Spatial and temporal patterns of Krüppel gene expression in early Drosophila embryos

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Abstract
Determination of the segmental pattern in the Drosophila larva occurs at the blastoderm stage of embryogenesis1-3. Many of the genes involved in the establishment of segmentation have been identified on the basis of mutations, and the cuticular phenotypes of the typically lethal mutant embryos have allowed the classification of the segmentation genes and the assignment of the domain of action for each of them4-7. Mutations of the gap class of segmentation genes result in deletions of contiguous segments in defined regions7. Kr is required specifically for thoracic and anterior abdominal segmentation7-10. Embryos homozygous for strong (amorphic) Kr alleles show a deletion of all three thoracic and five abdominal segments that are replaced by a partial duplication of posterior abdominal segments with reversed polarity7-10. In addition to the segmentation defect, the malpighian tubules are absent9. The phenotypes of a series of hypomorphic Kr alleles show progressively smaller gaps in the segmental pattern, with the weakest lacking meso- and metathorax and one abdominal segment8,10. That Kr+ gene requirement for normal segmentation is purely zygotic and is limited to the embryo period from blastoderm to extended germ-band stage has been inferred from combined morphological and genetic studies7,10 and from the developmental profile of Kr+ transcripts8. Here, we describe in situ hybridization experiments showing the spatial and temporal patterns of Kr gene expression in wild-type embryos and cytoplasm injection experiments that reveal localization of Kr+ activity in wild-type embryos and the spatial requirement for Kr+ activity in Kr embryos. We relate these experiments to Kr mutant phenotypes and to other genes involved in the establishment of the embryonic body plan.