Carl Troein, James C. W. Locke, Matthew S. Turner and Andrew J. Millar
Weather and seasons together demand complex biological clocks
Current Biology 19, 1961–1964 (2009)
Abstract:
The 24-hour rhythms of the circadian clock allow an organism to anticipate
daily environmental cycles, giving it a competitive advantage. Although
clock components show little protein sequence homology across phyla,
multiple feedback loops and light inputs are universal features of clock
networks. Why have circadian systems evolved such a complex structure? All
biological clocks entrain a set of regulatory genes to the environmental
cycle, in order to correctly time the expression of many downstream
processes. Thus the question becomes: What aspects of the environment, and
of the desired downstream regulation, are demanding the observed complexity?
To answer this, we have evolved gene regulatory networks in silico,
selecting for networks that correctly predict particular phases of the day
under light/dark cycles. Gradually increasing the realism of the
environmental cycles, we have tested the networks for the minimal
characteristics of clocks observed in nature: oscillation under constant
conditions, entrainment to light signals, and the presence of multiple
feedback loops and light inputs. Realistic circadian gene networks are found
to require a nontrivial combination of conditions, with seasonal differences
in photoperiod as a necessary but not sufficient component.