Interactions of the Polycomb group of genes with homeotic loci of Drosophila

Members of the Polycomb (Pc) group of genes are required for the correct determination of segment identity, and are thought to be negative regulators of the bithorax and Antennapedia complexes. This hypothesis has been tested molecularly for only some members of the Pc group. Here, we examine the distribution of Ultrabithorax (Ubx), Antennapedia (Antp), and Sex combs reduced (Scr) proteins in the epidermis, central nervous system, and midgut of embryos homozygous for mutations in ten Pc group genes. We show that zygotic loss of most Pc group genes causes ectopic expression of Ubx and Antp, but that there are differences in time and tissue-specificity. Five Pc group mutations lack midgut constrictions and also exhibit ectopic or suppressed Ubx expression and suppression of Antp expression. Distribution of Antp is upset earlier than distribution of Ubx in the central nervous system of every Pc group mutant affecting both genes. Loss of the zygotic products of Polycomb, extra sex combs, and Additional sex combs cause ectopic expression of Scr in epidermis, and all Pc group genes except Psc have suppressed Scr expression in the nervous system. These results are discussed with respect to the function of the Pc group.