Biology Reference
In-Depth Information
A
B
E.
coqui
B. americanus
Figure 9.3 Comparison between jaws of an E. coqui TS13 embryo and a tadpole. (A) The
E. coqui embryo has a froglike mouth, with a single keratinous egg tooth (arrow) in the
middle of the upper jaw. The egg tooth is used to hatch from the jelly capsule. (B) A
tadpole, here of the toad Bufo americanus, has rows of keratinous teeth and a keratinous
beak. The teeth and beak are used for scraping surfaces to collect decayed plant ma-
terial and other detritus. Note the very different shape of the tadpole jaw, compared
to that of E. coqui. These photographs are at the same magnification.
As yolk amounts in the egg increase to provide more and more of the
nutritional needs, structures and tissues, used by tadpoles for feeding, are
also lost. These include the tadpole-specific jaw cartilages, the suprarostral
and infrarostral (
Hanken, Klymkowsky, Summers, Seufert, & Ingebrigtsen,
1992; Kerney, Gross, &Hanken, 2010
), the larval pattern of the olfactory sys-
tem (
Jermakowicz et al., 2004
), and the long coiled gut, required by tadpoles
for their plant diet (
Buchholz et al., 2007
). Mutations affecting the
development of these structures can accumulate with no deleterious effect
on the animal. Indeed, we see the loss of these structures to a greater or lesser
degree among direct developing species (e.g.,
Kerney, Meegaskumbura,
Manamendra-Arachchi, & Hanken, 2007
).
The reductionor loss of all structures, used by tadpoles for feeding and swim-
ming, eliminates one of the regulatory functions of thyroid hormone, namely,
to cause destruction and resorption of tadpole-specific tissues. That leaves the
thyroid hormone role of promoting development of adult-specific structures.
3. THYROID HORMONE DEPENDENCY IN FROG DIRECT
DEVELOPMENT
Development from the embryo to the froglet in a direct developer
looks seamless, without the radical transition from a tadpole's morphology
to that of a frog (
Fig. 9.1
). Nonetheless, development to the froglet requires
