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In Drosophila, only one cell in a multicellular female germline cyst is specified as an oocyte and a similar process occurs in mammals. The symmetry-breaking cue for oocyte selection is provided by the fusome, a tubular structure connecting all cells in the cyst. The Drosophila spectraplakin Shot localises to the fusome and translates its asymmetry into a polarised microtubule network that is essential for oocyte specification, but how Shot recognises the fusome is unclear. Here, we demonstrate that the actin-binding domain (ABD) of Shot is necessary and sufficient to localise Shot to the fusome and mediates Shot function in oocyte specification together with the microtubule-binding domains. The calponin homology domain 1 of the Shot ABD recognises fusomal F-actin and requires calponin homology domain 2 to distinguish it from other forms of F-actin in the cyst. By contrast, the ABDs of utrophin, Fimbrin, Filamin, Lifeact and F-tractin do not recognise fusomal F-actin.We therefore propose that Shot propagates fusome asymmetry by recognising a specific conformational state of F-actin on the fusome. [ABSTRACT FROM AUTHOR]

The timing of Drosophila egg chamber development is controlled by a germline Delta signal that activates Notch in the follicle cells to induce them to cease proliferation and differentiate. Here, we report that follicle cells lacking the RNA-binding protein IMP go through one extra division owing to a delay in the Delta-dependent S2 cleavage of Notch. The timing of Notch activation has previously been shown to be controlled by cis-inhibition by Delta in the follicle cells, which is relieved when the miRNA pathway represses Delta expression. imp mutants are epistatic to Delta mutants and give an additive phenotype with belle and Dicer-1 mutants, indicating that IMP functions independently of both cis-inhibition and the miRNA pathway. We find that the imp phenotype is rescued by overexpression of Kuzbanian, the metalloprotease that mediates the Notch S2 cleavage. Furthermore, Kuzbanian is not enriched at the apical membrane in imp mutants, accumulating instead in late endosomes. Thus, IMP regulates Notch signalling by controlling the localisation of Kuzbanian to the apical domain, where Notch cleavage occurs, revealing a novel regulatory step in the Notch pathway. [ABSTRACT FROM AUTHOR]

The direction in which a cell divides is determined by the orientation of its mitotic spindle at metaphase. Spindle orientation is therefore important for a wide range of developmental processes, ranging from germline stem cell division to epithelial tissue homeostasis and regeneration. In multiple cell types in multiple animals, spindle orientation is controlled by a conserved biological machine that mediates a pulling force on astral microtubules. Restricting the localization of this machine to only specific regions of the cortex can thus determine how the mitotic spindle is oriented. As we review here, recent findings based on studies in tunicate, worm, fly and vertebrate cells have revealed that the mechanisms for mediating this restriction are surprisingly diverse. [ABSTRACT FROM AUTHOR]

In animal cells, mitotic spindles are oriented by the dynein/dynactin motor complex, which exerts a pulling force on astral microtubules. Dynein/dynactin localization depends on Mud/NUMA, which is typically recruited to the cortex by Pins/LGN. In Drosophila neuroblasts, the Inscuteable/Baz/Par-6/aPKC complex recruits Pins apically to induce vertical spindle orientation, whereas in epithelial cells Dlg recruits Pins laterally to orient the spindle horizontally. Here we investigate division orientation in the Drosophila imaginal wing disc epithelium. Live imaging reveals that spindle angles vary widely during prometaphase and metaphase, and therefore do not reliably predict division orientation. This finding prompted us to re-examine mutants that have been reported to disrupt division orientation in this tissue. Loss of Mud misorients divisions, but Inscuteable expression and aPKC, dlg and pins mutants have no effect. Furthermore, Mud localizes to the apical-lateral cortex of the wing epithelium independently of both Pins and cell cycle stage. Thus, Pins is not required in the wing disc because there are parallel mechanisms for Mud localization and hence spindle orientation, making it a more robust system than in other epithelia. [ABSTRACT FROM AUTHOR]

The Drosophila anterior-posterior axis is specified when the posterior follicle cells signal to polarise the oocyte, leading to the anterior/lateral localisation of the Par-6/aPKC complex and the posterior recruitment of Par-1, which induces a microtubule reorganisation that localises bicoid and oskar mRNAs. Here we show that oocyte polarity requires Slmb, the substrate specificity subunit of the SCF E3 ubiquitin ligase that targets proteins for degradation. The Par-6/aPKC complex is ectopically localised to the posterior of slmb mutant oocytes, and Par- 1 and oskar mRNA are mislocalised. Slmb appears to play a related role in epithelial follicle cells, as large slmb mutant clones disrupt epithelial organisation, whereas small clones show an expansion of the apical domain, with increased accumulation of apical polarity factors at the apical cortex. The levels of aPKC and Par-6 are significantly increased in slmb mutants, whereas Baz is slightly reduced. Thus, Slmb may induce the polarisation of the anteriorposterior axis of the oocyte by targeting the Par-6/aPKC complex for degradation at the oocyte posterior. Consistent with this, overexpression of the aPKC antagonist Lgl strongly rescues the polarity defects of slmb mutant germline clones. The role of Slmb in oocyte polarity raises an intriguing parallel with C. elegans axis formation, in which PAR-2 excludes the anterior PAR complex from the posterior cortex to induce polarity, but its function can be substituted by overexpressing Lgl. [ABSTRACT FROM AUTHOR]

Presents a study that identified novel genes required for Drosophila anteroposterior axis formation in a germline clone screen using green fluorescent protein (GFP)-Staufen. Definition of the anteroposterior axis of Drosophila; Description of a genetic screen in germline clones for mutants that disrupt the localization of GFP-Staufen in living oocytes; Conclusion of the study.

Discusses a study that investigated the role of PAR-1 kinase in the polarized microtubule cytoskeleton in Drosophila follicular epithelium. Importance of apicobasal polarity in the formation and morphogenesis of epithelia; Role of PAR proteins in the establishment of epithelial polarity; Results and discussion.

The specification of polar, main-body and stalk follicle cells in the germarium of the Drosophila ovary plays a key role in the formation of the egg chamber and polarisation of its anterior-posterior axis. High levels of Notch pathway activation, resulting from a germline Delta ligand signal, induce polar cells. Here we show that low Notch activation levels, originating from Delta expressed in the polar follicle cells, are required for stalk formation. The metalloprotease Kuzbanian-like, which cleaves and inactivates Delta, reduces the level of Delta signaling between follicle cells, thereby limiting the size of the stalk. We find that Notch activation is required in a continuous fashion to maintain the polar and stalk cell fates. We further demonstrate that mutual antagonism between the Notch and JAK/STAT signaling pathways provides a crucial facet of follicle cell patterning. Notch signaling in polar and main-body follicle cells inhibits JAK/STAT signaling by preventing STAT nuclear translocation, thereby restricting the influence of this pathway to stalk cells. Conversely, signaling by JAK/STAT reduces Notch signaling in the stalk. Thus, variations in the levels of Notch pathway activation, coupled with a continuous balance between the Notch and JAK/STAT pathways, specify the identity of the different follicle cell types and help establish the polarity of the egg chamber. [ABSTRACT FROM AUTHOR]

The Drosophila maternal effect gene oskar encodes the posterior determinant responsible for the formation of the posterior pole plasm in the egg, and thus of the abdomen and germline of the future fly. Previously identified oskar mutants give rise to offspring that lack both abdominal segments and a germline, thus defining the 'posterior group phenotype'. Common to these classical oskar alleles is that they all produce significant amounts of oskar mRNA. By contrast, two new oskar mutants in which oskar RNA levels are strongly reduced or undetectable are sterile, because of an early arrest of oogenesis. This egg-less phenotype is complemented by oskar nonsense mutant alleles, as well as by oskar transgenes, the protein-coding capacities of which have been annulled. Moreover, we show that expression of the oskar 3′ untranslated region (3′UTR) is sufficient to rescue the egg-less defect of the RNA null mutant. Our analysis thus reveals an unexpected role for oskar RNA during early oogenesis, independent of Oskar protein. These findings indicate that oskar RNA acts as a scaffold or regulatory RNA essential for development of the oocyte. [ABSTRACT FROM AUTHOR]