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Evolutionarily conserved developmental pathways
Diurnal rhythms, the biological rhythms that recur daily in response to each day's dark and light periods, require (1) a system for response to light, (2) the transduction of information to neurons whose activities fluctuate in response to the light dark cycle, (3) transduction of centrally generated signals to the periphery and (4) regulated behaviorial routines. It is not known how much of this system is evolutionarily conserved, but at the very least, the central element in the photoperiod response (the cycling of a neural clock) is known to be conserved between flies and mammals.
The gene controlling this neural clock in Drosophila, called period(per), is a Pas domain protein that, unlike most Pas domain proteins, has no basic helix-loop-helix domain. per transcription is regulated by a Period-Timeless heterodimer. Per first accumulates in the cytoplasm, increasing during daylight hours and reaching peak levels at night. It then enters the nucleus during a restricted part of the circadian cycle (near the middle of the dark period in the light-dark cycle). The delay between Per synthesis and entry into the nucleus is critical to the timing of the circadian cycle. By the end of the dark period, transcripts have reached their lowest level (Curtin, 1995).
The Clock protein of mammals (King, 1997), like Drosophila Period, is a Pas domain protein, but it also has a bHLH domain. The presence of Pas domains in both Drosophila and mammalian clock proteins suggests that the pivotal proteins that cycle in response to periods of light and dark are conserved in evolution. This conservation does not extend to the Neurospora, whose clock protein, called Frequency (Garceau, 1997), bears no sequence relationship to either the Drosophila or mammalian clock proteins. Nevertheless, Frequency levels fluctuate in response to light-dark cycles in a manner similar to that of Drosophila Period and mammalian Clock.
To what extent are other systems, also involved in photoperiod response, conserved? Like Period, mammalian Clock is transcibed in a number of neuronal and non-neuronal tissues. What are the functions of clock proteins in non-neuronal tissues? How much of the light reception and signal transduction apparatus is conserved, and what novel systems have evolved? Are there common genes regulated in Neurospora, flies and mammals? These are only a few of the questions that will drive research in the years to come.
Curtin, K., Huang, Z.J. and Rosbash, M. (1995). Temporally regulated nuclear entry of the Drosophila period protein contributes to the circadian clock. Neuron 14: 365-372. PubMed Citation: 7857645
Garceau, N. Y., et al. (1997). Alternative initiation of translation and time-specific phosphorylation yield multiple forms of the essential clockprotein Frequency. Cell 89: 469-476. PubMed Citation: 9150146
King, D. P., et al. (1997). Positional cloning of the mouse circadian clock gene. Cell 89: 641-653. PubMed Citation: 9160755
date revised: 1 July 97
Developmental Pathways conserved in Evolution
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