Bazooka is required for polarisation of the Drosophila anterior-posterior axis

Abstract

SUMMARYThe Drosophila anterior-posterior (AP) axis is determined by the polarisation of the stage 9 oocyte and the subsequent localisation of bicoid and oskar mRNAs to opposite poles of the cell. Oocyte polarity has been proposed to depend on the same PAR proteins that generate AP polarity in C. elegans, with a complex of Bazooka (Baz; Par-3), Par-6 and aPKC marking the anterior and lateral cortex, and Par-1 defining the posterior. The function of the Baz complex in oocyte polarity has remained unclear, however, because although baz-null mutants block oocyte determination, egg chambers that escape this early arrest usually develop normal polarity at stage 9. Here, we characterise a baz allele that produces a penetrant polarity phenotype at stage 9 without affecting oocyte determination, demonstrating that Baz is essential for axis formation. The dynamics of Baz, Par-6 and Par-1 localisation in the oocyte indicate that the axis is not polarised by a cortical contraction as in C. elegans, and instead suggest that repolarisation of the oocyte is triggered by posterior inactivation of aPKC or activation of Par-1. This initial asymmetry is then reinforced by mutual inhibition between the anterior Baz complex and posterior Par-1 and Lgl. Finally, we show that mutation of the aPKC phosphorylation site in Par-1 results in the uniform cortical localisation of Par-1 and the loss of cortical microtubules. Since nonphosphorylatable Par-1 is epistatic to uninhibitable Baz, Par-1 seems to function downstream of the other PAR proteins to polarise the oocyte microtubule cytoskeleton.

Multiple Roles for Egalitarian in Polarization of the Drosophila Egg Chamber

“…Furthermore, in par1 and baz/par3 mutants, osk mRNA is delocalized to a central focus (Shulman et al 2000;Tomancak et al 2000;Becalska and Gavis 2010;Doerflinger et al 2010). In some mutant alleles of these genes, bcd mRNA is relatively unaffected (Shulman et al 2000), whereas in others, bcd mRNA localizes along the lateral cortex in a similar manner to Egl depletion (Becalska and Gavis 2010;Doerflinger et al 2010).…”

“…In some mutant alleles of these genes, bcd mRNA is relatively unaffected (Shulman et al 2000), whereas in others, bcd mRNA localizes along the lateral cortex in a similar manner to Egl depletion (Becalska and Gavis 2010;Doerflinger et al 2010). In some mutant alleles of baz, the oocyte nucleus is retained at the posterior (Becalska and Gavis 2010;Doerflinger et al 2010), whereas in some mutants of par-1, migration of the oocyte nucleus is unaffected (Shulman et al 2000).…”

TheC. eleganshomolog ofDrosophilaLethal giant larvae functions redundantly with PAR-2 to maintain polarity in the early embryo

“…Recent studies have shown that Lgl is involved in the polarization of the A-P axis in the Drosophila oocyte (Doerflinger et al, 2010;Fichelson et al, 2010;Li et al, 2008;Tian and Deng, 2008). In the early oocyte, Lgl is required for the proper posterior translocation of cell fate determinants as well as the centrosomes (Fichelson et al, 2010;Tian and Deng, 2008).…”

Refining the role of Lgl, Dlg and Scrib in Tumor Suppression and Beyond: Learning from the Old Time Classics

“…Dlg, Scrib and Lgl are also important in follicular epithelium morphogenesis and subsequent polarization of the Drosophila oocyte, albeit in a different way as in other epithelia. The work of several groups has shown that Lgl is an essential regulator of posterior follicle cells and that phosphorylation of Lgl together with Par-1 and Par-3 is required for the posterior translocation of oocyte-specific proteins and germline determinants (Fichelson et al 2010, Doerflinger et al 2010. Mutation of the aPKC phosphorylation site in Par-1 results in the uniform cortical localization of Par-1 and the loss of cortical microtubules (Doerflinger et al 2010).…”

“…The work of several groups has shown that Lgl is an essential regulator of posterior follicle cells and that phosphorylation of Lgl together with Par-1 and Par-3 is required for the posterior translocation of oocyte-specific proteins and germline determinants (Fichelson et al 2010, Doerflinger et al 2010. Mutation of the aPKC phosphorylation site in Par-1 results in the uniform cortical localization of Par-1 and the loss of cortical microtubules (Doerflinger et al 2010). Dlg and Scrib are required differentially for patterning in both the anterior and posterior follicular epithelium (Li et al 2009) and genetically interact with Lgl for posterior follicle cell induction , suggesting a common regulatory pathway in this process.…”