Fig. 9.

A conserved regulatory framework for arthropod segmentation. (A) Schematic comparison of timing factor expression during Drosophila versus Tribolium segmentation. Kymographs depict expression along the ectodermal AP axis over time. Dotted lines mark blastoderm-to-germband transition. Neural and segment-polarity expression domains are not drawn. (B) Proposed scenario for the evolution of arthropod segmentation: ancestrally, timing factors regulated AP axis extension and ‘segmentation’ genes had neural functions (top); the evolution of short-germ segmentation involved segmentation genes coming under the spatiotemporal regulation of the timing factors within the germband (middle); the evolution of long-germ segmentation involved the timing factors being expressed earlier in development and the pair-rule genes being spatially regulated by the gap genes (bottom).