Fig. 1.

The core mechanism of speed regulation model is flexible, can pattern elongating and nonelongating tissues, and can explain short-germ to long-germ evolution in insects. (A) Core mechanism of speed regulation model: The speed of sequential activation of states (or fates) is regulated by the concentration of a speed regulator. Different states are shown in different colors (in the order of sequential transitioning): blue, red, green, gold, and brown. The speed regulator is shown in gray. (B) Speed regulation model can operate in a gradient-based mode to pattern nonelongating tissues (Left) and in a wavefront-based mode to pattern elongating tissues (Right). (C) AP fates (shown in different colors) are specified during two different phases of insect early development: blastoderm and germband. Most AP fates are specified during the germband stage in short-germ insects (Left), and during the blastoderm stage in long-germ insects (Right). In intermediate-germ insects (Middle), anterior fates are specified in the blastoderm, whereas posterior fates are specified in the germband. (D) Presumed expression of speed regulator (gray) in insects. Blastoderm can be patterned with the gradient-based mode of speed regulation model, whereas germband can be patterned with the wavefront-based mode. (E and F) Computer simulation of two strategies for short- to intermediate- to long-germ evolution based on the speed regulation model. (E) A short-germ insect can evolve into an intermediate germ by delaying the blastoder-to-germband transition; similarly, an intermediate-germ insect can evolve into a long germ by introducing a further delay to blastoderm-to-germ transition (Movie S1). (F) A short-germ insect can evolve into an intermediate germ by boosting the speed regulator; similarly, an intermediate-germ insect can evolve into a long germ by further boosting the speed regulator (Movie S2).