Biology Reference
In-Depth Information
CHAPTER EIGHT
Salamander Paedomorphosis:
Linking Thyroid Hormone to Life
History and Life Cycle Evolution
Carlena K. Johnson, S. Randal Voss
1
Department of Biology, Spinal Cord and Brain Injury Research Center, University of Kentucky, Lexington,
Kentucky, USA
1
Corresponding author: e-mail address: srvoss@uky.edu
Contents
1. What is Paedomorphosis and Why Salamanders?
230
2. Paedomorphosis and Salamander Life History Variation
234
3. Hormonal Basis of Salamander Metamorphosis
237
4. Hormonal Basis of Paedomorphosis
239
5. Paedomorphosis in the Mexican Axolotl
241
6. Genomic Analysis of Salamander Metamorphosis and Paedomorphosis
243
7. Genetic Analysis of Paedomorphosis
246
8. The Link Between met1 and TH Regulation of Metamorphosis
248
9. What Genes Map to Metamorphic Timing QTL?
249
10. Synthesis: Evolution of Salamander Paedomorphosis
250
References
252
Abstract
Many salamanders have biphasic life cycles with aquatic larval and terrestrial adult
phases. In these species, the transition between phases
—
metamorphosis
—
requires
thyroid hormone (TH) activation of transcriptional programs that cause regression of
larval traits and development of adult traits. During salamander evolution, TH signaling
pathways have been altered in biphasic species to yield paedomorphic salamanders
that retain larval traits and attain sexual maturity in larval aquatic habitats. We review
literature concerning the ecology, evolution, and hormonal regulation of metamorphic,
paedomorphic, and facultative salamander life histories. We then discuss recent micro-
array results that detail gene expression signatures of metamorphosis and paedomor-
phosis, and genetic results that establish TH responsiveness as a continuous trait with a
quantitative trait locus (QTL) basis. TH-responsive QTL from ambystomatid salamanders
explain variation in metamorphic timing, expression of metamorphosis versus paedo-
morphosis, and adult fitness traits. We propose a model for salamander life history evo-
lution that links adaptation to aquatic habitats with TH-responsive loci that
pleiotropically alter metamorphic timing and adult body size. Future studies that adopt
