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across the animal taxa. It is impossible to reasonably relate changes in genes/regula-
tory sequences or the evolution of new genes to the evolution of animal Bauplans. To
overcome this difficulty, the genetic toolkit hypothesis was developed. According to
this theory, the patterns of gene expression, especially of
Hox
genes, rather than the
number or the nature of genes, mold the Bauplan.
For the sake of argument, let us take for granted that
Hox
genes are the true deter-
minants of the body plan. Rather than resolving anything, this assertion raises a for-
midable question.
Hox
genes, like all the nonhousekeeping genes, are induced by
extracellular signals. The temporal and spatial patterns of
Hox
expression, and con-
sequently body plans, are determined externally, not by
Hox
genes themselves. The
question “What determines the body plan?” continues to haunt us. Whatever the real
role of
Hox
genes in the development of the Bauplan might be, it is essential from a
causal viewpoint to identify the ultimate origin of the information that determines
the spatial and temporal patterns of the expression of the
Hox
gene (and of all the
nonhousekeeping genes for that matter).
Metamorphosis is one of the most convenient biological phenomena for studying
and identifying the source of information for determining Bauplan development. It
gives us a glimpse into some aspects of development that are hidden in the egg or
uterus. Metamorphosis is a widespread life history trait observed from the simplest
metazoans (cnidarians) to vertebrates (amphibians), and a brief review of metamor-
phosis in some animal groups may be helpful in understanding the source of infor-
mation that enables this amazing biological phenomenon.
The 1-mm-long, free-living larval planula of the cnidarian
Hydractinia echi-
nata
, consisting of just 10,000 cells, has secretory neurons that specialize in detect-
ing a specific bacterial cue in their environment. When the planula detects the cue,
its neurons secrete a number of neuropeptides from a signal system (
Schmich et al.,
1998
), represented by two opposing groups of neuropeptides, called GLWamide and
RFamide neuropeptides, that stimulate and inhibit metamorphosis in
Hydra
, respec-
tively (
Katsukura et al., 2004
).
The free-swimming molluscan larvae have a functioning nervous system that
includes an apical sensory organ (ASO), whose neurons secrete various neurotrans-
mitters (dopamine, norepinephrine, etc.). Larvae respond to specific chemical cues
by entering metamorphosis and the receptors of these cues are ASO neurons. When
the ASO neurons are destroyed by UV irradiation, the larvae do not respond to the
cue (
Hadfield et al., 2000
). Experimental depletion of these neurotransmitters inhib-
its metamorphosis, and elevation of their levels induces it in competent
Phestilla
larvae (
Pires et al., 1997
). In the process of metamorphosis, ASO disappears (
Ruiz-
Jones and Hadfield, 2011
; see
Figure 3.44
), suggesting that its function is to deter-
mine the time of metamorphosis and initiate it.
In insects, metamorphosis is determined primarily by the temporal and quanti-
tative patterns of expression and interaction of juvenile hormone (JH) secreted by
corpora allata and ecdysone secreted from the prothoracic gland. Both hormones
are strictly cerebrally regulated by many neuropeptides, but primarily by PTTH for
ecdysone and allatostatins and allatotropins for JH.
An essential behavioral component of insect metamorphosis is ecdysis, the shed-
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