Localization in situ of polyhomeotic transcripts in Drosophila embryos reveals spatially restricted expression beginning at the blastoderm stage.

Polyhomeotic is a member of a group of genes, the Pc-group responsible for the maintenance of gene expression during development. In particular, the Pc-group of genes is involved in the correct expression of homeotic genes of the bithorax and Antennapedia complexes. Molecular analysis reveals that the Pc-group genes function relatively late in development, once homeotic gene expression has been correctly initiated. This initiation of homeotic gene expression depends on interaction between genes in the segmentation gene hierarchy, the gap and pair-rule genes. The in situ analysis presented here indicates that polyhomeotic transcripts are expressed in a spatially restricted fashion early in development. This blastoderm expression is under the control of genes in the segmentation hierarchy. Given these results, and the role of polyhomeotic in the correct maintenance of homeotic gene expression, I propose that polyhomeotic expression may relay the initiation of homeotic gene expression with other mechanisms involved in the maintenance of this expression, involving the other Pc-group genes.