  Continuous and Boolean models for the Drosophila segment polarity network
have shown that the system is able to maintain the wild-type pattern when
subjected to sustained changes in the interaction parameters and initial
conditions. Embryo development is likely to occur under fluctuating
environmental conditions. We use a well-established Boolean model to explore
the ability of the segment polarity network to resist transient changes. We
identify paths along which alternate unviable states are reached, and hence
critical nodes whose state changes lead the system away from the wild-type
state. We find that the system appears to be more sensitive to changes that
involve activation of normally inactive nodes. Through a simulation of the heat
shock response, we show how a localized perturbation in one parasegment is more
deleterious than a global perturbation affecting all parasegments. We identify
the sequence of events involved in the recovery of the system from a global
transient heat shock condition. Finally we discuss these results in terms of
the robustness of the system response.
