Practice question

Drosophila ovarian germline stem cells reside at anterior tip contacting cap cells that secrete Decapentaplegic and Glass bottom boat, Drosophila BMP homologs. BMP ligands bind Thickveins and Saxophone receptors, phosphorylating Mad which complexes with Medea to repress bag-of-marbles, the key differentiation factor. This short-range TGF-beta signal maintains GSC self-renewal only within one cell diameter of niche. As cystoblasts move posteriorly, BMP decays, Bam accumulates, and differentiation proceeds through four mitotic divisions with incomplete cytokinesis, demonstrating niche-dependent spatial control of stemness.