Patterning during pupal commitment of the epidermis in the butterfly, Precis coenia: the role of intercellular communication.

The commitment of cells to pupal development in the larvae of holometabolous insects can be prevented by treatment with juvenile hormone (JH) or a JH mimic during a critical period early in the last larval instar. By treating larvae of different ages with a JH mimic, pupal commitment of the epidermis of the butterfly, Precis coenia, was found to occur in a strict temporal and spatial progression, which was serially homologous and occurred independently in each segment. The mechanism underlying this sequence of pupal commitment was examined by cauterizing regions of the epidermis to observe the effects of local ablation on the pattern of pupal commitment revealed by treatment with the JH mimic. Cautery of the segmental site of origin of pupal commitment, the dorsal midline, suppressed pupal commitment in the rest of the operated segment, indicating that the midline has a special effect on commitment of the rest of the segment. Cautery off the midline produced asymmetries in the pattern of pupal commitment; when placed close to the midline, such cauteries prevented pupal commitment in the region "downstream" of the cautery, suggesting that a signal (diffusible or transducible) emanates from the midline. Finally, cautery of a circle around the midline inhibited pupal commitment only outside the circle, showing that cautery could act as a barrier to the passage of a signal coming from the midline. These results suggest that inductive as well as hormonal signals are involved in the regulation of pupal commitment in the epidermis of the lepidopteran, P. coenia.