P>Genetic and developmental alterations associated with the evolution of amphibian direct development remain largely unexplored. Specifically, little is known of the underlying expression of skeletal regulatory genes, which may reveal early modifications to cranial ontogeny in direct-developing species. We describe expression patterns of three key skeletal regulators (runx2, sox9, and bmp4) along with the cartilage-dominant collagen 2 alpha 1 gene (col2a1) during cranial development in the direct-developing anuran, Eleutherodactylus coqui. Expression patterns of these regulators reveal transient skeletogenic anlagen that correspond to larval cartilages, but which never fully form in E. coqui. Suprarostral anlagen in the frontonasal processes are detected through runx2, sox9, and bmp4 expression. Previous studies have described these cartilages as missing from Eleutherodactylus cranial ontogeny. These transcriptionally active suprarostral anlagen fuse to the more posterior cranial trabeculae before they are detectable with col2a1 staining or with the staining techniques used in earlier studies. Additionally, expression of sox9 fails to reveal an early anterior connection between the palatoquadrate and the neurocranium, which is detectable through sox9 staining in Xenopus laevis embryos (a metamorphosing species). Absence of this connection validates an instance of developmental repatterning, where the larval quadratocranial commissure cartilage is lost in E. coqui. Expression of runx2 reveals dermal-bone precursors several developmental stages before their detection with alizarin red. This early expression of runx2 correlates with the accelerated embryonic onset of bone formation characteristic of E. coqui and other direct-developing anurans, but which differs from the postembryonic bone formation of most metamorphosing species. Together these results provide an earlier depiction of cranial patterning in E. coqui by using earlier markers of skeletogenic cell differentiation. These data both validate and modify previously reported instances of larval recapitulation and developmental repatterning associated with the evolution of anuran direct development.
Wu, Y., Wang, Y., Jiang, K., Chen, X. & Hanken, J. (2010). Homoplastic evolution of external colouration in Asian stout newts (Pachytriton) inferred from molecular phylogeny.-Zoologica Scripta, 39, 9-22.The Asian stout newts of the genus Pachytriton (Salamandridae) inhabit montane streams in south-eastern China. Despite their abundance in the pet trade, the phylogeny and systematics of this genus are poorly understood. Colouration is often used to delimit species under the assumption that consistent chromatic differences characterize independent evolutionary lineages. We present the first phylogenetic study of Pachytriton that incorporates 2.35 kb of mitochondrial DNA (ND2, cytb) and 1.2 kb of nuclear sequence data (RAG-1) along with morphometric characters to infer evolutionary relationships and patterns of colour evolution among the three described species: Pachytriton brevipes, Pachytriton labiatus and Pachytriton archospotus. Our results support the monophyly of Pachytriton and recover P. archospotus as the sister taxon to P. brevipes. Monophyly of P. labiatus is significantly rejected: south-western populations are sister to the group of P. brevipes plus P. archospotus, whereas north-eastern populations nest with P. brevipes. The two geographic units are further separated by multivariate morphological analyses. South-western P. labiatus is the type species; misidentification of north-eastern populations as P. labiatus results from their similar colouration. An unspotted, dark brown dorsum is the likely ancestral state for the genus, whereas black-spotted colouration characterized the common ancestor of P. brevipes, P. archospotus, and north-eastern P. labiatus and was secondarily lost in the latter group. Homoplastic evolution and intraspecific variation render colour pattern in Pachytriton an unreliable character for delimiting species boundaries. North-eastern populations of P. labiatus are declining as the result of human collection and habitat destruction and are in urgent need of conservation protection.
A new species of fire-bellied newt, Cynops fudingensis, is described from northeastern Fujian Province, southeastern China. This species forms a well-supported clade with C. orientalis and C. orphicus based on molecular phylogenetic analysis of nucleotide sequences from mitochondrial DNA subunit two of NADH dehydrogenase and its two flanking tRNAs. Further, its genetic distance to each of the two previously described species is large. Principal-components analysis of external linear measurements differentiates the new species from C. orientalis and C. orphicus in morphological space. Geographically, C. fudingensis occupies the distribution gap between C. orientalis and C. orphicus.
We report two previously unknown populations of Asian warty newts (Salamandridae: Paramesotriton) in China. The first population, from southwestern Guangdong, is described as a new species, which is closely related to P. guangxiensis based on morphological and molecular data. The second new population, from western Hubei, is assigned to P. longliensis, which extends the known range of this species 400 km northwards. Limited genetic differentiation between P. longliensis and P. zhijinensis suggests that these two names may refer to the same (single) species.
This study characterizes regulatory elements of collagen2 alpha 7 (col2a1) in Xenopus that enable transgene expression in cartilage-forming chondrocytes. The reporters described in this study drive strong cartilage-specific gene expression, which will be a valuable tool for further investigations of Xenopus skeletal development. While endogenous col2a1 mRNA is expressed in many embryonic tissues, its expression becomes restricted to tadpole and adult chondrocytes. This chondrocyte-specific expression is recapitulated by col2a1 reporter constructs, which were tested through I-Scel meganuclease-mediated transgenesis. These constructs contain a portion of the Xenopus tropicalis col2a1 intron, which aligns to a cartilage-specific intronic enhancer that has been well characterized in mammals. Two overlapping regions of the first intron that are 1.5-Kb and 665-bp long, both of which contain this enhancer sequence, drove EGFP expression in both larval and adult chondrocytes when connected to an upstream promoter. However, neither a truncated 155-bp region that also contains the enhancer, nor a separate 347-bp intronic region that lacks it, was able to drive cartilaginous transgene expression. The two cartilage-specific transgenes are heritable in F1 progeny, which exhibit none of the background expression observed in the injected founders. This study is the first to use the I-Scel technique to characterize an enhancer element in Xenopus, and the first to generate chondrocyte-specific gene expression in a non-mammalian vertebrate. The creation of novel cartilage-specific gene expression provides a new tool for further studies of anuran skeletal development.
A new species of Asian warty newt, Paramesotriton ermizhaoi, is described from Guangxi Zhuang Autonomous Region in southern China. This species is easily distinguished from other congeners by external morphology and osteology. Phylogenetic analyses based on mitochondrial data place P. ermizhaoi as a possible sister taxon to P. hongkongensis and P. chinensis. The number of trunk vertebrae varies from 12 to 13 in the new species. The relationship between P. ermizhaoi and a potentially new Pachytriton species (Pachytriton C), and the Guangxi population of P. chinensis is briefly discussed.