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

Abstract

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 coordinated by the fusome, a unique endoplasmic reticulum-like organelle. Although many structural components of the fusome have been characterized, little is known about the genetic factors that control fusome growth and distribution between germ cells. Here, we identify the {beta}-importin, Tnpo-SR, as an important regulator of fusome morphogenesis in germline stem cells (GSCs) and their dividing daughters. Although Tnpo-SR does not aggregate at fusomes or centrosomes, Tnpo-SR null mutants fail to form proper fusomes and knock-down of Tnpo-SR reduces fusome accumulation, ultimately leading to cyst fragmentation and improper numbers of germ cells within an egg chamber. Tnpo-SR depletion disrupts microtubule organization during interphase and sub-cellular localization of the microtubule associated protein Asp. However, overexpression of asp is not sufficient to restore fusome size or cyst architecture when Tnpo-SR is depleted, suggesting that Tnpo-SR-dependent regulation of asp does not solely control fusome morphogenesis. Instead, we find that restoring fusome size by overexpressing the core fusome component hu-li tai shao or the polarity factor Par3/bazooka is sufficient to restore fusome area in Tnpo-SR-depleted GSCs. Moreover, restoration of fusome area in the absence of Tnpo-SR also rescues cyst organization and oocyte specification, suggesting that Tnpo-SR functions upstream of fusome structural component production. Taken together, these data functionally link nuclear import/export machinery to fusome morphogenesis during Drosophila germline cyst development. ARTICLE SUMMARYThe fusome plays a key role in cyst formation in the Drosophila ovarian germline, yet regulation of fusome growth and distribution between germ cells after mitosis remains understudied. Through loss-of-function analyses, the authors identify Tnpo-SR as a novel regulator of fusome morphogenesis. Tnpo-SR depletion reduces the core fusome structural component Hts in germline stem cells (GSCs) and disrupts microtubule organization and nucleation. Overexpression of Hts or Par3/bazooka in Tnpo-SR-depleted GSCs restores fusome size, which in turn rescues cyst organization and oocyte specification. These findings establish Tnpo-SR as an upstream regulator of Hts during fusome morphogenesis.