Precise spatial scaling in the early fly embryo*

Animals vary more in size than in proportions; a possible quantitative version of this observation would that organisms exhibit scaling, so that the dimensions of different body segments all vary in proportion to the overall size of the organism. If this were true, pattern formation in biological systems would be qualitatively different from that in the non-biological pattern forming systems that we understand, such as Rayleigh-Benard convection and directional solidification. A relatively pure version of this question is accessible in the early development of insect embryos, where a “blueprint” for the final segmented body plan is visible in striped patterns of gene expression. We measure the positions of these stripes in an ensemble of 100+ embryos from a laboratory strain of Drosophila melanogaster, under controlled conditions. These embryos vary in length by only 4% (rms), yet we can resolve the scaling of stripe positions with length. The resulting 1% accuracy of relative positions is so high as to exclude alternatives, such as combinations of unscaled signals from the two ends of the embryo.