Biology Reference
In-Depth Information
3.2. Metamorphosis variation in amphibians
Within amphibians, all species do not undergo such marked metamorphosis
as seen in Anurans such as
Xenopus
. For most Urodeles (salamanders and
newts), we have as yet no clear description of how THs and their receptors
trigger metamorphosis at the cellular and molecular levels. Even in classical
biphasic species such as
Pleurodeles waltl
in which metamorphosis is well
known at the morphological level (
Galien & Durocher, 1957
), it is not
known if and how the
Xenopus
model can be applied. This is a clear limi-
tation for understanding the frequent variations of metamorphosis that are
observed in several species of Urodeles.
The axolotl (
Ambystoma mexicanum
) is a facultatively neotenic Urodele that
belongs toa largecomplexof species (
Voss&Shaffer, 1997
).Under natural con-
dition, some species undergometamorphosis anddevelop the classical adult fea-
tures of a salamander, but many species of Ambystoma, and in particular
A. mexicanum
, acquire the ability to reproduce while keeping their larval fea-
tures. In axolotl, TRs are functional; they respond to TH, bind DNA, are
expressed at high levels in several tissues (
Safi et al., 2004
), and exogenous
T4 treatment on axolotl induces metamorphosis (
Brown, 1997
). The faculta-
tive neoteny of axolotl has been attributed to low levels of circulating THs due
toweakactivationof the thyroidaxis by thepituitary gland,when the rest ofTH
signalingpathway functionnormally (
Boorse&Denver, 2002; Safi et al., 2004
).
The mudpuppy (
Necturus maculosus
) is an example of an obligate paedo-
morphic amphibian. Obligate means that the adult of this species exhibits
the same morphological traits as the larvae of closely related species. Morpho-
logically, the mudpuppy resembles a typical Urodele larva with external gills
and an entirely aquatic life history. It was even described as an animal resistant
to the action of THs (
Yaoita, Shi, & Brown, 1990
). Nevertheless, it has been
shown that, as for
Xenopus
, mudpuppy TRs can bind DNA and fully respond
to THs (
Safi et al., 2006
). Moreover,
TR
is expressed in the gills (
Vlaeminck-
Guillem et al., 2006
) and is upregulated by TH, as during
Xenopus
metamor-
phosis, indicating that this animal is not resistant to TH. In
Necturus
, the visible
absence of metamorphosis is more likely due to changes in the genes regulated
by TH rather than by a global arrest of metamorphosis. Indeed, the intestine of
Necturus
resembles that of an adult
Xenopus
, suggesting that this organ un-
dergoes a postembryonic remodeling similar to the one occurring in
Xenopus
.
We thus suggest that
Necturus
undergoes a weak, or subtle, metamorphosis in
which the pathway controlling TH levels and TH tissue response is indeed
present with no obvious morphological changes. This example provides an
excellent illustration of similar situations that can be found in other species
of vertebrates, in particular, mammals and other amniotes.
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