ASYMMETRIC LOCALIZATION OF DROSOPHILA PAIR-RULE TRANSCRIPTS FROM DISPLACED NUCLEI - EVIDENCE FOR DIRECTIONAL NUCLEAR EXPORT

Drosophila pair-rule transcripts accumulate exclusively apical of the layer of peripheral nuclei in syncytial blastoderm stage embryos. Here , we use aneuploid embryos to test zygotic gene requirements for pair- rule transcript localization. As apical localization is maintained in all genotypes tested, the required components must be maternally encod ed. In aneuploid embryos with multiple layers of cortical nuclei, pair -rule transcripts lie apical of both superficial and internalized nucl ei. In the latter case, the transcripts are 'pseudo-apical', i.e. apic al of the nuclei from which they derive, but basal of superficial nucl ei. We show that internalized nuclei maintain their apico-basal nuclea r orientation, and that they lack the apical cytoskeletal assemblies w hich lie adjacent to superficial nuclei. These results support a mecha nism of localizing pair-rule transcripts by directional (vectorial) nu clear export.