A β-importin supports fusome growth and distribution in Drosophila ovarian germ cells

Germline cysts, or interconnected groups of germ cells, promote the synchronization of gamete development in vertebrates and invertebrates. In the Drosophila ovary, cyst formation is accompanied by growth of the fusome, a unique endoplasmic reticulum-like organelle. Although structural components of the fusome have been characterized, much less is known about the genetic factors that control fusome growth and distribution between germ cells. Here, we identify the β-importin, Tnpo-SR, as a regulator of fusome morphogenesis in germline stem cells (GSCs) and cysts. Although Tnpo-SR does not aggregate at fusomes or centrosomes, Tnpo-SR null mutants fail to form proper fusomes and depletion of Tnpo-SR alters fusome deposition and reduces fusome accumulation in GSCs. We demonstrate that Tnpo-SR-depletion disrupts microtubule organization and subcellular localization of the microtubule-associated protein Asp during interphase. Finally, we provide data suggesting that Tnpo-SR genetically interacts with microtubule-associated proteins and the cytoskeletal protein Hts, and likely functions indirectly and upstream of these key regulators of fusome formation, cyst architecture, and oocyte specification. Taken together, these data provide compelling evidence linking nuclear transport machinery to fusome morphogenesis and germline cyst development in Drosophila.