Abstract
Tissue replenishment from stem cells follows a precise cascade of events, during which stem cell daughters first proliferate by mitotic transit amplifying divisions and then enter terminal differentiation. Here we address how stem cell daughters are guided through the early steps of development. In Drosophila testes, somatic cyst cells enclose the proliferating and differentiating germline cells and the units of germline and surrounding cyst cells are commonly referred to as cysts. By characterizing flies with reduced or increased Epidermal Growth Factor (EGF) signaling we show that EGF triggers different responses in the cysts dependent on its dose. In addition to the previously reported requirement for EGF signaling in cyst formation, a low dose of EGF signaling is required for the progression of the germline cells through transit amplifying divisions, and a high dose of EGF signaling promotes terminal differentiation. Terminal differentiation was promoted in testes expressing a constitutively active EGF Receptor (EGFR) and in testes expressing both a secreted EGF and the EGFR in the cyst cells, but not in testes expressing either only EGF or only EGFR. We propose that as the cysts develop, a temporal signature of EGF signaling is created by the coordinated increase of both the production of active ligands by the germline cells and the amount of available receptor molecules on the cyst cells.
Highlights
Tissue homeostasis depends on adult stem cells that constantly self-renew and produce differentiated cells [1,2]
TA-divisions To investigate a dose-dependent effect of Epidermal Growth Factor (EGF) signaling we used the UAS/Gal4- system that allows for temporal control of tissuespecific expression of target genes by exposing Drosophila to different temperatures
We present genetic evidence that a low dose of EGF signaling is required for the early stages of germline development, the progression of the germline cells through TA-divisions, and that a high dose of EGF signaling promotes the entry of germline and cyst cells into terminal differentiation
Summary
Tissue homeostasis depends on adult stem cells that constantly self-renew and produce differentiated cells [1,2]. The germline stem cells (GSCs) are attached to a single group of post-mitotic, apical hub cells and enclosed by cytoplasmic extensions from two somatic stem cells, the cyst stem cells (CySCs, Figure 1A) [9,10]. Both stem cell populations undergo asymmetric mitotic cell divisions, producing gonialblasts and cyst cells respectively [11,12]. Cyst cells normally cease mitosis and form the germline microenvironment During this process, two cyst cells grow cytoplasmic extensions around one newly formed gonialblast [9,13,14,15]. Germline and cyst cells dissociate from each other only at the end of spermatogenesis for sperm individualization and release [8,24,25]
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