| Adult stem cells are essential for tissue homeostasis and repair, but little is known about their formation and regulation in mammals. Drosophila spermatogenesis parallels mammalian spermatogenesis, and is an ideal system for the genetic manipulation of stem cells in vivo. Germline stem cells (GSCs) and somatic cyst stem cells (CySCs) anchor to a group of stromal cells called the hub, and can be unambiguously identified using molecular markers. The hub secretes a ligand that activates Janus Kinase - Signal Transducer and Activator of Transcription (Jak-STAT) signaling, preventing differentiation of stem cells within the microenvironment, or niche.;To understand stem cell establishment during development, we examined the expression of GSC markers within Drosophila embryonic gonads. By the end of embryogenesis, a subset of primordial germ cells (PGCs) in the anterior of the gonad form functional germline stem cells (GSCs) by adhering to the embryonic hub and transducing Jak-STAT signaling. Sustained STAT activation is likely required for the PGC to GSC transition, and prevents differentiation of nascent GSCs. Thus, similar to stem cells in the mouse hair follicle epithelium, genes involved in the adult stem cell maintenance pathway also mediate stem cell establishment.;To study stem cell replenishment during tissue homeostasis and regeneration, we developed live imaging of intact Drosophila testes for up to 12 hours. During homeostasis, GSC divisions had stereotypically oriented division planes and usually resulted in asymmetric daughter cell fates. Surprisingly, interconnected GSC-daughter pairs that had divided to displace daughter cells away from the hub occasionally swiveled such that both daughters contacted the hub and adopted GSC identity. These symmetrical renewing events were the predominant mode of GSC replacement, and occurred more often during physiological stress. To study processes involved in regeneration, we established a novel genetic assay to deplete GSCs, causing the progeny of GSCs (spermatogonia) to dedifferentiate. Live imaging of this event showed spermatogonia moving, fragmenting, and dividing symmetrically as they reverted into GSCs while outcompeting CySCs for niche occupancy. Our data reveal that Drosophila GSCs, like mammalian stem cells, are not always confined to divide with strict asymmetric outcome, and support the emerging perspective that stem cell lineages exhibit remarkable flexibility. |
