Biology Reference
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mediating the larval developmental response to environmental conditions
(
Glennemeier & Denver, 2002b
).
Hayes (1997)
also reported an elevation
in whole-body CORT content in tadpoles caused by crowding. By con-
trast,
Belden,Rubbo,Wingfield,andKiesecker(2007)
did not find such
a relationship in a mesocosm study. Predation, temperature, photoperiod,
or other environmental factors could conceivably work through similar
neuroendocrine pathways to exert their effects on larval development.
If larvae have a means of detecting the state of environmental conditions,
through visual, chemical, or other sensory systems, then the neuroendo-
crine system is a likely pathway through which developmental responses
to the environment can operate.
While the hypothalamus and pituitary gland are required for metamor-
phosis through their control of thyroid and interrenal gland secretion, other
processes occurring at target tissues may influence metamorphic timing.
For example, the availability of biologically active TH is regulated within
tissues by the monodeiodinases.
Buchholz and Hayes (2005)
showed that
closely related species of spadefoot toads that differ in the duration of their
larval periods show strong differences in the tissue content of T
3
and T
4
,
and the sensitivity of their tissues to TH. They speculated that these
differences might be due to differences in TH uptake into cells and/or
TH metabolism. The expression of monodeiodinases enzymes could be
modified either directly or indirectly by environmental factors. CS
increases 5
0
-deiodinase activity, with the result that more of the active
hormone T
3
is generated. This regulatory relationship suggests that stress
and stress hormones could accelerate metamorphosis by upregulating
5
0
-deiodinase activity.
Tissue expression of TRs influences sensitivity to the TH signal. TH
receptor
b
is autoinduced in many tissues during metamorphosis, and evi-
dence suggests that this is required to drive metamorphosis (
Bagamasbad &
Denver, 2011; Laudet, 2011
).
Hollar, Choi, Grimm, and Buchholz (2011)
recently showed that TR expression level is negatively correlated with the
duration of the larval period in different species of spadefoot toad; that is,
higher TR equals shorter larval period.
Biosynthesis of TRs might be regulated directly or indirectly by environ-
mental factors, which could influence metamorphic timing. Currently, rel-
atively little is known about what factors, either physiological or
environmental, regulate nuclear receptor expression in any species
(
Bagamasbad & Denver, 2011
). As for monodeiodinases, evidence suggests
that CS can enhance TH action by upregulating TR expression, and so TR
