Evidence for the temporal regulation of insect segmentation by a conserved set of developmental transcription factors

11 Long-germ insects, such as the fruit fly Drosophila melanogaster, pattern their segments 12 simultaneously, whereas short germ insects, such as the beetle Tribolium castaneum, pattern 13 their segments sequentially, from anterior to posterior. While the two modes of segmentation 14 at first appear to be very different, many details of segmentation gene expression are 15 surprisingly similar between long-germ and short-germ species. Collectively, these 16 observations hint that insect segmentation may involve fairly conserved patterning 17 mechanisms, which occur within an evolutionarily malleable spatiotemporal framework. 18 Based on genetic and comparative evidence, we now propose that, in both Drosophila and 19 Tribolium embryos, the temporal progression of the segmentation process is regulated by a 20 temporal sequence of Caudal, Dichaete, and Odd-paired expression. These three transcription 21 factors are broadly expressed in segmenting tissues, providing spatiotemporal information 22 that intersects with the information provided by periodically-expressed segmentation genes 23 such as the pair-rule factors. However, they are deployed differently in long-germ versus 24 short-germ insects, acting as simple timers in Drosophila, but as smooth, retracting 25 wavefronts in Tribolium, compatible with either gap gene-based or oscillator-based generation 26 of periodicity, respectively. 27