The Drosophila optic lobe comprises a wide variety of neurons, which form laminar neuropiles with columnar units and topographic projections from the retina. The Drosophila optic lobe shares many structural characteristics with mammalian visual systems. However, little is known about the developmental mechanisms that produce neuronal diversity and organize the circuits in the primary region of the optic lobe, the medulla. Here, we describe the key features of the developing medulla and report novel phenomena that could accelerate our understanding of the Drosophila visual system. The identities of medulla neurons are pre-determined in the larval medulla primordium, which is subdivided into concentric zones characterized by the expression of four transcription factors: Drifter, Runt, Homothorax and Brain-specific homeobox (Bsh). The expression pattern of these factors correlates with the order of neuron production. Once the concentric zones are specified, the distribution of medulla neurons changes rapidly. Each type of medulla neuron exhibits an extensive but defined pattern of migration during pupal development. The results of clonal analysis suggest homothorax is required to specify the neuronal type by regulating various targets including Bsh and cell-adhesion molecules such as N-cadherin, while drifter regulates a subset of morphological features of Drifter-positive neurons. Thus, genes that show the concentric zones may form a genetic hierarchy to establish neuronal circuits in the medulla.

The brain consists of various types of neurons that are generated from neural stem cells; however, the mechanisms underlying neuronal diversity remain uncertain. A recent study demonstrated that the medulla, the largest component of the Drosophila optic lobe, is a suitable model system for brain development because it shares structural features with the mammalian brain and consists of a moderate number and various types of neurons. The concentric zones in the medulla primordium that are characterized by the expression of four transcription factors, including Homothorax (Hth), Brain-specific homeobox (Bsh), Runt (Run) and Drifter (Drf), correspond to types of medulla neurons. Here, we examine the mechanisms that temporally determine the neuronal types in the medulla primordium. For this purpose, we searched for transcription factors that are transiently expressed in a subset of medulla neuroblasts (NBs, neuronal stem cell-like neural precursor cells) and identified five candidates (Hth, Klumpfuss (Klu), Eyeless (Ey), Sloppy paired (Slp) and Dichaete (D)). The results of genetic experiments at least explain the temporal transition of the transcription factor expression in NBs in the order of Ey, Slp and D. Our results also suggest that expression of Hth, Klu and Ey in NBs trigger the production of Hth/Bsh-, Run- and Drf-positive neurons, respectively. These results suggest that medulla neuron types are specified in a birth order-dependent manner by the action of temporal transcription factors that are sequentially expressed in NBs.

A model for mitosis suggests that correct kinetochore-microtubule (KT-MT) attachments are stabilized by spatial separation of the attachment sites from Aurora B kinase through sister KT stretching. However, the spatiotemporal regulation of attachment stability during meiosis I (MI) in oocytes remains unclear. Here, we found that in mouse oocytes, Aurora B and C (B/C) are located in close proximity to KT-MT attachment sites after bivalent stretching due to an intrinsic property of the MI chromosomes. The Aurora B/C activity destabilizes correct attachments while allowing a considerable amount of incorrect attachments to form. KT-MT attachments are eventually stabilized through KT dephosphorylation by PP2A-B56 phosphatase, which is progressively recruited to KTs depending on the BubR1 phosphorylation resulting from the timer Cdk1 and independent of bivalent stretching. Thus, oocytes lack a mechanism for coordinating bivalent stretching and KT phosphoregulation during MI, which may explain the high frequency of KT-MT attachment errors.

The downregulation of E-cadherin by Src promotes epithelial to mesenchymal transition and tumorigenesis. However, a simple loss of cell adhesion is not sufficient to explain the diverse developmental roles of Src and metastatic behavior of viral Src-transformed cells. Here, we studied the functions of endogenous and activated forms of Drosophila Src in the context of tracheal epithelial development, during which extensive remodeling of adherens junctions takes place. We show that Src42A is selectively activated in the adherens junctions of epithelia undergoing morphogenesis. Src42A and Src64B are required for tracheal development and to increase the rate of adherens junction turnover. The activation of Src42A caused opposing effects: it reduced the E-cadherin protein level but stimulated transcription of the E-cadherin gene through the activation of Armadillo and TCF. This TCF-dependent pathway was essential for the maintenance of E-cadherin expression and for tissue integrity under conditions of high Src activity. Our data suggest that the two opposing outcomes of Src activation on E-cadherin facilitate the efficient exchange of adherens junctions, demonstrating the key role of Src in the maintenance of epithelial integrity. KEY WORDS:Src, E-cadherin, Armadillo, Drosophila, Trachea, Cancer Development 135, 1355Development 135, -1364Development 135, (2008Development 135, ) doi:10.1242 Riken Center for Developmental Biology, 2-2-3 Minatojima-minamimachi, Chuo-ku Kobe 650-0047, Japan. DEVELOPMENT 1356 strain carrying the trachea-specific btl-Gal4 driver and the UAS-GFPmoesin marker. An additional 142 lines lacking the rough eye phenotype were also tested. GS11022 (Src42A) and GS9618 (Src64B) were analyzed for further study. Fly stocks and geneticsWe used strong loss-of-function alleles of Src genes: Src42A 26-1 (Takahashi et al., 2005), Src42A myri (Tateno et al., 2000) and Src64B P1 (Dodson et al., 1998). The following strains were used in this study: trachealess enhancer trap line 1-eve-1 (Perrimon et al., 1991), UAS-wg (Lawrence et al., 1995), UAS-arm S10 (Pai et al., 1997), UAS-TCF⌬N (van de Wetering et al., 1997), UAS-E-cadherin-GFP (Oda and Tsukita, 1999b), UAS-D␣-catenin-GFP (Oda and Tsukita, 1999a), shg-lacZ , UAS-GFP-moesin (Chihara et al., 2003), YF (Tateno et al., 2000), UAS-Src42A-RNAi (NIG Stock Center) and btl-Gal4 (Shiga et al., 1996). Src42A DN was constructed by introducing the K295M mutation at the catalytic center of the kinase domain and was cloned into the pUAST vector (Brand and Perrimon, 1993). Immunostaining and imagingThe following primary antibodies were used: rat anti-Esg (Fuse et al., 1994); mouse anti-tracheal luminal antigen 2A12, mouse anti-Armadillo N27A1 and mouse anti-septin 4C9H4 (Developmental Studies Hybridoma Bank); rabbit anti-␤-galactosidase (Cappel); mouse anti-GFP B-2 and rabbit anti-GFP (MBL); rabbit anti-Src PY418 (Biosource International); rat anti-E-cadherin (DCAD2) (Oda et al., 1994) and rabbit anti-Src42A (Takahashi et al., 2005). Chicken anti-Src42A antibody ...

The corepressor complex that includes Ebi and SMRTER is a target of epidermal growth factor (EGF) and Notch signaling pathways and regulates Delta (Dl)‐mediated induction of support cells adjacent to photoreceptor neurons of the Drosophila eye. We describe a mechanism by which the Ebi/SMRTER corepressor complex maintains Dl expression. We identified a gene, charlatan (chn), which encodes a C2H2‐type zinc‐finger protein resembling human neuronal restricted silencing factor/repressor element RE‐1 silencing transcription factor (NRSF/REST). The Ebi/SMRTER corepressor complex represses chn transcription by competing with the activation complex that includes the Notch intracellular domain (NICD). Chn represses Dl expression and is critical for the initiation of eye development. Thus, under EGF signaling, double negative regulation mediated by the Ebi/SMRTER corepressor complex and an NRSF/REST‐like factor, Chn, maintains inductive activity in developing photoreceptor cells by promoting Dl expression.