INTRODUCTION The orbit/mast gene of Drosophila encodes a microtubule- associated protein required for correct mitotic spindle organisation in embryos, larval neuroblasts and cultured cells

The orbit/mast gene of Drosophila encodes a microtubuleassociated protein required for correct mitotic spindle organisation in embryos, larval neuroblasts and cultured cells (Inoue et al., 2000; Lemos et al., 2000). The Orbit/Mast protein is required to maintain spindle bi-polarity and to ensure the attachment of kinetochores to microtubules to facilitate chromosome congression (Maiato et al., 2002). The mammalian counterpart of Orbit is known as CLASP and, together with its interacting partner CLIP-170, has been proposed to contribute to the stabilisation of the plus ends of microtubules at their cortical attachment sites in interphase cells (Akhmanova et al., 2001). Ultimately, this leads to the establishment of cellular asymmetry of crucial importance in processes such as wound healing-induced fibroblast motility (Akhmanova et al., 2001; Allan and Näthke, 2001; McNally, 2001). The emerging picture is that CLASP mediates microtubule based cellular processes in both interphase and mitosis and yet little is known of how the CLASPs, either alone and/or in association with other proteins, assist in establishing polarity. Oogenesis in Drosophila is an attractive developmental process for studying microtubule-based polarisation events, as it involves asymmetric cell divisions that are important in establishing cell fate, and also polarised interand intracellular transport phenomena required for the oocyte to differentiate. A specialised organelle, the fusome (reviewed by Telfner, 1975; Büning, 1994; de Cuevas et al., 1997; McKearin, 1997) plays a determinative role in imposing asymmetry upon dividing stem cells and cysts, as it associates with only one pole of the mitotic spindle at every mitosis (Storto and King, 1989; Lin and Spradling, 1995; McGrail and Hays, 1997) and later is partitioned unequally between daughter cells (de Cuevas and Sradling, 1998). The fusome comprises membrane skeletal proteins such as the αand β-spectrins, ankyrin, the adducinlike HtsF (the fusome specific product of the hu-li tai shao gene) (Yue and Spradling, 1992), Bam (bag-of-marbles) 901 Development 130, 901-915 © 2003 The Company of Biologists Ltd doi:10.1242/dev.00315