Biology Reference
In-Depth Information
data discussed above from other animals, interactions between maternal
thyroid hormone and its receptor could play roles in early embryos. If so,
E. coqui
may utilize those interactions to promote the early development
of eyes, limbs, and other structures.
Alternatively, if activation roles for thyroid hormone are not found in
embryos of other animals, very simple evolutionary changes could have gen-
erated activation roles in embryos of
E. coqui
. For example, the thyroid hor-
mone inactivating enzyme D3 is expressed in many places in the
S. tropicalis
early embryo. A reduction of D3 expression would allow maternal thyroid
hormone to interact with its receptor and change gene repression by
unliganded receptor to gene activation by liganded receptor. A more thor-
ough examination of the components of the thyroid hormone regulation
system in
E. coqui
may reveal significant differences compared to tadpole spe-
cies. These differences could positively regulate the development of eyes,
limbs, and other tissues.
7. QUESTIONS FOR THE FUTURE
Direct developing frogs provide many questions for future investiga-
tion of metamorphosis and its evolution. Prime among these is whether thy-
roid hormone regulates events of early development in
E. coqui
as well as in
embryos of fish, frogs with tadpoles, and birds. An important corollary to this
question is whether thyroid hormone is involved in utilization of yolk in
embryonic tissues of many animals and even in the yolk sac of birds and other
amniotes. With regard to this latter possibility, thyroid hormone receptor
b
is upregulated in chicken yolk sac at day 9 of incubation, and expression
remains elevated through the major period of yolk utilization (
Forrest,
Sj¨berg, & Vennstr¨m, 1990
).
Another question is how the patterns of expression of deiodinases compare
between
E. coqui
and
X. laevis
or
S. tropicalis
. The activities of deiodinases affect
tissue sensitivity to thyroid hormone, so differences in their expression in
E. coqui
may contribute to precocious development of specific tissues and
structures. With respect to tissue sensitivity, there is the question of whether
CORT played a role in the repeated evolution of frog direct development.
Finally, many of the novel features of frog direct development, such as
cryptic metamorphosis and nutritional endoderm, are known from only one
species,
E. coqui
. The multiple origins will allow exploration of variation in
the evolution of this derived life history.
