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time during ontogeny (
Rosenkilde & Ussing, 1996
).
Galton (1992)
suggested that T4 levels might never eclipse a threshold during development
to stimulate brain regions to increase activity of the thyroid gland.
Kuhn
et al. (2005)
similarly proposed that brain T3 availability is the limiting factor
for metamorphic onset. They found that DIO2 activity in the brain, which is
required to generate T3, requires appropriate doses of corticoids and T4.
Thus, stimulation of both the HPA and HPI axes is required for metamor-
phosis, and in fact, it is possible to induce metamorphosis in
A. mexicanum
if
submetamorphic levels of T4 and corticoids are administered together. Be-
cause injection of amniote CRH stimulates corticotropin but not thyrotro-
pin release,
Kuhn et al. (2005)
implicated the thyrotroph CRH receptor as a
likely candidate gene for paedomorphosis in
A. mexicanum
. However,
researchers have demonstrated T4 increases after stimulating hypothalamic
regions in neotenic tiger salamanders (
A. tigrinum
) and
A. mexicanum
; this
could seemingly only occur if downstream thyrotrophin signaling pathways
are functional (
Norris & Gern, 1976; Rosenkilde & Ussing, 1996
).
It is also possible that a recently discovered TSH signaling pathway may
be implicated in salamander paedomorphosis. Studies of neuroendocrine
regulation of amniote reproductive cycles showed that TSH from the
pars
tuberalis
(PT) of the anterior pituitary gland is released to signal DIO2 tran-
scription in the hypothalamus (
Hanon et al., 2008, Nakao et al., 2008
). This
increases local T3 concentrations and thus stimulates canonical TH signaling
pathways. Surprisingly, these studies show that the flow of information may
be bidirectional between the pituitary and hypothalamus. If a similar PT sig-
naling pathway is necessary for metamorphosis, this pathway would not be
activated in
A. mexicanum
because of a block in TSH release. This might also
explain why
A. mexicanum
does not show strong seasonality in breeding and
can breed multiple times a year. Whether paedomorphosis involves a PT
signaling mechanism or a classical TSH signaling mechanism, the metamor-
phic block in
A. mexicanum
is likely associated with hypothalamus and
pituitary function (
Kuhn et al., 2005; Laudet, 2011
;
Norris et al., 1973
;
Prahlad & DeLanney, 1965; Rosenkilde & Ussing, 1996
).
6. GENOMIC ANALYSIS OF SALAMANDER
METAMORPHOSIS AND PAEDOMORPHOSIS
Most mechanistic studies of metamorphosis and paedomorphosis have
focused on physiology and to a lesser extent biochemistry. Recently, micro-
arrays developed for
A. mexicanum
have been used to characterize global
transcriptional responses of metamorphic and paedomorphic salamanders.
