The reinforcement-extinction process of selector gene activity: a positive feed-back loop and cell-cell interactions in Ultrabithorax patterning.

Most viable alleles of homeotic genes cause partial transformations within given lineages in a topographically specific fashion. We study this phenomenon as a way to understand the normal mechanisms involved in the spatial regulation of homeotic gene expression. To this end we have investigated the distribution of Ultrabithorax (Ubx) proteins in imaginai discs mutant for hypomorphic and neomorphic alleles of Ubx and alleles of trans-acting genes. We find that the morphological discontinuities observed in the adult transformations are caused by corresponding new patterns of the Ubx proteins in the imaginai anlagen. These novel patterns of Ubx proteins are understood as a consequence of a process of reinforcement-extinction of Ubx expression. The evidence suggesting that this process results from a positive feed-back loop and cell-cell interactions is discussed.