SPECTROSOMES AND FUSOMES ANCHOR MITOTIC SPINDLES DURING ASYMMETRIC GERM-CELL DIVISIONS AND FACILITATE THE FORMATION OF A POLARIZED MICROTUBULE ARRAY FOR OOCYTE SPECIFICATION IN DROSOPHILA

In the Drosophila ovary, membrane skeletal proteins such as the adduci n-like Hts protein(s), spectrin, and ankyrin are found in the spectros ome, an organelle in germline stem cells (GSC) and their differentiate d daughter cells (cystoblasts). These proteins are also components of the fusome, a cytoplasmic structure that spans the cystoblast's progen y that develop to form a germline cyst consisting of 15 nurse cells an d an oocyte. Spectrosomes and fusomes are associated with one pole of spindles during mitosis and are implicated in cyst formation and oocyt e differentiation. Here we show that the asymmetric behavior of the sp ectrosome persists throughout the cell cycle of GSC. Eliminating the s pectrosome by the hrs' mutation leads to randomized spindle orientatio n, suggesting that the spectrosome anchors the spindle to ensure the a symmetry of GSC division; eliminating the fusome in developing cysts r esults in defective spindles and randomized spindle orientation as wel l as asynchronous and reduced cystocyte divisions. These observations suggest that fusomes are required for the proper formation and asymmet ric orientation of mitotic spindles. Moreover, they reinforce the noti on that fusomes are required for the four synchronous divisions of the cystoblast leading to cyst formation. In hts' cysts which lack fusome s and fail to incorporate a hts gene product(s) into ring canals follo wing cyst formation, polarized microtubule networks do not form, the d ynamics of cytoplasmic dynein is disrupted, and oskar and orb RNAs fai l to be transported to the future oocyte. These observations support t he proposed role of fusomes and ring canals in organizing a polarized microtubule-based transport system for RNA localization that leads to oocyte differentiation. (C) 1997 Academic Press.