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genetic and genomic approaches will further establish salamanders as ideal models for
investigating TH signaling mechanisms that regulate postembryonic development and
the expression of alternate life histories.
Salamanders are one of three primary groups of amphibians, the other two
being caecilians and anurans. Ancestrally, all three groups trace their origins
to ancestors that present biphasic life cycles with an aquatic larval phase and
a more terrestrial adult phase ( Duellman & Trueb, 1986 ). However, alter-
nate modes of development subsequently evolved within all three groups.
Interestingly, ancestral vestiges of metamorphosis are observed during early
development of direct developing anurans and salamanders ( Callery &
Elinson, 2000; Kerney, Blackburn, Muller, & Hanken, 2012 ), suggesting
shared evolutionary potential for radical and early shifts in the timing of
metamorphosis. Radical, later shifts in metamorphic timing are only ob-
served in salamanders. In the most extreme cases, metamorphosis has been
abolished completely, yielding bizarre larval-form adults with completely
aquatic life cycles ( Gould, 1977; Shaffer & Voss, 1996 ). These unique pae-
domorphic forms are ideal for investigating mechanisms of thyroid hormone
(TH) regulation that are associated with adaptive delays in metamorphic
timing and the evolution of novel life histories.
1. WHAT IS PAEDOMORPHOSIS AND WHY
SALAMANDERS?
Paedomorphosis is a heterochronic term that describes a specific pat-
tern of evolution, the retention of ancestral juvenile traits in the adult stage of
a derived species ( Gould, 1977 ). In this meaning, paedomorphosis references
an evolutionary change in developmental timing between an ancestor and
descendant species. However, paedomorphosis is often used in a more gen-
eral sense to describe the retention of larval morphological traits in adult sal-
amanders, irrespective of phylogeny. The evolution or expression of
paedomorphosis is clearly associated with TH, the primary metamorphic
hormone in anurans, salamanders, and some fish. In species that undergo
a metamorphosis, TH induces the regression of larval traits and the devel-
opment of traits typical of a more terrestrial adult ( Fig. 8.1 ). As we discuss
in more detail below, many paedomorphic salamanders can be induced to
undergo partial or complete metamorphosis by simply placing them in a bath
of TH. This suggests the possibility that paedomorphosis evolves “simply”
by blocking the synthesis, secretion, or reception of TH in target cells ( Page,
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