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 encoded. In aneuploid embryos with multiple layers or cortical nuclei, pair‐rule transcripts lie apical of both superficial and internalized nuclei. In the latter case, the transcripts are ‘pseudo‐apical’, i.e. apical of the nuclei from which they derive, but basal of superficial nuclei. We show that internalized nuclei maintain their apico‐basal nuclear orientation, and that they lack the apical cytoskeletal assemblies which lie adjacent to superficial nuclei. These results support a mechanism of localizing pair‐rule transcripts by directional (vectorial) nuclear export.