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a chordate Bauplan with a tail, fins, notochord, dorsal neural cord, and gill clefts.
When the larva detects an environmental cue, it enters metamorphosis. In contrast
with other metamorphosizing organisms, the transition from the larval stage to the
juvenile stage in ascidians is associated with a developmental and behavioral degra-
dation: larval muscles, the tail, fins, and the dorsal nerve cord are lost, and the brain
becomes a neural ganglion. In response, they shift to a sedentary life by attaching to
a solid substrate. For these reasons, it is said that ascidians go through a retrogres-
sive metamorphosis. In the early 1980s, it was concluded that the metamorphosis in
ascidians is controlled by the nervous system ( Burke, 1983 ).
Free-swimming amphibian larvae have fishlike bodies, vertically flattened tails,
internal gills, cartilaginous skeletons, etc. They are induced to enter metamorphosis
by various internal cues, such as growth beyond a neurally determined threshold, and
external cues, such as water temperature and availability, photoperiod, etc. ( Denver,
1997 ). These external signals are received, integrated, and processed in the amphib-
ian CNS. The information for starting metamorphosis in amphibians flows via the
hypothalamus-pituitary-thyroid axis. In a simple form, the updated signal cascade
for metamorphosis in amphibians looks as follows:
Signals from the nonhypothalamic brain → hypothalamic thyroid-releasing hormone
(TRH) → pituitary thyroid-stimulating hormone (TSH) → thyroid hormone (TH) → matrix
metalloproteinases (MMPs) → integrins → signals for gene expression and apoptosis
Along the TRH, another hypothalamic neuropeptide, the corticotropin-releasing
hormone (CRH) also stimulates secretion of TSH and the TH. Injections of this neu-
ropeptide accelerate metamorphosis in several amphibian species ( Denver, 1999 ).
The neuroendocrine mechanism of metamorphosis is conserved across amphibian
taxa ( Denver, 2002 ).
Metamorphosis induces radical changes in amphibian morphology, including the
development of lungs, shortening of intestines, repositioning of eyes, restructuring of
the nervous system to adapt for the new way of life, gradual loss of the tail, etc.
With the exception of ascidians, the development of the species-specific body
plan during metamorphosis is preceded by an evolutionarily lower body plan. In
contrast, the metamorphosis of ascidians leads to an adult morphology, physiology,
and behavior that is simpler than that of the larva's. Metamorphosis is an amazing
example of the dexterity of animals to switch to different developmental programs.
This certainly contradicts the prevailing opinion that an egg or a zygote is provided
with a program that determines development up to the adult stage. This gains more
significance when one remembers the ease with which some metamorphosizing
amphibians can switch to a direct mode of development, or even skip metamorpho-
sis altogether. Is it possible that the same egg/zygote contains the programs for two
different Bauplans, and sometimes even a program for skipping its species-specific
Bauplan?
Metamorphosizing species, besides their own developmental program, have incor-
porated and executed ancestral developmental programs. Amazingly, like biological
Houdinis, they shift the gears of development both forward (insects and amphibians)
and backward (ascidians).
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