Live imaging of the Drosophila ovarian niche shows spectrosome and centrosome dynamics during asymmetric germline stem cell division.

Gema Villa-Fombuena,Patricia Rojas-Ríos,Miriam Marín-Menguiano,María Lobo-Pecellín,Acaimo González-Reyes

doi:10.1242/dev.199716

Gema Villa-Fombuena, Patricia Rojas-Ríos + Show 3 more

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https://doi.org/10.1242/dev.199716

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Abstract

ABSTRACTDrosophila female germline stem cells (GSCs) are found inside the cellular niche at the tip of the ovary. They undergo asymmetric divisions to renew the stem cell lineage and to produce sibling cystoblasts that will in turn enter differentiation. GSCs and cystoblasts contain spectrosomes, membranous structures essential for orientation of the mitotic spindle and that, particularly in GSCs, change shape depending on the cell cycle phase. Using live imaging and a fusion protein of GFP and the spectrosome component Par-1, we follow the complete spectrosome cycle throughout GSC division and quantify the relative duration of the different spectrosome shapes. We also determine that the Par-1 kinase shuttles between the spectrosome and the cytoplasm during mitosis and observe the continuous addition of new material to the GSC and cystoblast spectrosomes. Next, we use the Fly-FUCCI tool to define, in live and fixed tissues, that GSCs have a shorter G1 compared with the G2 phase. The observation of centrosomes in dividing GSCs allowed us to determine that centrosomes separate very early in G1, before centriole duplication. Furthermore, we show that the anterior centrosome associates with the spectrosome only during mitosis and that, upon mitotic spindle assembly, it translocates to the cell cortex, where it remains anchored until centrosome separation. Finally, we demonstrate that the asymmetric division of GSCs is not an intrinsic property of these cells, as the spectrosome of GSC-like cells located outside of the niche can divide symmetrically. Thus, GSCs display unique properties during division, a behaviour influenced by the surrounding niche.

Highlights

Germ cells are the gamete precursors and, key components of sexual reproduction
Polarised vesicle transport in the germline stem cells (GSCs) Here, we show that the asymmetric self-renewing division of live GSCs is reflected in the behaviour of the spectrosome and the inheritance of the post-abscission MB, as the GSCs remaining in the niche retain most of the spectrosome material and inherit the MB remnant
In the case of the anterior spectrosome adjacent to the cap cells (CpCs) rosette, it is known that the active form of the small GTPase Rac accumulates at the niche-GSC interface and brings about the microtubule binding protein Apc2 to orientate the mitotic spindle (Lu et al, 2012)

Summary

Introduction

Germ cells are the gamete precursors and, key components of sexual reproduction. The GSC niche contains three cell types of somatic origin: terminal filament cells (TFCs), cap cells (CpCs) and anterior escort cells (ECs; Fig. 1A) These niche cells provide GSCs with signals and physical support to prevent entry into differentiation (Eliazer and Buszczak, 2011). The organisation of the ovarian niche is very well established and includes a specialised extracellular matrix, a terminal filament (TF) of 8-10 cells, a rosette of 6-8 CpCs connected to the TF via the ‘transition cell’ (TC) and 2-3 ECs placed in close contact with the CpCs (Díaz-Torres et al, 2021; Panchal et al, 2017; Wang and PageMcCaw, 2018) This microenvironment orchestrates short-range signalling and provides physical space to maintain 2-4 GSCs per niche. The CB spectrosome grows into a branched figure called a fusome, characteristic of differentiating germline cysts (de Cuevas and Spradling, 1998; Ong and Tan, 2010)

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