Different requirements for homeotic genes in the soma and germ line of Drosophila.

Some homeotic genes of Drosophila are active only in particular developmental compartments ("selector" genes) whereas others are active in all compartments and are responsible for regulating selector gene expression. Using pole cell transplants to generate mosaic females with mutant germ lines, we examine the extent to which genes of both classes are required during oogenesis. We find that none of four selector genes (Ultrabithorax+, Antennapedia+, Sex combs reduced+, and engrailed+) is required for the development of viable eggs, and none contributes detectable product to the fertilized zygote. Similarly, neither of two regulatory genes (Polycomb+ and extra sex combs+) are required for normal oogenesis. However, both are active in the germ line and contribute functional gene product to the embryo. These results suggest that selector genes are inactive in the germ line where they may be regulated by a different mechanism than in the soma.