Biology Reference
In-Depth Information
D2 catalyzes the conversion of the thyroxine (T
4
)tothemoreacive
3,5,3
0
-triiodothyronine (T
3
). Both effects of CORT increase tissue sensitivity
to thyroid hormone, so in addition to CRF stimulation of the thyroid gland,
CORT changes target tissue sensitivity.
The environmental pressure for direct development is to obtain a safer
place for the embryos to develop. Ponds are unsafe, so frogs generally lay
thousands of eggs of which no more than a couple survive to adulthood.
In tropical areas, not only does the high humidity provide moist opportu-
nities for development on land, but also ponds may be even more dangerous,
increasing stress levels in tadpoles. The interplays at several levels in tadpoles
between the thyroid hormone system and the CORT stress system could be
the nexus that has facilitated the multiple evolutionary origins of anuran
direct development.
5. PRECOCIOUS APPEARANCE OF FROG FEATURES IN
DIRECT DEVELOPERS
Along with the deletion of most tadpole-specific structures in direct
developers, some frog-like structures develop precociously. Prominent eyes
and limbs are two features which distinguish most frogs from tadpoles, and
both of these develop in indirect developers in response to thyroid hormone
at metamorphosis (
Brown &Cai, 2007; Hoskins, 1990; Mann &Holt, 2001;
Marsh-Armstrong, Huang, Remo, Liu, & Brown, 1999; Rot-Nikcevic &
Wassersug, 2004; Schreiber et al., 2001; Shi, 2000
). Large eyes and limbs
develop early in direct developers (
Fig. 9.1
), and this timing is before the
thyroid gland becomes functional in
E. coqui
(
Jennings & Hanken, 1998
).
5.1. Eyes
The eyes of tadpoles are generally smaller relative to the head compared to
frogs. The tadpole optic nerve, connecting the retina to the tectum and thal-
amus in the brain, crosses over completely at the optic chiasm (
Mann &
Holt, 2001
). In response to thyroid hormone in
X. laevis
, cells on the ventral
side of the retina proliferate and generate neural connections to the thalamus
on the same side as the eye (
Hoskins, 1986, 1990; Marsh-Armstrong et al.,
1999
). This new ipsilateral projection contributes to bilateral vision for the
frog. The preferential action of thyroid hormone on the ventral retina is due
to the expression of
type III deiodinase
(
D3
) in the dorsal retina (
Marsh-
Armstrong et al., 1999
). D3 inactivates both T
4
and T
3
, preventing interac-
tion with their receptor.
