Two new species of lungless salamanders (Plethodontidae) are described from montane habitats of eastern Costa Rica and adjacent western Panama. Bolitoglossa gomezi and B. bramei are distinguished from each other and from other salamander species in this remote area in adult body size, external proportions, foot webbing, tooth counts, and/or external coloration. Both new species are assigned to the B. subpalmata species group, subgenus Eladinea. A newly identified specimen of Bolitoglossa anthracina-only the fourth known specimen of this rare species in collections-is reported from the same region. Salamander species diversity along the border between Costa Rica and Panama is exceptionally large, at present comprising 22 named and two unnamed forms.
Widespread and persistent marker expression is a prerequisite for many transgenic applications, including chimeric transplantation studies. Although existing transgenic tools for the clawed frog, Xenopus laevis, offer a number of promoters that drive widespread expression during embryonic stages, obtaining transgene expression through metamorphosis and into differentiated adult tissues has been difficult to achieve with this species. Here we report the application of the murine ROSA26 promoter in Xenopus. GFP is expressed in every transgenic tissue and cell type examined at post-metamorphic stages. Furthermore, transgenic ROSA26:GFP frogs develop normally, with no apparent differences in growth or morphology relative to wild-type frogs. ROSA26 transgenes may be used as a reliable marker for embryonic fate-mapping of adult structures in Xenopus laevis. Utility of this transgenic line is illustrated by its use in a chimeric grafting study that demonstrates the derivation of the adult bony jaw from embryonic cranial neural crest.
As a step toward resolving the developmental origin of the ossified skull in adult anurans, we performed a series of cell labeling and grafting studies of the cranial neural crest (CNC) in the clawed frog, Xenopus laevis. We employ an indelible, fixative-stable fluorescent dextran as a cell marker to follow migration of the three embryonic streams of cranial neural crest and to directly assess their contributions to the bony skull vault, which forms weeks after hatching. The three streams maintain distinct boundaries in the developing embryo. Their cells proliferate widely through subsequent larval (tadpole) development, albeit in regionally distinct portions of the head. At metamorphosis, each stream contributes to the large frontoparietal bone, which is the primary constituent of the skull vault in adult anurans. The streams give rise to regionally distinct portions of the bone, thereby preserving their earlier relative position anteroposteriorly within the embryonic neural ridge. These data, when combined with comparable experimental observations from other model species, provide insights into the ancestral pattern of cranial development in tetrapod vertebrates as well as the origin of differences reported between birds and mammals. (c) 2005 Wiley-Liss, Inc.
Contemporary studies of vertebrate cranial development document the essential role played by the embryonic neural crest as both a source of adult tissues and a locus of cranial form and patterning. Yet corresponding and basic features of cranial evolution, such as the extent of conservation vs. variation among species in the contribution of the neural crest to specific structures, remain to be adequately resolved. Investigation of these features requires comparable data from species that are both phylogenetically appropriate and taxonomically diverse. One key group are amphibians, which are uniquely able to inform our understanding of the ancestral patterns of ontogeny in fishes and tetrapods as well as the evolution of presumably derived patterns reported for amniotes. Recent data support the hypothesis that a prominent contribution of the neural crest to cranial skeletal and muscular connective tissues is a fundamental property that evolved early in vertebrate history and is retained in living forms. The contribution of the neural crest to skull bones appears to be more evolutionarily labile than that of cartilages, although significance of the limited comparative data is difficult to establish at present. Results underline the importance of accurate and reliable homology assessments for evaluating the contrasting patterns of derivation reported for the three principal tetrapod models: mouse, chicken and frog.
A new species of Bolitoglossa is described from the Cordillera Central of central Panama. This is a large black species, which is likely related to the B. (Eladinea) schizodactyla group of Costa Rica and Panama. The new species, known from a single specimen, differs from other large black Bolitoglossa in having white pigmentation on the lower face and on the ventral portions of the head and chest. It also has more interdigital webbing than most members of the B. (E.) schizodactyla group.
Several populations of large (adult standard length, 43-134 mm) black salamanders of the widespread neotropical genus Bolitoglossa (Plethodontidae) are known from the cordilleras of western Panama and Costa Rica. These populations constitute at least seven species, including two recently described (B. anthracina, B. copia), one described long ago that remains poorly known (B. nigrescens), and three described herein as new. The long recognized, Aide-ranging B. robusta, which is distinguished by a pale, pigmented ring around the tail base and a unique combination of maxillary and vomerine tooth counts, may occur sympatrically with four of the other taxa. Differences in head and body form, adult size, cranial osteology, and maxillary and vomerine tooth counts separate all recognized taxa from one another. These results confirm and indeed increase the exceedingly high diversity of salamander species known from the Cordillera Talamanca-Baru of Costa Rica and Panama, diversity that now rivals that found anywhere else in the tropics.
We describe two new species of salamanders of the genus Pseudoeurycea from mountains in the northern part of the state of Oaxaca, Mexico. Pseudoeurycea papenfussi, a large, muscular member of the P gadovii group, occurs near the peaks (just below 3000 m) of the highest mountains of the Sierra de Juarez. It is related to P smithi, a more southerly species, and possibly to P aquatica, another species from Oaxaca. Pseudoeurycea obesa, a rotund member of the P leprosa group, is known only from the type locality in the Sierra Mazateca at the northernmost extremity of Oaxaca. It is related to P. werleri and P mystax, which are known from more southern parts of Oaxaca. These descriptions bring to 27 the number of species of salamanders known from Oaxaca. Most of these species are endemic to the state and are known only from regions that are undergoing rapid habitat modification and destruction.
Recent surveys of the plethodontid salamander fauna of Oaxaca, Mexico, disclosed the existence of a new, morphologically distinct arboreal species of the genus Pseudoeurycea. The new species, described here, is from the Sierra Mazateca in northern Oaxaca. Sequences of 1833 base pairs of the 16S, cytochrome b and ND4 mitochondrial DNA ( mtDNA) genes from the new taxon were used to assess its phylogenetic position. Previous phylogenetic analyses based on mtDNA supported recognition of four clades within Pseudoeurycea: P. bellii, P. gadovii, P. juarezi and P. leprosa-Lineatriton species groups. One additional species, P. unguidentis, was not closely allied to any of the four groups. Re-analysis including the additional sequences reported here establishes a sister-group relationship between the new species and P. unguidentis. Moreover, it supports this clade as part of the P. juarezi species group.
The neural crest is a population of multipotent stem cells unique to vertebrates. In the head, cranial neural crest (CNC) cells make an assortment of differentiated cell types and tissues, including neurons, melanocytes, cartilage, and bone. The earliest understanding of the developmental potentiality of CNC cells came from classic studies using amphibian embryos. Fate maps generated from these studies have been largely validated in recent years. However, a fate map for the most late-developing structures in amphibians, and especially anurans (frogs), has never been produced. One such tissue type, skull bone, has been among the most difficult tissues to study due to the long time required for its development during anuran metamorphosis, which in some species may not occur until several months, or even years, after hatching. We report a relatively simple technique for studying this elusive population of neural crest-derived osteogenic (bone-forming) cells in Xenopus laevis by using fluorescently labeled dextran conjugates. (C) 2004 Wiley-Liss, Inc.
Loss of limb skeletal elements is a recurring theme in tetrapod evolution, but the developmental mechanisms underlying this phenomenon remain largely unknown. The Australian lizard genus Hemiergis offers an excellent model system to study limb reduction among closely related, naturally occurring populations with different numbers of digits. Evolutionary digit loss in Hemiergis does not result from simple truncation of a pentadactyl skeletal developmental program. Rather, the duration of embryonic expression of the patterning molecule Sonic hedgehog (SHH) is shortened in limbs with reduced numbers of digits, and is correlated with decreased cell proliferation in the posterior aspect of the limb. Moreover, this comparative analysis suggests an early role for SHH in specification of digit identity and later importance in maintaining cell proliferation and survival. Subtle changes in spatial or temporal regulation of SHH may alter proliferation and patterning of the developing limb, thereby effecting divergence in adult limb morphology among closely related species. In contrast, expression of MSX and Distal-less proteins were similar among embryos from different populations. (C) 2003 Wiley Liss, Inc.
The timing and pattern of cranial neural crest cell emergence and migration in the Mexican axolotl, Ambystoma mexicanum, are assessed using scanning electron microscopy (SEM). Cranial neural crest cells emerge and begin to migrate at the time of neural fold closure and soon form three distinct streams. The most anterior (mandibular) stream emerges first, at the level of the mesencephalon. Cells in this stream migrate rostroventrally around the optic vesicle. The second (hyoid) and third (branchial) streams emerge in close succession at the level of the rhombencephalon and extend ventrolaterally. Cells forming the hyoid stream migrate rostral to the otic vesicle, whereas the branchial stream divides into two parallel streams, which migrate caudal to the otic vesicle. At later stages (stage 26 onwards) the cranial neural crest cells disperse into the adjacent mesoderm and can no longer be followed by dissection and SEM. The pattern of cranial neural crest emergence and migration, and division into migratory streams is similar to that in other amphibians and in the Australian lungfish (Neoceratodus forsteri). Emergence of crest cells from the neural tube, relative to the time of neural tube closure, occurs relatively late in comparison to anurans, but much earlier than in the Australian lungfish. These results establish a morphological foundation for studies in progress on the further development and fate of cranial neural crest cells in the Mexican axolotl, as well as for studies of the role of cranial neural crest in cranial patterning.
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.
The contribution of cranial neural crest cells to the development and patterning of cranial muscles in amphibians was investigated in the phylogenetically basal and morphologically generalized frog, Bombina orientalis. Experimental methods included fluorescent marking of premigratory cranial neural crest and extirpation of individual migratory streams. Neural crest cells contributed to the connective tissue component, but not the myofibers, of many larval muscles within the first two branchial arches (mandibular and hyoid), and complex changes in muscle patterning followed neural crest extirpation. Connective tissue components of individual muscles of either arch originate from the particular crest migratory stream that is associated with that arch, and this relationship is maintained regardless of the segmental identity-or embryonic derivation-of associated skeletal components. These developmental relations define a pattern of segmentation in the head of larval anurans that is similar to that previously described in the domestic chicken, the only vertebrate that has been thoroughly investigated in this respect. The fundamental role of the neural crest in patterning skeleton and musculature may represent a primitive feature of cranial development in vertebrates. Moreover, the corresponding developmental processes and cell fates appear to be conserved even when major evolutionary innovations-such as the novel cartilages and muscles of anuran larvae-result in major differences in cranial form. (C) 2001 Academic Press.
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 (D1x) 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. (C) 2001 Wiley-Liss, Inc.