To examine the requirements for X-chromosomal transcription during precellular stages of Drosophila embryogenesis, attached X-chromosomes and XY translocations were used to generate embryos deficient for large cytologically defined regions of that chromosome. Embryos that lack all X-chromosome material (‘nullo-X embryos’) develop normally to the cycle-14 syncytial blastoderm stage, but fail to partition their nuclei to single cells during cellularization. The cellularization defects can first be detected in the abnormal distribution of cortical actin and nuclei during early cycle 14. The same defects are produced by deletions of only a single region on the X-chromosome, between 6F and 7A. Nullo-X embryos carrying a duplication of this region cellularize and develop normally to the onset of gastrulation.

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