Maternal-effect genes that alter the fate map of the Drosophila blastoderm embryo

The pattern of segmentation in the Drosophila embryo is controlled by at least 25 zygotically active genes and at least 20 maternally active genes. We have examined the pattern of expression of the protein product of the zygotically active segmentation gene fushi tarazu (ftz) at the cellular blastoderm stage in progeny of mutant females homozygous for each of six maternal-effect segmentation genes to observe the early effects of the maternal-effect genes on zygotic gene expression. The genes included exuperantia (a member of the anterior class of maternal-effect segmentation genes); staufen and vasa (members of the posterior class); and torso, trunk, and fs(1)N (members of the terminal class). Mutations in the genes caused a disruption of the normal pattern of ftz stripes in regions of the embryo where gene activity is known to be required. The ftz stripes provide a marker for segmental determination at the cellular blastoderm stage, making it possible to correlate aberrant patterns of ftz protein with defects in cuticle morphology at the end of embryogenesis. ftz protein expression in progeny of females mutant for combinations of the above genes was also examined. The changes in the ftz pattern in progeny of females doubly mutant for genes of the anterior and terminal classes or of the posterior and terminal classes can largely be understood as the result of the additive effects of the single mutations. In contrast, clearly nonadditive effects on the ftz pattern were seen when a mutation in a gene of the anterior class (exuperantia) was combined with mutations in posterior class genes.