A molecular screen for polar-localised maternal RNAs in the early embryo of Drosophila.

Localised, maternally synthesised RNAs and proteins play an important role in an early animal embryogenesis. In Drosophila, genetic screens have recovered a number of maternal effect loci that encode localised products in the embryo. However, only a third of Drosophila's genes have been genetically mutated. Consequently, we conducted a molecular screen for polar-localised RNAs in the early Drosophila embryo in order to identify additional maternal molecules that carry out spatially restricted functions during early embryogenesis. Total RNA was purified from anterior or posterior poles cut off early Drosophila embryos. These RNAs were used to construct directionally cloned anterior and posterior cDNA libraries which were used in a differential screen for cDNAs representing maternal RNAs localised to one or other pole of the embryo. Five such clones were identified, representing cyclin B RNA, Hsp83 RNA, 28S ribosomal RNA, mitochondrial cytochrome c oxidase subunit one RNA and mitochondrial 16S large ribosomal RNA. Mutations in the loci encoding these RNAs have not been recovered in genetic screens, confirming that our molecular approach complements genetic strategies for identifying maternal molecules that carry out spatially restricted functions in the early embryo. We consider the possible biological significance of localisation of each of these species of transcripts as well as the mechanism of their localisation, and discuss the potential use of our cDNA libraries in screens for rarer localised RNAs.