Biology Reference
In-Depth Information
morphologically similar paedomorphic salamanders (with external gills or
gill slits, no eyelids, larval pigmentation, etc.) responded differently to
TH. Members of the family Proteidae, such as the American mudpuppy
(
Necturus maculosus
), are known to be refractory to TH, such that treating
them with exogenous or injected TH does not induce morphological meta-
morphosis (
Swingle, 1922
).
Larsen (1968)
proposed that peripheral tissues of
N. maculosus
had lost sensitivity to TH, given that it exhibits a morpholog-
ically normal thyroid gland that had been tested for functionality via trans-
plant experiments (
Charipper, 1929; Grant, 1930; Swingle, 1922
). This
prompted investigations into the functionality of the animal's deiodinase
activity and TR expression.
Galton (1985)
found that deiodinase activity
capable of converting T4 to T3 was present within
N. maculosus
, and
Safi
et al. (2006)
showed that
N. maculosus
expresses functional TRs that bind
to DNA and activate transcription in response to TH treatment, including
transcriptional changes of classical TR target genes like
stromelysin 3
(
mmp11
)
and
thrb
(
Safi et al., 2006; Vlaeminck-Guillem et al., 2006
). Additionally,
in situ
hybridization has been used to show expression of
thra
and
thrb
in
central nervous system, epithelia of the digestive tract, myocardium, and
skeletal muscle (
Vlaeminck-Guillem et al., 2006
). These studies suggest that
the paedomorphic phenotype exhibited by
N. maculosus
may be due to the
loss of key TH response genes needed to induce morphological changes,
even though juveniles experience a postembryonic period of high TH
sensitivity. Apparently, individuals undergo natural transcriptional and bio-
chemical changes during a “cryptic metamorphosis” that does not result in
gross morphological alterations (
Laudet, 2011; Vlaeminck-Guillem et al.,
2006
). Species of the families Cryptobranchidae and Amphiumidae are
responsive to THs in some respects, but not others. They often reabsorb
external gills and develop adult characteristics of some features, but do
not develop eyelids and retain gill slits as mature adults, indicating that some
metamorphic processes may have been decoupled from TH regulation
(
Larson et al., 2003
). When species of these families are treated with high
doses of TH, often the only result is skin shedding, a trait that is regulated
by TH in metamorphic species, though otherwise the individual continues
to exhibit paedomorphic characteristics (
Dent, 1968; Fox, 1984
). It was
originally thought that the Texas blind salamander (
Eurycea rathbuni
, for-
merly assigned to the genus
Typhlomolge
) was the only known vertebrate
to lack a thyroid gland (
Emerson, 1905
). However,
Gorbman (1957)
later
showed the presence of a thyroid gland in
E. rathbuni
and
Dundee (1957)
showed that an individual of this species underwent some morphological
