A re-inducible genetic cascade patterns the anterior-posterior axis of insects in a threshold-free fashion

Gap genes mediate the division of the anterior-posterior axis of insects into different fates through regulating downstream hox genes. Decades of tinkering the segmentation gene network of the long-germ fruit fly Drosophila melanogaster led to the conclusion that gap genes are regulated (at least initially) through a threshold-based French Flag model, guided by both anteriorly- and posteriorly-localized morphogen gradients (bicoid and caudal, respectively). In this paper, we show that the expression patterns of gap genes in the intermediate-germ beetle Tribolium castaneum are mediated by a self-regulatory and threshold-free Speed Regulation mechanism, guided by a posterior gradient of the transcription factor caudal. We show this by re-inducing the gap gene cascade at arbitrary points in time by simply re-inducing the leading gene in the gap gene cascade (namely, hunchback). This demonstrates that the gap gene network is self-regulatory and is primarily under the control of a posterior morphogen in short- and intermediate-germ insects.