  In a developing embryo, information about the position of cells is encoded in
the concentrations of "morphogen" molecules. In the fruit fly, the local
concentrations of just a handful of proteins encoded by the gap genes are
sufficient to specify position with a precision comparable to the spacing
between cells along the anterior--posterior axis. This matches the precision of
downstream events such as the striped patterns of expression in the pair-rule
genes, but is not quite sufficient to define unique identities for individual
cells. We demonstrate theoretically that this information gap can be bridged if
positional errors are spatially correlated, with relatively long correlation
lengths. We then show experimentally that these correlations are present, with
the required strength, in the fluctuating positions of the pair-rule stripes,
and this can be traced back to the gap genes. Taking account of these
correlations, the available information matches the information needed for
unique cellular specification, within error bars of ~2%. These observation
support a precisionist view of information flow through the underlying genetic
networks, in which accurate signals are available from the start and preserved
as they are transformed into the final spatial patterns.
