MODELS OF BIOLOGICAL PATTERN-FORMATION - COMMON MECHANISM IN PLANT AND ANIMAL DEVELOPMENT

Earlier proposed models for primary pattern formation, for gene activa tion and for segmentation are summarized and compared with recent mole cular-genetic observations. A model for head, foot, tentacle and bud f ormation in Hydra illustrates that complex patterns can be reliably ge nerated. Stable cell determination requires autocatalytic (autoregulat ory) genes. Segmentation in insects has been proposed to result from a reiteration of (at least three) cell states. Their patterning is achi eved by a mutual activation of cell states that locally exclude each o ther. A model for accretion of new segments by proliferation at the po sterior pole is proposed that accounts for the generation of a periodi c and a sequential pattern in register with each other. The assumption of a process analogous to segmentation in plants can account for the initiation of leaves with an intrinsic polarity that eventually leads to the upper and lower leaf surfaces. The model accounts also for the formation of axillary buds in correct relation to a leaf and for the m uch smaller spacing of leaves within a whorl when compared with the sp acing between two successive whorls along the shoot. It is concluded t hat the generation of complex structures in distantly related organism s may be based on similar mechanisms.