Hanken, J. & Wake, D.B., 1998.
Biology of tiny animals: Systematics of the minute salamanders (Thorius : Plethodontidae) from Veracruz and Puebla, Mexico, with descriptions of five new species.
Copeia , pp. 312-345.
AbstractMinute plethodontid salamanders, genus Thorius, are far more diverse taxonomically than has been recognized previously. Populations of these salamanders from the Mexican states of Veracruz and Puebla are assigned to 10 species, five of which are described as new. Combinations of morphological and allozymic characters are used to sort the species and to make initial assessments of relationships. Valid existing names include Thorius pennatulus, T., troglodytes, T. dubitus, and T. schmidti. Thorius narismagnus; from the Sierra de Los Tuxtlas, which previously was considered to be a disjunct subspecies of T., pennatulus, is elevated to species rank. Thorius maxillabrochus is treated as a subjective junior synonym of the sympatric T.:schmidti. New taxa include Thorius lunaris, T. magnipes, T. minydemus, T. munificus, and T. spilogaster: All 10 species can be distinguished by morphological characters, but the distinctiveness of the taxa is bolstered by allozymic characters and by extensive sympatry. As many as three, and possibly four, species occur in sympatry, with some evidence of segregation by microhabitat (arboreal vs terrestrial). Adult body sizes span the range known for the genus, from very small in T., pennatulus (maturing at < 16 mm standard length) to large in T. lunaris (adults reaching > 31 mm). Collectively these species display a Hide elevational distribution, from less than 1000 m (T. pennatulus, T. narismagnus) to more than 3000 m (T. lunaris, T., spilogaster). Discovery of these new species adds to the rich herpetological diversity of east-central Mexico and underscores its importance as a principal center of radiation of tropical plethodontid salamanders.
PDF Richardson, M.K., et al., 1998.
Limb development and evolution: a frog embryo with no apical ectodermal ridge (AER).
Journal of Anatomy , 192 , pp. 379-390.
AbstractThe treefrog Eleutlrerodactylus coqui is a direct developer-it has no tadpole stage. The limb buds develop earlier than in metamorphosing species (indirect developers, such as Xenopus laevis). Previous molecular studies suggest that at least some mechanisms of limb development in E. coqui are similar to those of other vertebrates and we wished to see how limb morphogenesis in this species compares with that in other vertebrates. We found that the hind limb buds are larger and more advanced than the forelimbs at all stages examined, thus differing from the typical amniote pattern. The limb buds were also small compared to those in the chick. Scanning and transmission electron microscopy showed that although the apical ectoderm is thickened, there was no apical ectodermal ridge (AER). In addition, the limb buds lacked the dorsoventral flattening seen in many amniotes. These findings could suggest a mechanical function for the AER in maintaining dorsoventral flattening, although not all data are consistent with this view. Removal of distal ectoderm from E. coqui hindlimb buds does not stop outgrowth, although it does produce anterior defects in the skeletal pattern. The defects are less severe when the excisions are performed earlier. These results contrast with the chick, in which AER excision leads to loss of distal structures. We suggest that an AER was present in the common ancestor of anurans and amniotes and has been lost in at least some direct developers including E. coqui.
PDF Jennings, D.H. & Hanken, J., 1998.
Mechanistic basis of life history evolution in anuran amphibians: Thyroid gland development in the direct-developing frog, Eleutherodactylus coqui.
General and Comparative Endocrinology , 111 , pp. 225-232.
AbstractDirect development is a widespread, alternative life history in Recent amphibians. There is no free-living, aquatic larva; adult features form in the embryo and are present at hatching. The mechanistic bases of direct development remain relatively unexplored. The current study describes the embryonic ontogeny of the thyroid gland in the direct-developing frog Eleutherodactylus coqui (Leptodactylidae) and quantifies histological changes that occur in the gland after its initial appearance. The thyroid gland of E. coqui is first apparent at Townsend-Stewart stage 10, approximately two-thirds of the way through embryogenesis. Soon after this the thyroid begins to accumulate follicular colloid. Quantitative analyses of thyroid histology reveal embryonic peaks in two measures, follicle number and follicle volume, which are followed by declines in these measures prior to hatching. These peaks in thyroid activity in E. coqui are correlated with morphological changes that are directly comparable to metamorphic changes in frogs that retain the ancestral, biphasic life history. In metamorphic taxa, a histologically identifiable thyroid gland does not form until the larval period, well after hatching. Nevertheless, measures of thyroid histology observed in E. coqui follow the pattern reported for metamorphosing amphibians. The present results support the hypothesis that the evolution of direct development in anurans is associated with precocious development and activity of the thyroid axis. (C) 1998 Academic Press.
PDF Richardson, M.K., et al., 1998.
Somite number and vertebrate evolution.
Development , 125 , pp. 151-160.
AbstractVariation in segment number is an important but neglected feature of vertebrate evolution, Some vertebrates have as few as six trunk vertebrae, while others have hundreds. We examine this phenomenon in relation to recent models of evolution and development. Surprisingly, differences in vertebral number are foreshadowed by different somite counts at the tailbud stage, thought to be a highly conserved (phylotypic) stage, Somite number therefore violates the 'developmental hourglass' model, We argue that this is because somitogenesis shows uncoupling or dissociation from the conserved positional field encoded by genes of the zootype.Several other systems show this kind of dissociation, including limbs and feathers. Bmp-7 expression patterns demonstrate dissociation in the chick pharyngeal arches, This makes it difficult to recognise a common stage of pharyngeal development or 'pharyngula' in ail species, Rhombomere number is more stable during evolution than somite number, possibly because segmentation and positional specification in the hindbrain are relatively interdependent, Although developmental mechanisms are strongly conserved, dissociation allows at least some major evolutionary changes to be generated in phylotypic stages.
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