Biology Reference
In-Depth Information
CHAPTER NINE
Metamorphosis in a Frog That
Does Not Have a Tadpole
Richard P. Elinson
1
Department of Biological Sciences, Duquesne University, Pittsburgh, Pennsylvania, USA
1
Corresponding author: e-mail address: elinson@duq.edu
Contents
1.
Introduction
260
2. Loss of Tadpole Structures in Frog Direct Development
260
3. Thyroid Hormone Dependency in Frog Direct Development
263
4. Mechanisms Underlying Tadpole Deletion
265
5. Precocious Appearance of Frog Features in Direct Developers
267
5.1 Eyes
267
5.2 Limbs
268
6. A Possible Role for Maternal Thyroid Hormone and Thyroid Hormone Receptor
269
7. Questions for the Future
271
References
272
Abstract
The evolutionary removal of the tadpole from the frog life history is a very successful
strategy, particularly in the tropics. These direct developers form limbs and a frog-like
head early in embryogenesis, and they have reduced or lost tadpole-specific structures,
like gills, a long, coiled intestine, and tadpole teeth and jaws. Despite the apparently con-
tinuous development to the frog morphology, the direct developer, Eleutherodactylus
coqui, undergoes a cryptic metamorphosis requiring thyroid hormone. As in Xenopus
laevis, there is a stimulation by corticotrophin-releasing factor (CRF) and an upregulation
of thyroid hormone receptor
b
(thrb). In addition to changes in skin and muscle, thyroid
hormone stimulates yolk utilization for froglet growth from a novel tissue, the nutritional
endoderm. The activities of CRF and corticosterone (CORT) in metamorphosis may pro-
vide the basis for the multiple evolutionary origins of direct development in anuran am-
phibians. Potential roles for maternally supplied thyroid hormone and its receptor and
for deiodinases in regulating tissue sensitivity to thyroid hormone should be the subjects
of future investigations.
