of thyroid hormone in flounder metamorphosis was studied by use of thyroxine and the antithyroidal agent, thiourea (TU). T4 stimulated metamorphosis of pelagic larvae, producing miniatures of naturally metamorphosed benthic juveniles. In contrast, TU induced metamorphic stasis, resulting in giant pelagic larvae. These results indicate that the thyroid plays a role in flounder metamorphosis comparable to its well-known developmental role in amphibian metamorphosis.
Direct developing frogs, like Eleutherodactylus coqui, have deleted the tadpole from their life history. Limb buds appear early and develop continuously through embryonic life. The capacity for autonomous development of E. coqui limb buds was tested by explanation and transplantation. When limb buds were explanted to in vitro culture, they progressed for a few stages and then arrested. When limb buds were transplanted to embryos of a typical tadpole species (Rana pipiens), the buds formed legs with knees, digits, and some cartilage, but they did not elongate. Since limb buds transplanted to various sites on E. coqui embryos tended to develop completely, the limited development of the explants in vitro and the transplants to R. pipiens suggests the presence of a systemic factor in the embryo involved in the normal continuous growth of E. coqui legs. Attempts to demonstrate a role for thyroid hormone have thus far been unsuccessful.
Thyroid hormone, acting through thyroid hormone receptors (TRs), plays an important role in amphibian metamorphosis and vertebrate development. To identify where and when TR beta 1 promoter is activated during fetal life, we carried out an in vivo functional study of a 1.3 kilobase (kb) TR beta 1 gene promoter using transgenic mice that express the beta-galactosidase gene under control of the TR beta 1 promoter. Transactivation of the gene was determined by blue staining of tissues after incubation with X-gal. High expression of transgene was detected in the limbs and face of the 12.5-day-old fetus (12.5 F) and 14.5 F, reminiscent of the changes occurring during amphibian metamorphosis, and this disappeared at 17.5 F. The expression was confined to the tip of finger bones, between fingers in the limb buds, and was detected in the root of whisker follicles, nose, and around the eyes. Signal was detected in the oral cavity, nasal cavity, lung, and urogenital sinus of 14.5 F, and disappeared at 17.5 F. Signal was detected in the midbrain and auditory vesicles of 9.5 F but was reduced between 12.5F and 17.5F, and there was no expression in the cerebral cortex layer of 0 days old neonates (PO). Expression was detected in the cortex after P5. There was signal in the cerebral cortex, cerebellum, kidney, and liver of adult mice. TR beta 1 messenger RNA was detected by RT-PCR in the developing limbs and face. Transgene expression in the interdigital tissues, which regress during development, suggests that TR beta 1 is expressed in mammals in areas undergoing apoptosis as well as in areas undergoing differentiation.
Eleutherodactylus coqui develops directly from a large 3.5-mm egg to a froglet, without an intervening tadpole stage. We have examined the development of the body wall, a structure whose behavior has been altered in this derived development. In an event that is unusual for amphibian embryos, the yolk mass is secondarily surrounded by the body wall, which originates near the embryo's trunk. The epidermis of the body wall is marked by melanophores, and the rectus abdominis, which will form the ventral musculature, is near its leading edge. As the body wall expands, the epidermis, melanophores, and rectus abdominis all move from the dorsal side to close over the yolk at the ventral midline. The original ectoderm over the yolk undergoes apoptosis, as it is replaced by body wall epidermis. Intact muscles are not required for ventral closure of the body wall, despite their normal presence near the advancing edge. Comparative examination of embryos of Xenopus laevis and Rana pipiens suggests that ventral closure does not occur in species with tadpoles. The expansion of dorsal tissues over the yolk, as illustrated by E. coqui, may have been important in the origin of amniote embryos.
The direct-developing frog, Eleutherodactylus coqui, has eliminated the tadpole stage from its ontogeny, and lacks many larval characters. We demonstrate that the dermal folds of E. coqui are homologous with the opercular folds of metamorphosing frogs. In both E. coqui and its metamorphic counterparts the opercular folds grow over the developing forelimb before perforating to free the entrapped limb. Opercular perforation in E. coqui occurs even in the absence of the forelimb but shows no signs of thyroid hormone dependence. The condensation of E. coqui development appears due to the excision of the extended larval period of developmental stasis. Analysis of opercular development, when viewed in conjunction with other developmental characters, suggests the ontogenetic period in the ancestral Eleutherodactylus life-history from which the tadpole was likely eliminated.
Although the gut is homologous among different vertebrates, morphological differences exist between different species. The most obvious variation in the guts of extant vertebrates appears in the stomach. To investigate the evolution of this structure, we compared the histology of the stomach and gastrointestinal tract in amphibian (Xenopus laevis), avian (Gallus gallus), and mammalian (Mus musculus) organisms, and defined the expression patterns of several genes within the developing guts of these lineages. In all three groups, we find that the anterior portion of the stomach has a similar glandular histology as well as a common embryonic expression of the secreted factors Wnt5a and BMP-4. Likewise, within the amniote lineages, the posterior nonglandular stomach and pyloric sphincter regions are also comparable in both histological and molecular phenotypes. The posterior stomach expresses Six2, BMPR1B, and Barx1, whereas the pyloric sphincter expresses Nkx2.5. Although the adult Xenopus stomach exhibits both glandular and aglandular regions and a distinct pyloric sphincter similar to that of the amniotic vertebrates, the histology of the Xenopus tadpole gut shows less distinct variation in differentiation in this region, which is most likely a derived condition. The molecular signature of the embryonic Xenopus gut correlates with the more derived morphology of the larval phase. We conclude that the global patterning of the gut is remarkably similar among the different vertebrate lineages. The distinct compartments of gene expression that we find in the gut be necessary for the unique morphological specializations that distinguish the stomachs from terrestrial vertebrates.