Skeletal development in Xenopus laevis (Anura: Pipidae).

Postembryonic skeletal development of the pipid frog Xenopus laevis is described from cleared-and-stained whole-mount specimens and sectioned material representing Nieuwkoop and Faber developmental Stages 46-65, plus postmetamorphic individuals up to 6 months old. An assessment of variation of skeletogenesis within a single population of larvae and comparison with earlier studies revealed that the timing, but not the sequence, of skeletal development in X. laevis is more variable than previously reported and poorly correlated with the development of external morphology. Examination of chondrocranial development indicates that the rostral cartilages of X. laevis are homologous with the suprarostral cartilages of non-pipoid anurans, and suggests that the peculiar chondrocranium of this taxon is derived from a more generalized pattern typical of non-pipoid frogs. Derived features of skeletal development not previously reported for X. laevis include 1) bipartite formation of the palatoquadrate; 2) precocious formation of the adult mandible; 3) origin of the angulosplenial from two centers of ossification; 4) complete erosion of the orbital cartilage during the later stages of metamorphosis; 5) development of the sphenethmoid as a membrane, rather than an endochondral bone; and 6) a pattern of timing of ossification that more closely coincides with that of the pelobatid frog Spea than that recorded for neobatrachian species.     

Tooth replacement in adult Xenopus laevis (Amphibia: Anura)

To determine the time scale of tooth replacement in adult Xenopus laevis (Daudin), three large females of similar size were kept in aquaria at 25 °C for ten weeks. They were anaesthetized twice weekly with MS 222 and impressions of their upper jaws were taken using thin sheets of dental gold-casting wax. Because the erupted tips of the teeth were small (100 μm), the impressions were enlarged by projection so that the presence or absence of a tooth at each locus in the jaw could be recorded. Each half of each animal's jaw was analysed separately and a statistical analysis of the records yielded results for the duration of the Replacement Cycle and Functional Life of the teeth. The range of the median Replacement Cycle time between specimens was 910–1,010h, that of the Functional Life 580–700 h and that of the Gap Period (the time over which loci were unoccupied by functional teeth) 230–420 h. A tentative time scale for the complete tooth development cycle (from tooth germ initiation to complete resorption) was calculated by extrapolation from the results and ranged from 59.07 to 71.29 days.     

Patterns of spatial and temporal cranial muscle development in the African clawed frog, Xenopus laevis (Anura: Pipidae)

The African Clawed Toad, Xenopus laevis, has been a major vertebrate model organism for developmental studies for half a century. Because most studies have focused on the early stages of development, this has had the effect that many aspects of organogenesis and later development remain relatively poorly known in this species. In particular, little is known about cranial muscle development even at the level of morphology and histological differentiation of muscle anlagen and muscle fibers. In this study, we document the morphogenesis and histological differentiation of cranial muscles in X. laevis. We provide a detailed account of the timing of development for each of the cranial muscles, and also describe a new muscle, the m. transversus anterior. The cranial musculature of X. laevis larvae generally develops in a rostrocaudal sequence. The first muscles to differentiate are the extrinsic eye muscles. Muscles of the mandibular and hyoid arches develop almost simultaneously, and are followed by the muscles of the branchial arches and the larynx, and by the mm. geniohyoideus and rectus cervicis. Despite the fact that differentiation starts at different stages in the different muscles, most are fully developed at Stage 14. These baseline data on the timing of muscle differentiation in the X. laevis can serve as a foundation for comparative studies of heterochronic changes in cranial muscle development in frogs and other lissamphibians.     

Mechanical properties of the skin of Xenopus laevis (Anura,Amphibia)

The skin of the aquatic pipid frog, Xenopus laevis, was examined for specific biomechanical features: (1) thickness, (2) maximal strain at break (ε_f), (3) tensile strength (σ_m), (4) modulus of elasticity (E, stiffness), and (5) the area under the stress-strain curve (W) (breaking energy, toughness). Skin freshly removed from dorsal, ventral, and lateral areas of the body was subjected to uniaxial tension. In both sexes, the dorsal skin is thicker than the ventral. The skin of male frogs was consistently thinner in all body regions than that of females. Most biomechanical parameters showed a considerable range of values in both males (ε_f = 59–63%, σ_m = 15–16.5 MPa, E = 33.5–38.4 MPa, W = 3.8–4.5 MJ/m³) and females (ε_f = 102–126%, σ_m = 11.5 MPa, E = 10.4–12 MPa, W = 5.2–6.7 MJ/m³). The disparate ε_f values in males (low) and females (high) might reflect sexual dimorphism. Static stress-strain curves were typicxally J-shaped; with the exception of “toe,” the curves rose approximately linearly with increasing strain. The skin of X. laevis, although heterogeneous in structure, possesses features similar to those found in tissues with aligned collagen fibers such as tendons or fish skin. However, in anurans, the skin seems to play a more passive mechanical role during locomotion than in fish. © 1995 Wiley-Liss, Inc.     

Xenopus laevis larvae (Amphibia,Anura) as model suspension feeders

Larvae of the South African clawed frog, Xenopus laevis (Daudin), are efficient, obligate suspension feeders. We examine the relationship between the ambient particle concentration offered these larvae as food and their filtering, ingestion, and buccal pumping rates. We demonstrate that: (i) the larvae can sense and respond to a broad range of particle concentrations, down to 0.2 mg 1^–1 (dry weight); (ii) their metabolic needs theoretically can be met by particle concentrations as low as 5 mg 1^–1; and (iii) their patterns of regulation of filtering and ingestion fit predictions from certain models used to describe zooplankton feeding dynamics. Two such models are discussed: the modified Monod (Michaelis-Menten) model, with a lower threshold below which the tadpoles do not feed, and an energy optimization model. Both the models and the observed behavior of the tadpoles allow for stability of populations of food organisms. Tadpole feeding dynamics apparently are compatible with both the predictions and assumptions of these models, suggesting similar regulation of feeding by tadpoles and zooplankton. However, the size, morphology, and behavior of X. laevis larvae make their feeding regulation uniquely accessible to direct observation.Contribution No. 223, Center for Great Lakes Studies, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin.Contribution No. 223, Center for Great Lakes Studies, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin.     

Evolution of globin expression in the genus Xenopus (Anura: Pipidae)

Comparison of electrophoretic globin phenotypes of 18 different Xenopus taxa reveals four different basic types of banding patterns. Each type includes species that also are similar in their morphological, cytogenetical, and biochemical characteristics. Three of these patterns reflect distinct evolutionary lines, while the fourth may be interpreted as the intersection of two of these lines. The composition of the basic pattern of the highly polyploid species is consistent with an allopolyploid origin of most of these species. The number of distinct globin polypeptides--four in the only extant diploid species, X. tropicalis, and five or more in most of the tetraploid species, including X. laevis--suggests that primordial globin genes had undergone duplication either before or after the tetraploidization event. Finally, the individual globin phenotypes are excellent molecular markers that are of great help in identifying the various species but not the subspecies.      

The morphology and distribution of Xenopus vestitus (Anura: Pipidae) in Central Africa

Studies on the collections of the Musée Royal de l'Afrique Centrale and the Institut Royal des Sciences Naturelles de Belgique have shown that Xenopus kigesiensis Tinsley, 1973 is synonymous with X. vestitus Laurent, 1972. Previously unrecognized representatives of the species have been found in the museum collections and these considerably enlarge the known geographical distribution of X. vestitus. Samples have been recorded principally from highland areas adjacent to the western Rift in Zaire, Uganda and Ruanda; comprehensive morphological analysis demonstrates relatively limited intraspecific variation throughout the known range.
The distribution of X. vestitus overlaps with that of three other Xenopus taxa: X. laevis victorianus, X. bunyoniensis and a new Xenopus species. There is evidence from a large number of collections from widely separated localities that X. vestitus is commonly sympatric with X. l. victorianus. On the other hand, it is postulated that X. vestitus has recently invaded Lake Mutanda, Uganda, and has replaced the original X. bunyoniensis population during the past 30 years. The museum record provides a potential origin of this invasion: X. vestitus populations are well documented at Rutshuru, only 25 km downstream from Lake Mutanda.
A comparison of the morphological characters of X. vestitus, X. l. victorianus and X. bunyoniensis is presented to provide information on the systematic relationships and possible ecological adaptations of the overlapping species.     

Anomalous rates of evolution of pancreatic polypeptide and peptide tyrosine-tyrosine (PYY) in a tetraploid frog, Xenopus laevis (Anura:Pipidae)

The South African clawed frog Xenopus laevis is believed to have arisen as a result of a tetraploidization event occurring approximately 30 million years ago. Two molecular forms of pancreatic polypeptide (PP) have been isolated from an extract of the pancreas of this species and two molecular forms of peptide tyrosine-tyrosine (PYY) from the intestine. Despite the fact that the amino acid sequence of PP has, in general, been very poorly conserved during the evolution of tetrapods (only Pro(5), Pro(8), Gly(9), Ala(12), Tyr(27), Arg(33) and Arg(35) are invariant among species studied so far), the two Xenopus PPs differ by only a single amino acid substition (Asp(22)-->Glu). In contrast the two molecular forms of PYY differ by six amino acid substitutions (Glu(15)-->Gln, Thr(18)-->Ala, Leu(21)-->Met, Ile(22)-->Thr, Ile(28)-->Val, Val(31)-->Ile). The data imply that strong evolutionary pressure is acting to conserve the functional domain in both genes encoding PP and so suggest that PP may have an important physiological role in amphibians (although the nature of this role has yet to be determined). The more rapid mutation of the functional domain in the genes encoding PYY, a peptide whose amino acid sequence has been quite well conserved in tetrapods and whose physiological significance is well established, suggests that one of the PYY genes may be evolving towards a new function or towards becoming a pseudogene.     

The epidermis and its degeneration in the larval tail and adult body of Rana temporaria and Xenopus laevis (Amphibia: Anura)

The tail epidermis of the larva and the body epidermis of adults of Rana temporaria and Xenopus laevis are described in terms of electron microscopy.
The activity of lysosomes (determined by the localization of acid phosphatase) in relation to autolysis and the process of cellular cornification, is considered during the periods of climactic disappearance of the larval tail and skin sloughing of adults. The results obtained generally correspond for both genera.
Larval tail epidermal cells completely disappear at metamorphic climax; those of the adult, which are shed, are replaced throughout life after each periodic sloughing. Nevertheless the mechanisms of their epidermal cell loss are comparable, though the level of lysosomal activity in larval tail epidermal cells is higher than in the adult body epidermis. This higher activity of lysosomal enzymes may facilitate the heavy necrosis which ensues in the larval tail at metamorphic climax.     

Thyroid hormones in the skeletogenesis and accessory sources of endogenous hormones in Xenopus laevis (Amphibia; Anura) ontogeny: Experimental evidence

Skeletal development was studied in normal and goitrogen-treated Xenopus laevis tadpoles reared under thyroid hormone (TH) deficiency. Early stages of skeletal development proceed similarly in both groups. Later stages are retarded or completely arrested in goitrogen-treated tadpoles. After goitrogen-treated tadpoles were transferred into pure water or into a medium containing both goitrogen and exogenous TH, tadpoles resumed development. Consequently, late stages of skeletogenesis are TH-dependent and TH-induced. Athyroid X. laevis “giant tadpoles” described in literature differ from goitrogen-arrested tadpoles in that they have features which require TH to appear. The appearance of TH-depended features in giant tadpoles indicates the occurrence of the additional sources of TH other than thyroid gland.     

Histological microstructure of the claws of the African clawed frog, Xenopus laevis (Anura: Pipidae): implications for the evolution of claws in tetrapods

Claws are consistent components of amniote anatomy and may thus be implicated in the success of the amniote invasion of land. However, the evolutionary origin of these structures in tetrapods is unclear. Claws are present in certain extant non-amniotes, such as Xenopus laevis, the African clawed frog. The histology of the soft tissue component of the claws of X. laevis is described and compared with the amniote condition in order to gain new information on the question of homology of claws in these two groups based on patterns of keratinization.The X. laevis claw sheath is composed of a localized thickening of the corneous region of the epidermis that envelops the terminal phalanx. Noted differences between the non-cornified layers of the epidermis of the claw and non-claw region are the overall grainier appearance of the cells and an increased abundance of desmosomes in the intermediate spinosus cells. The biochemical identity of the sheath keratin(s) is inferred to be different from that of non-claw region epidermis, based on histological differences and differences in stain affinity between the two regions. The microstructure of the frog claw differs from that of amniotes in several respects, including the lack of a specified zone of growth near the base of the claw. Amphibians and amniotes, therefore, have very different patterns of claw sheath growth. Observations do not support homology of claws on a structural level in these two groups; however, further experimental work may confirm a conserved pattern of cornification in these structures in tetrapods.     

The functional significance of the pterygomaxillary ligament in Xenopus Iaevis (Amphibia: Anura)

A pterygomaxillary ligament, which runs postero-inferiorly from the posterior end of the maxilla to the pterygoid, is described macroscopically and microscopically in the aquatic anuran amphibian Xenopus laevis (Daudin). The quadratojugal is absent in this species, and consequently the short maxilla has no posterior articulation and is potentially mobile. However, the pterygomaxillary ligament limits upward and lateral displacement of the maxilla when pressure is applied upwards during closure of the lower jaw. The ligament absorbs forces applied to the posterior end of the maxilla and permits their dissipation via the pterygoid to the neurocranium.     

The natural stimulus for spawning in Xenopus laevis (Amphibia)

I have already reported (Savage, 1965), preliminary experiments in which "natural spawning", that is, spawning by frogs kept under as natural conditions as aquaria allow, and by frogs not injected or manipulated in any other way, had been induced by a variety of external stimuli, including unialgal cultures of various kinds.
These experiments were planned and executed using as a guide, at every possible point, the statistical investigations of Rana temporaria obtained entirely from field data (Savage, 1935, 1961). The reasons for using Xenopus laevis and not R. temporaria were purely practical. R. temporaria breeds but once a year, and has other disadvantages, but X. laevis is a good laboratory animal.
Since conclusions drawn from mathematical relations in one species have led without important deviation to the expected experimental results in another, it is unlikely that what I now have to report is peculiar to these two species, selected because of availability in one case and ease of working in another. The conclusions are probably quite general. The earlier conclusions have been extended, active materials have been isolated, and other factors relevant to reproduction have been found.     

Development and differentiation of the brain ventricular system in tadpoles of Xenopus laeris (Daudin) (Amphibia, Anura)

The development and the differentiation of the ventricular system of the brain of tadpoles of the South African Clawed Toad, Xenopus laevis (Daudin), is studied by light microscopy (stages 45 to 66) and scanning and transmission electron microscopy (stages 50 to 66). Special interest is paid to the ependymal structures of the foramen of Monroe, the ventricles of the diencephalon, the mesencephalon, and the rhombencephalon, and to the ependymal of the central canal and the choroid plexus of the third and fourth ventricle. At early developmental stages the lower two thirds of the ventricles are dominated by blebs, cytoplasmatic protrusions of the ependymal cells. During the development they become reduced and replaced by cilia. The number of cilia and microvilli increases strongly towards the end of the metamorphosis. The surface structures demonstrated by scanning electron microscopy are discussed in respect to morphology and physiology.     

The Bearing of Filter Feeding on the Water Pumping Mechanism of Xenopus Tadpoles (Anura: Pipidae)

Abstract New findings on the functional anatomy of filter feeding have permitted some clarification of water flow through tadopoles. The dorsal velar apparatus projects downward from the roof of the buccopharynx and consists of three mucosal pads and three vanes on both sides of the midline. At least four functions may be ascribed to the dorsal velar apparatus. 1. Each pad and its complementary vane fit so flush into each of the three bilateral filter cavities that exhalent water must flow in thin sheets over the filter plates before its exit via the pharyngeal clefts. 2. Like hydrofoils, the vanes deflect water and prevent strong currents from disorganizing the mucous food cords. 3. When tadpoles cease water pumping, the plugging effect of the dorsal velar apparatus in the filter cavities helps to occlude the pharyngeal clefts. 4. Secretions of dorsal velar glands, especially on the surface of the velar pads, may assist the mucus of the ventral velum to entrap suspended food. Other findings concern the M3b muscle, which helps to elevate the buccopharyngeal floor during expiration; the lack of cilia, but presence of low glandular elevations on the ventral velum; the reduction of the exits from the pharyngeal clefts 2 and 3 into a single valvular exit controlled by the B2b and B2c muscles; and the probable absence of a mechanical feeding function for the oral tentacles.     

Speciation of Protopolystoma Bychowsky, 1957 (Monogenea: Polystomatidae) in hosts of the genus shape Xenopus (Anura: Pipidae)

The taxonomy, geographical distribution and hostrange of the polystomatid genus ProtopolystomaBychowsky, 1957 are reviewed. P. xenopodis(Price, 1943) and five new species are recognised,which occur in clawed toads ( Xenopus spp.)throughout subsaharan Africa. Of the two clawed toadsubgenera, Xenopus and Silurana, only theformer is infected. Protopolystoma spp. aredifferentiated by morphological variation of the gut,large hamulus and penis armature. P.xenopodis is found in Xenopus laevis subspeciesin South Africa, Transkei, Zimbabwe, DemocraticRepublic of Congo (D.R.C.), Rwanda, Uganda, Kenya andCameroon ( X. l. poweri and X. l.sudanensis are new host records). It also occurs inintroduced populations of X. l. laevis in theUnited States (southern California) and United Kingdom(South Wales). In subsaharan Africa the speciesdisplays significant, but continuous, geographicalvariation of penis spine size between southernpopulations in X. l. laevis and those in morenortherly host subspecies. Data on the natural hostrange of this parasite were complemented by anexperimental study of host-specificity in the southernform. This can produce patent infections in X.l. victorianus and X. gilli, but not X.wittei nor X. (Silurana) tropicalis. P.simplicis n. sp. is endemic to central and eastAfrican areas, infecting X. laevis subspecies ineastern D.R.C., Rwanda, Uganda and western Kenya, X. wittei-like hosts in eastern D.R.C., westernUganda, Rwanda and Burundi, X. vestitus inwestern Uganda and Xenopus sp. at Nairobi,Kenya. P. ramulosus n. sp. occurs in X.fraseri-like toads in eastern D.R.C. (Gabon andCameroon are also possible literature records), and P. fissilis n. sp. is found in X. fraseri-and X. wittei-like species in Cameroon andeastern D.R.C., and in southern Rwanda, respectively. Two Protopolystoma taxa are found in X.muelleri populations now suspected to representdistinct species: P. occidentalis n. sp. occursin X. muelleri (western form) in Ghana, Togo,Nigeria and Cameroon, while P. orientalis n. sp.is found in X. muelleri (eastern form) in SouthAfrica, Zimbabwe and Tanzania. The allopatricallydistributed species P. ramulosus, P.simplicis, P. occidentalis and P.orientalis form a relatively homogenous grouping withsome interspecific morphological overlap. These taxaare distinguished from P. xenopodis by penisspine morphology and from P. fissilis by hamulusroot form and aspects of gut morphology. Unidentified Protopolystoma sp. have been recorded in X. clivii in Ethiopia, X. fraseri aff. inCameroon and Xenopus sp. in Kenya and Tanzania. At some localities, single host species were infectedby two representatives of Protopolystoma. P. fissilis was recorded in eastern D.R.C. with P. ramulosus, with Protopolystoma sp. inCameroon in X. fraseri-like hosts and with P. simplicis in X. wittei-like hosts in Rwanda. P. xenopodis co-occurred with P. simplicisin X. laevis subspecies through central and eastAfrica.     

The mating calls of 12 species and sub-species of the genus Xenopus (Amphibia : Anura)

The principal acoustic characteristics of the mating calls of 12 species and sub-species of the genus Xenopus were determined: X. laevis laevis, X. laevis petersi, X. laevis victorianus, X. gilli, X. muelleri, X. borealis, X. clivii, X. fraseri, X. ruwenzoriensis, X. wittei, X. vestitus and X. tropicalis . These calls are very specific, especially among species whose hybrids are particularly viable in the laboratory. Twin species have not been discovered. Most of the species emit calls with high harmonic frequencies of 16 kHz; these frequencies reach 80 and 150 kHz in X. I. laevis and X. ruwenzoriensis , the ultra-sound level. This was a previously unknown phenomenon in the Batrachians.     

Muscle development in the abdominal region of Larval Hylidae (Amphibia: Anura)

Larval muscle development in the abdominal region of five species of hylid frogs (Scinax nasicum, S. fuscovarium, Hyla andina, Phyllomedusa boliviana, Gastrotheca gracilis) was studied using differential staining techniques. These five species represent three major hylid subfamilies. The development of the main abdominal muscles, the rectus abdominis, the two lateral muscles (obliquus externus and transversus), and the lateral pectoralis abdominalis is described. The number of myotomes of the rectus abdominis varies between five and six, and the abdominal muscles associated with the rectus abdominis (obliquus externus, pectoralis abdominalis, and rectus cervicis) vary interspecifically in time of appearance and configuration. The presence of gaps in the configuration of the rectus abdominis has been related to the lotic habits of the larvae. However, our observations indicate the presence of such gaps in larvae that inhabit lentic environments as well. These results suggest that the presence of these gaps is unrelated to larval habitat. There are relatively small differences in muscle morphology among these closely related species, which apparently cannot be explained by morphological adaptations related to their ecology. In the species studied, the number of elements that form the abdominal musculature in larvae is equal to that observed in adults. Likewise, the general morphology of the muscles is ontogenetically conserved. This suggests that both the axial skeleton and musculature are more ontogenetically conserved in relation to the substantial changes that are observed in the skull and head muscles of developing anurans. J. Morphol. 241:275–282, 1999. © 1999 Wiley‐Liss, Inc.     

DNA synthesis during larval development and compensatory growth in the mesonephros of Xenopus laevis.

Autoradiography following tritiated thymidine administration to Xenopus laevis tadpoles of stages 45–48 of larval development has revealed that, as the cells of the mesonephric kidney differentiate during organogenesis, there is a marked decrease in the percentage of cells synthesizing DNA (from 100% at stage 45 to less than 9% at stage 48). In the adult this figure is of the order of 0.1%. This reduced DNA synthetic activity was found to take place in the cells of both the proximal and distal tubules of the nephrons. Special mucous cells which serve as markers of distal tubules were not observed to synthesize DNA after the onset of their differentiation at stage 48 of larval development.Through the partial extirpation of tissue in one kidney of adult Xenopus laevis males, DNA synthesis was reactivated in differentiated cells. The increased DNA synthetic activity following partial unilateral nephrectomy was found to be maximal after 6 days of recovery when 19% of cells synthesize DNA. This increased DNA synthetic activity was found to occur only in the cells of the distal tubules, both mucous and nonmucous cells, while the cells of the proximal tubules did not respond to this reactivation.The apparent inability of proximal tubule cells to synthesize DNA following partial unilateral nephrectomy is discussed.