Msps/XMAP215 Is Essential for Oocyte Specification in the Drosophila Ovary

Cell fate determination is fundamental to development. In the Drosophila ovary, a 16-cell germline cyst forms in which two pro-oocytes arise; one of the pro-oocytes is selected as the oocyte, while the remaining 15 cells differentiate into nurse cells. Here, we identify Mini spindles (Msps), a Drosophila XMAP215-family microtubule polymerase, as a key regulator of oocyte fate specification. msps mRNA and Msps protein are enriched in the pro-oocytes prior to oocyte specification. Elimination of Msps prevents oocyte specification and disrupts the accumulation of the oocyte marker Orb, producing egg chambers with 16 nurse cells. Conversely, optogenetic recruitment of Msps increases microtubule polymerization and promotes Orb accumulation in nurse cells, suggesting that Msps activity is sufficient to initiate key molecular events associated with oocyte specification. Furthermore, Msps associates with the germline-specific organelles, the spectrosome and fusome, and becomes asymmetrically distributed, biasing the two pro-oocytes toward oocyte fate. We propose that an Msps–dynein feedback loop amplifies small initial differences between the two pro-oocytes to specify a single oocyte.