cappuccino and spire: two unique maternal-effect loci required for both the anteroposterior and dorsoventral patterns of the Drosophila embryo.

Abstract

cappuccino and spire are unique Drosophila maternal-effect loci that participate in pattern formation in both the anteroposterior and dorsoventral axes of the early embryo. Mutant females produce embryos lacking pole cells, polar granules, and normal abdominal segmentation. They share these defects with the posterior group of maternal-effect genes. Although embryos are defective in abdominal segmentation, in double mutant combinations with Bicaudal D, abdominal segments can be formed in the anterior half of the egg. This indicates that embryos produced by mutant females contain the 'posterior determinant' required for abdominal segmentation (Nüsslein-Volhard et al. 1987) and suggests that the wild-type gene products are not required for production of the posterior determinant but, rather, for its localization or stabilization. The vasa protein, a component of polar granules, is not localized at the posterior pole of mutant egg chambers or embryos, providing additional support for the hypothesis that localization to or stabilization of substances at the posterior pole of the egg chamber is defective in mutant females. Females mutant for the strongest alleles also produce dorsalized embryos. Phenotypic analysis reveals that these dorsalized embryos also have abdominal segmentation defects. The mutant phenotypes can be ordered in a series of increasing severity. Pole cell formation is most sensitive to loss of functional gene products, followed by abdominal segmentation, whereas normal dorsoventral patterning is the least sensitive to loss of functional gene products. In addition, mutant females contain egg chambers that appear to be dorsalized, resulting in the production of eggs with dorsalized eggshells. Germ-line mosaics indicate that cappuccino and spire are required in the oocyte-nurse cell complex. This suggests that the eggshell phenotype results from altered pattern in the underlying germ cell. Also, we defined the epistatic relationships between several early patterning loci, on the basis of an analysis of the eggs and embryos produced by females doubly mutant for cappuccino or spire and other loci that affect the pattern of both the egg and the embryo. On the basis of our current knowledge of the genes involved in this process, we formulated a working model for the early steps in dorsoventral patterning.