Biology Reference
In-Depth Information
circadian pathways exert temporal control over the production and release of ecdysone.
We will then take a look at molecular and genetic data that revealed the presence of two
circadian clocks, located in the brain and the prothoracic gland, that regulate eclosion
rhythms in Drosophila. In this context, we will also review recent developments that
examined how the ecdysone hierarchy delays the differentiation of the crustacean car-
dioactive peptide (CCAP) neurons, an event that is critical for the timing of ecdysis and
eclosion. Finally, we will discuss some recent findings that transformed our understand-
ing of JH function.
1. GENERAL CONSIDERATIONS: THE PROBLEM OF TIME
IN DEVELOPMENT
It appears that everything must exist within the framework of time,
and thus it comes as no surprise that life itself is inexorably intertwined with
the passing of time. While this may apply to both for our daily lives as well as
the grand scale of evolution, we will deal in this review with a more narrow
question: How do organisms keep track of time so that they can ensure the
proper chronology of distinct developmental events?
When we consider how time intersects with biology, two major catego-
ries come to mind. First, the ability of organisms to adapt to natural rhythms,
such as earth's rotation that results in day and night (circadian rhythms), or
the changes of the seasons, forcing plants and animals alike to adapt to annual
cycles of the climate and different lengths of daylight (photoperiodism).
Very different from these cyclical processes is the second category, which
only applies to developing organisms: How are developmental programs
controlled in such a fashion that they are turned on and off only when they
are supposed to? Given the complexity of coordinating multiple parallel pro-
grams during the development of higher organisms, one would expect that
any given sequence of events must follow a specific order, and that the pre-
cocious or delayed execution of developmental events could have poten-
tially fatal consequences.
At first glance one might object and argue that developmental programs
are “hardwired” and that the sequence of these events may be genetically
programmed, much like the domino effect. If true, this would eliminate
the need for a time-keeping mechanism during development. While many
developmental events will follow indeed such a genetically determined
sequence, a developing organism must have the ability to integrate environ-
mental variables, such as nutrient availability, temperature, and the daily
changes in daylight, which in turn may affect
the timing of certain
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