Anterior-posterior axis specification in Drosophila oocytes: identification of novel bicoid and oskar mRNA localization factors.

The Drosophila melanogaster anterior-posterior axis is established during oogenesis by the localization of bicoid and oskar mRNAs to the anterior and posterior poles of the oocyte. Although genetic screens have identified some trans-acting factors required for the localization of these transcripts, other factors may have been missed because they also function at other stages of oogenesis. To circumvent this problem, we performed a screen for revertants and dominant suppressors of the bicaudal phenotype caused by expressing Miranda-GFP in the female germline. Miranda mislocalizes oskar mRNA/Staufen complexes to the oocyte anterior by coupling them to the bicoid localization pathway, resulting in the formation of an anterior abdomen in place of the head. In one class of revertants, Miranda still binds Staufen/oskar mRNA complexes, but does not localize to the anterior, identifying an anterior targeting domain at the N terminus of Miranda. This has an almost identical sequence to the N terminus of vertebrate RHAMM, which is also a large coiled-coil protein, suggesting that it may be a divergent Miranda ortholog. In addition, we recovered 30 dominant suppressors, including multiple alleles of the spectroplakin, short stop, a lethal complementation group that prevents oskar mRNA anchoring, and a female sterile complementation group that disrupts the anterior localization of bicoid mRNA in late oogenesis. One of the single allele suppressors proved to be a mutation in the actin nucleator, Cappuccino, revealing a previously unrecognized function of Cappuccino in pole plasm anchoring and the induction of actin filaments by Long Oskar protein.