Biology Reference
In-Depth Information
3.3. Direct development in amphibians
One of the most striking examples of heterochrony of metamorphosis comes
from the terrestrial frog Eleutherodactylus coqui (see Chapter 5 in this issue). In
this species, a small froglet hatches directly from the egg without any free-
living tadpole stage. This situation is called direct development. In this species,
some of the larval features, as lateral line organs or some cranial features do not
appear at all during development but others, such as the tail, are maintained
within the egg and are transformed before hatching ( Callery & Elinson, 2000 ).
E. coqui and Xenopus exhibit similar TRs expression profile. TR
expression is
detectable at early developmental stages and remains fairly constant during de-
velopment. There is a peak of TR
a
at the same time as the organ remodeling
occurs ( Callery & Elinson, 2000 ). T3 treatment increases tail resorption in the
developing E. coqui ( Kulkarni, Singamsetty, & Buchholz, 2010 ), while goitro-
gen inhibits E. coqui development and their effect can be rescued by T3 treat-
ment ( Callery & Elinson, 2000 ), as for classical amphibian metamorphosis in
Xenopus . One of the features of the E. coqui egg is the large size of the yolk
compared to other amphibian species. For instance, E. coqui eggs are 20 times
the volume of X. laevis egg, mainly due to the yolk size ( Buchholz et al., 2007 ),
whereas the adult E. coqui is smaller than X. laevis , indicating that important
energy reserves seem necessary for direct development. Thus, in this species, a
true metamorphosis occurs in the embryonic period, within the egg. This can
also be used as a proxy to explain the situation in other vertebrate species in
which no metamorphosis is visible.
b
4. TH-REGULATED METAMORPHOSIS IS AN ANCESTRAL
CHORDATE CHARACTER
The molecular characterization of metamorphosis in model species
( Xenopus but also some teleost fishes like the flatfishes) has shown that this
process is governed in all these species by TH that triggers the changes
observed at the morphological and physiological levels via their receptors
TR
. Recent analyses of metamorphosis conducted in invertebrate
chordates, specifically in the cephalochordate amphioxus ( Branchiostoma ), have
revealed that in this basal chordate, metamorphosis is also triggered by TH and
its receptor ( Paris et al., 2008, 2010 ). Amphioxus undergoes metamorphosis
with a spectacular morphological change: the asymmetric larval mouth and
gill slits becoming symmetric in the adult. This is accompanied by an ecolog-
ical change, as the pelagic larva becomes a benthic filter-feeding adult living in
sand. Amphioxus exhibits a peak of its unique TR expression just before
metamorphosis, although the canonical forms of TH (i.e., T3 and T4) do
a
and TR
b
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