ments of insects had been posited to be under the control of saw-tooth-shaped g r a d i e n t ~ ( ' ~ 3 ~ ~ ) 1A). However, the (Fig. formation of graded distributions that have discontinuities and that also show the correct regulatory behaviour, is difficult (or perhaps impossible) to achieve by molecular-realistic interactions. In contrast, we have proposed that segmentation results from qualitative and not from quantitative differences. According to this view, the internal pattern of a segment consists of a sequence of different cell states (Fig. 1B). As mentioned above, stable cell states result from selfactivation and mutual competition of genes. If two (or more) such states not only exclude each other locally but activate each other over longer ranges, these cell states need each other within a close neighbourhood, with local exclusiveness ensuring that the two states do not merge(19).According to this model, segmentation requires the following molecular ingredients (Fig. 1C): (i) Genes (or more general feedback loops) must exist that have a positive feedback on their own activation. (ii) These activities are locally exclusive; only one of the alternative genes can be active in a given cell. (iii) Long-ranging molecules
BioEssays – Wiley
Published: Sep 1, 1994
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