Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1182 Chapter 21: Development of Multicellular Organisms
model, the nuclear-to-cytoplasmic ratio might be measured through the titration
of a transcription repressor against the increasing amount of nuclear DNA. The
total amount of repressor would stay constant during cleavage divisions, but the
amount of repressor per genome would decrease, falling by a half with each round
of DNA synthesis, until loss of repression allowed the zygotic genome to become
transcriptionally active. The newly synthesized transcripts include the miRNAs
that recognize many of the transcripts deposited in the egg by the mother, directing
their translational repression and rapid degradation.
Hormonal Signals Coordinate the Timing of Developmental
Transitions
We have so far emphasized timing mechanisms that operate locally and separately
in the different parts of the embryo, or in specific subsystems of the molecular
control machinery. Evolution has tuned each of these largely independent
processes to run at an appropriate rate, matched to the needs of the organism as
a whole. For some purposes, however, this is not enough: a global coordinating
signal is required. This is especially true where changes have to occur throughout
the body in response to a cue that depends on the environment. For example,
when an insect or amphibian undergoes metamorphosis—the transition from
larva to adult—almost every part of the body is transformed. The timing of metamorphosis
depends on external factors such as the supply of food, which determines
when the animal reaches an appropriate size. All the bodily changes have
to be triggered together at the right time, even though they are occurring in widely
separated sites. The coordination in such cases is provided by hormones—signal
molecules that spread throughout the body.
The metamorphosis of amphibians provides a spectacular example. During
this developmental transition, amphibians switch from an aquatic to a terrestrial
life. Larva-specific organs such as gills and tail disappear, and adult-specific
organs such as legs form. This dramatic transformation is triggered by thyroid
hormone, produced in the thyroid gland. If the gland is removed or if thyroid hormone
action is blocked, metamorphosis does not occur, although growth continues,
producing a giant tadpole. Conversely, a dose of thyroid hormone given to a
tadpole by an experimenter can trigger metamorphosis prematurely.
The thyroid hormone is distributed through the vascular system and induces
changes throughout the animal by binding to intracellular nuclear hormone
receptors, which regulate hundreds of genes. This does not mean, however, that
target tissues all respond in the same way to the hormone: organs differ not only
in their levels of thyroid hormone receptors and levels of extracellular proteins
that locally regulate the amount of active hormone, but also in the sets of genes
that respond. Thyroid hormone induces muscle in the limbs to grow and muscle
in the tail to die. The timing of the responses also differs: for example, the legs
form early in response to a very low concentration of circulating hormone, but it
requires a high level of the hormone to induce resorption of the tail.
A surge of thyroid hormone triggers metamorphosis, but how is the timing of
the surge controlled? One mechanism depends on coupling hormone synthesis
to the size of the thyroid gland, which reflects the size of the tadpole. Only when
the gland attains a certain size does it produce enough thyroid hormone to initiate
metamorphosis. However, environmental cues other than nutrition also play a
part: conditions such as temperature and light are sensed by the nervous system,
which regulates the secretion of another tier of hormones (neurohormones) that
stimulate the secretion of thyroid hormone. Thus, tadpole-intrinsic factors such
as size combine with environmental factors to determine when metamorphosis
begins.
Environmental Cues Determine the Time of Flowering
Another striking example of environmentally controlled developmental timing is
the flowering of plants. Flowering involves a transformation of the behavior of the