Fig. 7. Model depicting the regulatory interactions of the posterior gene network in Periplaneta americana.

(A) In the posterior region of the post-blastoderm embryo there are two clusters of cells expressing Pa-Wnt1 (red). Pa-Wnt1 activates the expression of zygotic Pa-cad (green) in the neighbouring cells. This activation is required for the posterior organisation and establishment of the growth zone (GZ). Pa-Dl has no functional role at the posterior end at this stage. (B) During germ band elongation the embryo grows from the posterior and new segments are laid down sequentially from the GZ. The arc of Pa-Wnt1 at the posterior GZ (P) has two main functions: 1) maintaining Pa-cad expression in a broad domain (green) in the mid-GZ (M) and 2) promoting Pa-Dl expression (purple) in the posterior tip (PT). Upon activation, Pa-Dl forms a positive feedback loop with Pa-Wnt1, thereby maintaining the expression of each other. As germ band elongation proceeds cyclic waves of Pa-Dl expression emerge from the posterior tip and pass through the Pa-cad domain (dashed arrow). During this process Pa-cad dampens the expression of Pa-Dl, thus inhibiting the formation of segmental stripes but allowing the wave of expression to travel through. Pa-Dl stripes form outside the Pa-cad domain in the anterior GZ (A) and activate segmental stripes of Pa-en (blue). Pa-en then activates a segment polarity gene network involving Pa-Wnt1 eventually leading to segment border formation. Thus, the interaction between the posterior gene network and the N-dependent oscillator regulates growth and segmentation in Periplaneta during germ band elongation.