Krüppel requirement for knirps enhancement reflects overlapping gap gene activities in the Drosophila embryo

SEGMENTAL pattern formation in Drosophila proceeds in a hierarchical manner whereby the embryo is stepwise divided into progressively finer regions until it reaches its final metameric form1,2. Maternal genes initiate this process by imparting on the egg a distinct antero-posterior polarity and by directing from the two polar centres the activities of the zygotic genes3. The anterior system is strictly dependent on the product of the maternal gene bicoid (bcd), without which all pattern elements in the anterior region of the embryo fail to develop4. The posterior system seems to lack such a morphogen. Rather, the known posterior maternal determinants simply define the boundaries within which abdominal segmentation can occur, and the process that actively generates the abdominal body pattern may be entirely due to the interactions between the zygotic genes5–7. The most likely candidates among the zygotic genes that could fulfil the role of initiating the posterior pattern-forming process are the gap genes, as they are the first segmentation genes to be expressed in the embryo8–10. Here we describe the interactions between the gap genes Kruppel (Kr), knirps (kni) and tailless (tll). We show thatkniexpression is repressed by til activity, whereas it is directly enhanced by Kr activity. Thus, Kr activity is present throughout the domain of kni expression and forms a long-range protein gradient, which in combination with kni activity is required for abdominal segmentation of the embryo.