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In vertebrates with complex, biphasic, life cycles, larvae have a distinct morphology and ecological preferences compared to metamorphosed juveniles and adults. In amphibians, abrupt and rapid metamorphic changes transform aquatic larvae to terrestrial juveniles. The main aim of this study is to test whether, relative to larval stages, metamorphosis (1) resets the pattern of variation between ontogenetic stages and species, (2) constrains intraspecific morphological variability, and (3) similar to the "hour-glass" model reduces morphological disparity. We explore postembryonic ontogenetic trajectories of head shape (from hatching to completed metamorphosis) of two well-defined, morphologically distinct Triturus newts species and their F1 hybrids. Variation in head shape is quantified and compared on two levels: dynamic (across ontogenetic stages) and static (at a particular stage). Our results show that the ontogenetic trajectories diverge early during development and continue to diverge throughout larval stages and metamorphosis. The high within-group variance and the largest disparity level (between-group variance) characterize the metamorphosed stage. Hence, our results indicate that metamorphosis does not canalize head shape variation generated during larval development and that metamorphosed phenotype is not more constrained relative to larval ones. Therefore, metamorphosis cannot be regarded as a developmental constraint, at least not for salamander head shape.

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Ben Wielstra introduces Eurasian newts of the genus Triturus.

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Peripheral nerve injuries are frequently observed and successful treatment depends mainly on the injury type, location of the damage, and the elapsed time prior to treatment. The regenerative capacity is limited only to the embryonic period in many mammalian tissues, but urodele amphibians do not lose this feature during adulthood. The main purpose of this study is to define the recovery period after serious sciatic nerve damage of a urodele amphibian, Triturus ivanbureschi. Experimental transection damage was performed on the sciatic nerves of T. ivanbureschi specimens. The recovery period of sciatic nerves were investigated by walking track analysis, electrophysiological recordings, and bottom-up proteomic strategies at different time points during a 35-day period. A total of 34 proteins were identified related to the nerve regeneration process. This study showed that the expression levels of certain proteins differ between distal and proximal nerve endings during the regeneration period. In distal nerve stumps, transport proteins, growth factors, signal, and regulatory molecules are highly expressed, whereas in proximal nerve stumps, neurite elongation proteins, and cytoskeletal proteins are highly expressed.

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Multilocus molecular data play a pivotal role in diagnosing cryptic species (i.e. genetically distinct but morphologically similar species). A multilocus phylogeographic survey has provided compelling evidence that Triturus ivanbureschi sensu lato comprises two distinct gene pools with restricted gene flow. We conclude that this taxon had better be treated as two distinct (albeit morphologically cryptic) species. The name T. ivanbureschi should be restricted to the western species, which is distributed in western Asiatic Turkey plus the south-eastern Balkan Peninsula. No name is as yet available for the eastern species, which is distributed in northern Asiatic Turkey. We propose the name T. anatolicus sp. nov. for the eastern species and provide a formal species description.

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By using immunohistochemical techniques applied to confocal microscopy, the presence of aquaporin 3 water channel in the epidermis of Triturus italicus (Amphibia, Urodela) has been shown. We analysed the expression of aquaporin 3 (AQP3) during the larval, pre-metamorphic and adult phases; we also showed the localization of the water-channel protein AQP3 in free-swimming conditions and during aestivation in parallel with histological analysis of the skin, focusing on the possible relationship between protein expression and terrestrial habitats. Our results indicate that aquaporin is produced as the epidermis modifies during the functional maturation phase starting at the climax. Moreover, our data suggest an increase in enzyme expression in aestivating newts emphasizing the putative functional importance of differential expression related to a distinct phase of the biological cycle.

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Two stages can be distinguished in the differentiation of myotomal muscle fibres in Triturus vulgaris. In the first stage only synchronously differentiating myotomal cells are engaged; in the second stage mesenchymal cells also take part in the process. Myotomal cells (primary myoblasts) fuse to form 2-3 nucleate myotubes. Only in the caudal part of the embryo mononucleate myotubes persist. The mononucleate myotubes, like polynucleate ones, occupy the whole length of the myotome. The differentiation of myotubes is accompanied by vitellolysis. At further development stages mesenchymal cells enter the intermyotomal fissure, after which they migrate to the myotomes, between the myotubes. The cells that remain in the intermyotomal fissures retain their fibroblastic potential (they synthesise collagen). Their daughter cells adjoining the myotubes acquire myogenic abilities. Their myoblastic potential is evidenced by their ability to fuse with the myotube. Fusion of secondary myoblasts (of mesenchymal origin) with the myotube results in further growth of the myotubes. In T. vulgaris myotomal myotubes and muscle fibres developing from them are of myotomal-mesenchymal origin.

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Natural and artificial hybrids represent an important source of material for developmental and evolutionary studies of urodeles. We review the available literature on hybrid salamanders, emphasizing the unique developmental insights that these organisms provide. Of particular interest is the application of new molecular tools to identify DNA markers for traditional characters in developmental research, and we discuss our own results using Bulk Segregant Analysis to identify RAPD markers for the white phenotype in the axolotl. We pay particular attention to the inferences that can be drawn from the many disparate crosses between ambystomatid salamanders that vary in their metamorphic response. These crossing experiments suggest that 1) metamorphosis is dominant to paedomorphosis, 2) that different ambystomatids use different genetic mechanisms to block metamorphosis and become sexually mature, larval paedomorphs, and 3) metamorphosis may be controlled by a few genetic loci. As increasingly sophisticated molecular approaches are applied to these and other hybrid crossing schemes, it should be possible to understand the mechanistic basis of a wide variety of developmental characters that differentiate urodele species.

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Some cytological and developmental aspects of the forebrain gray matter were studied in the control and the hypothyroid larvae of Triturus cristatus carnifex. The mitotic cell percentages in the periventricular germinal layer, the fusiform shape of the recently divided neuronal cells and the transitory gray matter cleavage, are characteristic of the immature status of the central nervous system (CNS) in this species. The hypothyroidism obtained during two months of thiourea treatment slows down the development in all the forebrain areas of the experimental larvae. Nevertheless, the nucleus olfactorius anterior and the corpus striatum, which have a longer mitotic activity with respect to the other neuronal structures, are more deeply affected by thyroid atrophy. Thyroid activity and forebrain maturation are discussed on the basis of the literature and our data with regard to their influence on behavior and evolution.

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After amputation of both forelimb of young, postmetamorphic Triturus alpestris through the middle of the zeugopod, the left and right regenerates grew asynchronously. As a result of oral administration of vitamin A palmitate (250 IU/gbw/day), from the 4th to the 13th dpa, the growth rate of the right regenerates was relatively higher than that of the left ones. Moreover, the percentage of skeletal abnormalities as well as proximodistal duplications (proximalizations), induced by vitamin A, was nearly twice as high in the right regenerates as compared to their left counterparts.

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During the morphogenesis of the adrenal gland of Triturus cristatus, a cranio-caudal differentiation is observed together with a migration of the two cell types composing the adrenal gland: the steroidogenic cells and the chromaffin cells. During the cranio-caudal differentiation the two cell type gradually occupy an increasingly posterior position on the mesonephros until they are distributed, in the adult, along the whole kidney. The migration brings the cells from dorsal or dorso-lateral position, with respect to the venous vessels, to the ventral surface of the kidney, an arrangement typical of the adult.

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The Golgi technique was used to study the morphology of spinal motoneurons at various stages in the early development of swimming behaviour in embryos and larvae of the palmate newt, Triturus helveticus ((Razoumowsky). The earliest motoneurons stained appeared to be associated with the Mauthner-cell system. The overall morphology of these "primary" motoneurons seems to be similar throughout the lower vertebrates and the distinctive characteristics found in earlier descriptions of those from caudate amphibia were probably due to misinterpretation. At about the time of hatching and development of low-frequency swimming behaviour, other motoneurons were found to innervate the axial musculature, cells with a central morphology different from those of the "primary" type. It was found likely that these "secondary" motoneurons innervate a separate muscle system concerned with tonic and "slow phasic" activity, while "fast phasic" acitivity in rapid swimming is supplied by "primary" cells.

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Gonad primordium of Ambystoma mexicanum when grafted at tail-bud stage on Triturus alpestris is indefinately tolerated. Testis or ovaries derived from those grafted embryonic gonads follow their differentiaion and reach sexual maturity. Axolotl juvenile ovaries transplanted in Triturus alpestris are also tolerated; the ovocytes achieve the entire vitellogenesis but are not able to be laid.

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