  The first cell fate decisions in the developing fly embryo are made very
rapidly : hunchback genes decide in a few minutes whether a given nucleus
follows the anterior or the posterior developmental blueprint by reading out
the positional information encoded in the Bicoid morphogen. This developmental
system constitutes a prototypical instance of the broad spectrum of regulatory
decision processes that combine speed and accuracy. Traditional arguments based
on fixed-time sampling of Bicoid concentration indicate that an accurate
readout is not possible within the short times observed experimentally. This
raises the general issue of how speed-accuracy tradeoffs are achieved. Here, we
compare fixed-time sampling strategies to decisions made on-the-fly, which are
based on updating and comparing the likelihoods of being at an anterior or a
posterior location. We found that these more efficient schemes can complete
reliable cell fate decisions even within the very short embryological
timescales. We discuss the influence of promoter architectures on the mean
decision time and decision error rate and present concrete promoter
architectures that allow for the fast readout of the morphogen. Lastly, we
formulate explicit predictions for new experiments involving Bicoid mutants.
