Pattern stability in the insect segment : I. Pattern reconstitution by intercalary regeneration and cell sorting inDysdercus intermedius Dist.

1. The mechanisms of pattern reconstitution in the abdominal segment of insects were studied by transplantations between the wild type and a colour mutant of the bugDysdercus intermedius. In this species, anterior and posterior regions of the segment differ in pigmentation. Thus transplants are marked by donor genotype and by region-specific pigmentation. Moreover, the translucent cuticle allows direct and continuous observation of the behaviour of host and transplant cells. 2. Transplants rotated by 90° re-rotate so as to approach or even restore the original relations with their surroundings, whereas transplants completely surrounded by cells from another segment region tend to contract. Both observations indicate differences in adhesiveness between cells from different regions of a segment. 3. When transplants rotated by 180° or shifted to different antero-posterior levels in a segment cannot approach their original situations by rerotation, then conspicuous folds appear before moults in the regions where cells from different segment levels meet, and these regions become pigmented according to the levels which were lacking in between. This indicates that an intercalary regenerate is formed which eliminates the discontinuity in positional values created by transplantation. 4. The intercalary regenerate forms essentially from cells representing the more posterior segment level, irrespective of whether these are host or transplant cells. 5. It is suggested that pattern reconstitution in the abdominal segment can be explained in terms of cell behaviour, assuming cell sorting and intercalary regeneration. 6. This interpretation is discussed in the light of a current hypothesis which assumes that displaced cells are re-programmed in situ under the influence of a diffusible morphogen.