Precise Spatial Scaling in the Early Fly Embryo

The fruit fly embryo is a particularly well-suited system for studying the problem of biological pattern scaling. Embryos from a single inbred strain vary in length by 4%, yet structural markers such as the cephalic furrow are positioned with ~1% accuracy in scaled coordinates. The body plan along the anterior axis of the embryo emerges from a cascaded gene regulatory network: maternal inputs drive mutually interacting gap genes which in turn drive striped patterns of expression in pair rule genes. Previous work has shown that positions of the pair rule stripes, which are distributed along the length of the embryo, scale to embryo length with 1% precision or better. Here we show that boundaries of gap gene expression also scale, and more strongly that local gene expression levels provide information about relative position but no additional information about absolute position or embryo length. In contrast, we do not see scaling of the maternal inputs. These results point toward models in which the gap gene network has a near-degenerate manifold of solutions that can be "pinned" by the maternal inputs.