Extracellular Spreading Of Wingless Is Required For Drosophila Oogenesis

Recent studies have investigated whether the Wnt family of extracellular ligands can signal at long range, spreading from their source and acting as morphogens, or whether they signal only in a juxtacrine manner to neighboring cells. The original evidence for long-range Wnt signaling arose from studies of Wg, a Drosophila Wnt protein, which patterns the wing disc over several cell diameters from a central source of Wg ligand. However, the requirement of long-range Wg for patterning was called into question when it was reported that replacing the secreted protein Wg with a membrane-tethered version, NRT-Wg, results in flies with normally patterned wings. We and others previously reported that Wg spreads in the ovary about 50 mu m or 5 cell diameters, from the cap cells to the follicle stem cells (FSCs) and that Wg stimulates FSC proliferation. We used the NRT-wg flies to analyze the consequence of tethering Wg to the cap cells. NRT-wg homozygous flies are sickly, but we found that hemizygous NRT-wg/null flies, carrying only one copy of tethered Wingless, were significantly healthier. Despite their overall improved health, these hemizygous flies displayed dramatic reductions in fertility and in FSC proliferation. Further, FSC proliferation was nearly undetectable when the wg locus was converted to NRT-wg only in adults, and the resulting germarium phenotype was consistent with a previously reported wg loss-of-function phenotype. We conclude that Wg protein spreads from its source cells in the germarium to promote FSC proliferation.Author summaryWingless (Wg)/Wnt proteins act as important signals between cells in many contexts. For decades, studies in the Drosophila wing established that Wg signals to distant cells, implying that Wg spreads extracellularly. However, studies in other tissues and organisms have found Wnt ligands signal in a juxtacrine manner, to neighboring cells. Recently the importance of Wg spreading was re-evaluated in the fly wing, spurred by the finding that membrane-tethered Wg, unable to spread from its source cell, can substitute for Wg. These findings fueled a search for other tissues where Wg extracellular spreading is required. The nature of Wg signaling in Drosophila oogenesis has been unclear. In the germarium a visible gradient of Wg spans similar to 50 mu m, reaching from its source to follicle stem cells, but it has been argued that Wg signals from neighboring cells to the stem cells. In this study, we tested the role of Wg spreading by analyzing oogenesis in the tethered-Wg flies. Two copies of tethered Wg cause a non-specific Wg toxicity; however, even when the dose tethered Wg is reduced, ovaries have negligible follicle stem cell proliferation and produce few eggs. Thus, extracellular Wg spreading is essential for follicle stem cell proliferation and oogenesis.