Publications

2001
Brame, A.H., et al., 2001. New species of large black salamander, genus Bolitoglossa (Plethodontidae) from western Panama. Copeia , pp. 700-704.Abstract

A new species of plethodontid salamander is one of several large black species found in the Cordillera Talamanca-Baru of Panama and Costa Rica. Bolitoglossa anthracina sp. nov. differs from others in this group in having a very large number of maxillary teeth and a moderate number of vomerine teeth.

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Hanken, J. & Wake, D.B., 2001. A seventh species of minute salamander (Thorius : Plethodontidae) from the Sierra de Juarez, Oaxaca, Mexico. Herpetologica , 57 , pp. 515-523.Abstract

We describe a new terrestrial species of minute lungless salamander of the Mexican genus Thorius (Plethodontidae) from montane pine-oak forests in the Sierra de Juarez of north central Oaxaca, Mexico. The new species is distinguished from congeners by a combination of body size, external morphology, osteology, and dental traits, and it is well differentiated genetically from other named species for which data are available. This is the seventh endemic species of Thorius reported from the Sierra de Juarez, and known localities are geographically isolated from those of all other species. Discovery of another new species of plethodontid salamander from Oaxaca enhances the state's standing as a preeminent center of herpetological diversity within both Mexico and Mesoamerica.

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2000
Hanken, J., 2000. A review of Tadpoles: the Biology of Anuran Larvae. Evol. Devel. , 2 , pp. 174-175. PDF
Carl, T.F., et al., 2000. Green fluorescent protein used to assess cranial neural crest derivatives in the frog, Xenopus laevis. In C. O. Jacobson & L. Olsson, ed. Regulatory Processes in Development: The Legacy of Sven Hörstadius (1898-1996). London. London: Wenner-Gren International Series, Portland Press, pp. 167-172.Abstract

We used RNA encoding green fluorescent protein (GFP) to study the migration and larval derivatives of cranial neural crest cells in the metamorphosing frog, Xenopus laevis. GFP provides an intrinsic cell-lineage marker that is retained after cell division. Moreover, because GFP label introduced at the one-cell stage continues to be expressed well after hatching, it offers a reliable and effective method for assessing the embryonic derivation of many larval, and possibly even adult, tissues in amphibians as well as other vertebrates. Basic patterns of cranial neural crest migration and derivation in X. laevis defined using GFP (including contributions to many larval cranial cartilages) arc similar to those documented in previous studies that used conventional vital stains, lineage markers, and ablation techniques. However, preliminary results also suggest the neural crest derivation of additional components of the larval anuran head, e.g., cranial bone, whose embryonic origins have proven much more difficult to resolve with other methods.

1999
Hanken, J., 1999. An alternative evolutionary synthesis [review of C.D. Schlichting and M. Pigliucci, Phenotypic Evolution: A Reaction Norm Perspective]. Trends Ecol. Evol. , 14 , pp. 162-163. PDF
Hanken, J., 1999. Larvae in amphibian development and evolution. In The Origin and Evolution of Larval Forms. San Diego. San Diego: Academic Press, pp. 61-108. PDF
Hanken, J., 1999. 4,780 and counting. Natural History , 108 , pp. 82-82. PDF
Carl, T.F., et al., 1999. Inhibition of neural crest migration in Xenopus using anti-sense-slug RNA. Developmental Biology , 213 , pp. 101-115.Abstract

Based primarily on studies in the chick, it has been assumed that the zinc finger transcription factor Slug is required for neural crest migration. In the mouse, however, Slug is not expressed in the premigratory neural crest, which forms normally in Slug -/- animals. To study the role of Slug in Xenopus laevis, we used the injection of XSlug antisense RNA and tissue transplantation. Injection of Slug antisense RNA did not suppress the early expression of the related gene XSnail, but led to reduced expression of both XSlug and XSnail in later stage embryos, whereas the expression of another neural crest marker, XTwist, was not affected. Down-regulation of XSlug and XSnail was associated with the inhibition of neural crest cell migration and the reduction or loss of many neural crest derivatives. In particular, the formation of rostral cartilages was often highly aberrant, whereas the posterior cartilages were less frequently affected. The effects of Slug antisense RNA on neural crest migration and cartilage formation were rescued by the injection of either XSlug or XSnail mRNA. These studies indicate that XSlug is required for neural crest migration, that XSlug and XSnail may be functionally redundant, and that both genes are required to maintain each other's expression in the neural crest development of xenopus laevis. (C) 1999 Academic Press.

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Hanken, J., 1999. Modern museums are far from fossilized. Nature , 400 , pp. 13-13. PDF
Hanken, J., Wake, D.B. & Freeman, H.L., 1999. Three new species of minute salamanders (Thorius : Plethodontidae) from Guerrero, Mexico, including the report of a novel dental polymorphism in urodeles. Copeia , pp. 917-931.Abstract

Three new species of minute lungless salamanders of the Mexican genus Thorius (Plethodontidae) are described from montane forests in the Sierra Madre del Sur of Guerrero. Each species is distinguished from congeners by a combination of body size, external morphology, osteology, dental traits, and proteins. Thorius omiltemi and T. grandis are among the largest species within the genus; standard length (SL) approaches or exceeds 30 mm in many adults. Thorius infernalis is much smaller (SL < 19 mm). Adult T. grandis display an extreme, unique sexual dimorphism involving the presence/absence of maxillary teeth and several related features of cranial osteology. Protein (allozyme) data for T. omiltemi and T. grandis reveal substantial levels of genetic differentiation relative to species in Veracruz, Puebla, and Oaxaca. Comparable genetic data are unavailable for T. infernalis, The three species collectively define a broad elevational range, from high elevation T. omiltemi and T. grandis (2200-2700 m and 2495-3360 m, respectively) to lower montane T. infernalis (1140 m), Description of several additional species of plethodontid salamanders from central montane Guerrero underscores the region's rich herpetological diversity, which includes many endemic species of both amphibians and reptiles.

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Hanken, J., 1999. Why are there so many new amphibian species when amphibians are declining?. Trends in Ecology & Evolution , 14 , pp. 7-8. PDF
1998
Hanken, J., 1998. Beauty beyond belief. Natural History , 107 , pp. 56-59. PDF
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.Abstract

Minute 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.

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Richardson, M.K., et al., 1998. Haeckel, embryos, and evolution. Science , 280 , pp. 983-985. PDF
Hanken, J. & Richardson, M.K., 1998. Haeckel's embryos. Science , 279 , pp. 1288-1288. 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.Abstract

The 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.

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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.Abstract

Direct 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.

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Richardson, M.K., et al., 1998. Phylotypic stage theory. Trends in Ecology & Evolution , 13 , pp. 158-158. PDF
Richardson, M.K., et al., 1998. Somite number and vertebrate evolution. Development , 125 , pp. 151-160.Abstract

Variation 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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1997
Hanken, J., et al., 1997. Jaw muscle development as evidence for embryonic repatterning in direct-developing frogs. Proceedings of the Royal Society B-Biological Sciences , 264 , pp. 1349-1354.Abstract

The Puerto Rican direct-developing frog Eleutherodactylus coqui (Leptodactylidae) displays a novel mode of jaw muscle development for anuran amphibians. Unlike metamorphosing species, several larval-specific features never form in E. coqui; embryonic muscle primordia initially assume an abbreviated, mid-metamorphic configuration that is soon remodelled to form the adult morphology before hatching. Also lacking are both the distinct population of larval myofibres and the conspicuous, larval-to-adult myofibre turnover that are characteristic of muscle development in metamorphosing species. These modifications are part of a comprehensive alteration in embryonic cranial patterning that has accompanied life history evolution in this highly speciose lineage. Embryonic 'repatterning' in Eleutherodactylus may reflect underlying developmental mechanisms that mediate the integrated evolution of complex structures. Such mechanisms may also facilitate, in organisms with a primitively complex life cycle, the evolutionary dissociation of embryonic, larval, and adult features.

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