Maternal genes required for the anterior localization of bicoid activity in the embryo of Drosophila

The maternal gene bicoid (bcd) organizes anterior development by means of an activity localized at the anterior tip of the embryo. The mutant phenotypes of swallow swa and exuperantia (exu) share with weak bcd alleles the lack of anterior head structures. Cytoplasmic transplantations reveal a much reduced bcd + activity at the anterior tip of swa and exu embryos. In contrast to weak bcd alleles, removal of anterior cytoplasm has very little effect on the swa or exu phenotype. We suggest that in embryos mutant for exu or swa the bcd activity is not restricted to the anterior tip but extends toward more posterior positions. These conclusions are supported by changes in the fate maps of bcd, exu, and swa mutant embryos and by the ectopic expression of anterior structures in certain double-mutant phenotypes.