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feature of all vertebrates. This allows us to propose that TH may play an important role in
coordinating the postembryonic development of apparently nonmetamorphosing ver-
tebrates such as mammals or sauropsids. Indeed, the observations of thyroid hormone
levels in mammals and sauropsids draw interesting parallels with what is observed dur-
ing amphibian metamorphosis. At the physiological level, the increase of thyroid hor-
mone signaling is required for the normal development particularly for the intestine and
the brain. At the behavioral level, a peak of TH often precedes the autonomy of the
young from parental care. At the ecological level, offspring with a TH peak close to
birth/hatching tends to be precocial young whereas offspring with a TH peak long after
birth/hatching tends to be altricial young. Taken together, these observations in amni-
otes, which are not considered as undergoing metamorphosis during their develop-
ment, are consistent with the idea of a late developmental step controlled by TH
and allowing the accession to the adult ecological niche. Thus, according to this view,
at the molecular level all vertebrates undergo a period of remodeling controlled by TH
that is reminiscent of metamorphosis.
1. INTRODUCTION
Metamorphosis is classically defined as a spectacular and abrupt post-
embryonic transformation of a larva into a juvenile ( Shi, 2000 ). Metamor-
phosis has received most attention in vertebrate model species such as the
toad Xenopus laevis but also some teleost fishes like the flatfishes, where thy-
roid hormones (TH) produced by the thyroid gland, trigger the changes ob-
served at the morphological and physiological levels. However, despite its
main association with amphibians and teleost fish, it is a really widespread
life history transition, with most animals undergoing some form of meta-
morphosis. Our recent analyses of metamorphosis in invertebrate chordates,
specifically in amphioxus, have revealed that metamorphosis is triggered by a
TH derivative and its receptor ( Paris et al., 2008 ). This has led to the sug-
gestion that TH-regulated metamorphosis is a shared ancestral character
common to all chordates. According to this view, metamorphosis in frogs
is an extreme example of a widespread life history transition and all verte-
brates, including mammals, do pass during their postembryonic develop-
ment through a remodeling period governed by TH that is extremely
similar, at the molecular and endocrine levels, to metamorphosis (see
Laudet, 2011 for a review).
In this review, we examine the role of TH during postembryonic devel-
opment by comparing, across vertebrate species, what we know about TH
levels and their roles in the newborn and the young. If THs are well studied
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