Jak-STAT regulation of male germline stem cell establishment during Drosophila embryogenesis

lines and bowl affect the specification of cyst stem cells and niche cells in the Drosophila testis

To function properly, tissue-specific stem cells must reside in a niche. The Drosophila testis niche is one of few niches studied in vivo. Here, a single niche, comprising ten hub cells, maintains both germline stem cells (GSC) and somatic stem cells (CySC). Here, we show that lines is an essential CySC factor. Surprisingly, lines-depleted CySCs adopted several characteristics of hub cells, including the recruitment of new CySCs. This led us to examine the developmental relationship between CySCs and hub cells. In contrast to a previous report, we did not observe significant conversion of steady-state CySC progeny to hub fate. However, we found that these two cell types derive from a common precursor pool during gonadogenesis. Furthermore, lines mutant embryos exhibited gonads containing excess hub cells, indicating that lines represses hub cell fate during gonadogenesis. In many tissues, lines acts antagonistically to bowl, and we found that this is true for hub specification, establishing bowl as a positively acting factor in the development of the testis niche.

magu is required for germline stem cell self-renewal through BMP signaling in the Drosophila testis

Understanding how stem cells are maintained in their microenvironment (the niche) is vital for their application in regenerative medicine. Studies of Drosophila male germline stem cells (GSCs) have served as a paradigm in niche-stem cell biology. It is known that the BMP and JAK-STAT pathways are necessary for the maintenance of GSCs in the testis (Kawase et al., 2004; Kiger et al., 2001; Schulz et al., 2004; Shivdasani and Ingham, 2003; Tulina and Matunis, 2001). However, our recent work strongly suggests that BMP signaling is the primary pathway leading to GSC self-renewal (Leatherman and DiNardo, 2010). Here we show that magu controls GSC maintenance by modulating the BMP pathway. We found that magu was specifically expressed from hub cells, and accumulated at the testis tip. Testes from magu mutants exhibited a reduced number of GSCs, yet maintained a normal population of somatic stem cells and hub cells. Additionally, BMP pathway activity was reduced, whereas JAK-STAT activation was retained in mutant testes. Finally, GSC loss caused by the magu mutation could be suppressed by overactivating the BMP pathway in the germline.

The endoderm specifies the mesodermal niche for the germline in Drosophila via Delta-Notch signaling

Interactions between niche cells and stem cells are vital for proper control over stem cell self-renewal and differentiation. However, there are few tissues where the initial establishment of a niche has been studied. The Drosophila testis houses two stem cell populations, which each lie adjacent to somatic niche cells. Although these niche cells sustain spermatogenesis throughout life, it is not understood how their fate is established. Here, we show that Notch signaling is necessary to specify niche cell fate in the developing gonad. Surprisingly, our results indicate that adjacent endoderm is the source of the Notch-activating ligand Delta. We also find that niche cell specification occurs earlier than anticipated, well before the expression of extant markers for niche cell fate. This work further suggests that endoderm plays a dual role in germline development. The endoderm assists both in delivering germ cells to the somatic gonadal mesoderm, and in specifying the niche where these cells will subsequently develop as stem cells. Because in mammals primordial germ cells also track through endoderm on their way to the genital ridge, our work raises the possibility that conserved mechanisms are employed to regulate germline niche formation.

Identification of adult midgut precursors in Drosophila

The adult Drosophila midgut is thought to arise from an endodermal rudiment specified during embryogenesis. Previous studies have reported the presence of individual cells termed adult midgut precursors (AMPs) as well as "midgut islands" or "islets" in embryonic and larval midgut tissue. Yet the precise relationship between progenitor cell populations and the cells of the adult midgut has not been characterized. Using a combination of molecular markers and directed cell lineage tracing, we provide evidence that the adult midgut arises from a molecularly distinct population of single cells present by the embryonic/larval transition. AMPs reside in a distinct basal position in the larval midgut where they remain through all subsequent larval and pupal stages and into adulthood. At least five phases of AMP activity are associated with the stepwise process of midgut formation. Our data shows that during larval stages AMPs give rise to the presumptive adult epithelium; during pupal stages AMPs contribute to the final size, cell number and form. Finally, a genetic screen has led to the identification of the Ecdysone receptor as a regulator of AMP expansion.

Evolution and spermatogenesis

Sexual reproduction depends on the production of haploid gametes, and their fusion to form diploid zygotes. Here, we discuss sperm production and function in a molecular and functional evolutionary context, drawing predominantly from studies in model organisms (mice, Drosophila, Caenorhabditis elegans). We consider the mechanisms involved in establishing and maintaining a germline stem cell population in testes, as well as the factors that regulate their contribution to the pool of differentiating cells. These processes involve considerable interaction between the germline and the soma, and we focus on regulatory signalling events in a variety of organisms. The male germline has a unique transcriptional profile, including expression of many testis-specific genes. The evolutionary pressures associated with gene duplication and acquisition of testis function are discussed in the context of genome organization and transcriptional regulation. Post-meiotic differentiation of spermatids involves very dramatic changes in cell shape and acquisition of highly specialized features. We discuss the variety of sperm motility mechanisms and how various reproductive strategies are associated with the diversity of sperm forms found in animals.

Notch and Egfr signaling act antagonistically to regulate germ-line stem cell niche formation in Drosophila male embryonic gonads

Germ-line stem cells (GSCs) are maintained by the somatic microenvironment, or GSC niche, which ensures that GSCs can both self-renew and produce functional gametes. However, it remains unclear how the proper niche size and location are regulated within the developing gonads. In the Drosophila testis, the hub cells that form the GSC niche are derived from a subset of somatic gonadal precursors (SGPs) in the anterior portion of the embryonic gonad. Here we show that Notch signaling induces hub differentiation. Notch is activated in almost all SGPs in the male embryonic gonad, but Epidermal growth factor receptor (Egfr) is activated in posterior SGPs to repress hub differentiation, thereby restricting the expansion of hub differentiation in the embryonic gonad. We further show that Egfr is activated in posterior SGPs by Spitz ligand secreted from primordial germ cells (PGCs), whereas the Notch ligand Serrate is expressed in SGPs. This suggests that varying the number of PGCs alters niche size. Indeed, a decrease in the number of PGCs causes ectopic hub differentiation, which consequently increases their opportunity to recruit PGCs as GSCs. When ectopic hub differentiation is repressed, the decreased number of PGCs fails to become GSCs. Thus, we propose that SGPs sense PGC number via signals from PGCs to SGPs that modulate niche size, and that this serves as a mechanism for securing GSCs.