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indicating the activity of thyroid hormone at this stage. CRF also upregulates
thrb
expression (
Kulkarni et al., 2010
). This expression is likely due to stimu-
lation of the central neuroendocrine axis leading to increased circulating
thyroid hormone, but an alternative exists at the peripheral tissue level. In
X. laevis
tadpoles, corticosterone (CORT), derived from interrenal glands in
response to CRF, can synergize with thyroid hormone to increase the expres-
sion of
thrb
(
Bonett, Hoopfer, & Denver, 2010
).
In addition to the upregulation of
thrb
associated with cryptic metamor-
phosis, another receptor,
thra
, is constitutively expressed through develop-
ment, and mRNAs for both
thra
and
thrb
are present in unfertilized oocytes
(
Callery & Elinson, 2000a
). Potential roles for these maternally expressed
receptors will be considered later.
4. MECHANISMS UNDERLYING TADPOLE DELETION
Two hypotheses have been presented to account for the evolutionary
loss of the tadpole from the life history of the direct developing frog: preco-
cious thyroid activation versus larval reduction. The hypothesis of precocious
thyroid activation is that development and activation of the thyroid gland oc-
cur earlier in development (
Hanken, Jennings, & Olsson, 1997; Jennings &
Hanken, 1998
). This early activation causes early metamorphosis. The
hypothesis of larval reduction is that the animal spends less time as a larva,
leading eventually to the joining of metamorphic events with embryonic ones
(
Fig. 9.4
)(
Callery & Elinson, 2000b; Callery et al., 2001
). Essentially, preco-
cious thyroid activation states that the earlier activation of the thyroid gland
is the cause of larval reduction, while larval reduction states that the shorter
larval period is the cause of earlier activation of the thyroid gland.
When the larval period is reduced, there is less time for the tadpole to feed
and to grow. Consequently, reduction of the larval period requires both an
increase in yolk content of the egg and the reduction of froglet size at meta-
morphosis (
Callery et al., 2001
). The Costa Rican golden toad,
Bufo periglenes
,
likely recently extinct, illustrates a potential intermediate in tadpole deletion.
It has a facultatively nonfeeding larva (
Crump, 1989
). Large eggs develop into
tadpoles, which in the presence of food, will eat, grow, and metamorphose. If
there is little food, the tadpoles have a sufficient amount of yolk to metamor-
phose at a small size without feeding and to try their luck on land.
A difficulty with the precocious thyroid activation hypothesis is that ex-
posure of tadpoles to thyroid hormone leads to metamorphic changes but
generally does not lead to normal metamorphosis. For example, treatment
of young
X. laevis
tadpoles with thyroid hormone causes elongation of
