Practice question

Germline stem cells in Drosophila ovary and testis are maintained by niche-derived BMP signals. Cap cells secrete Decapentaplegic and Glass bottom boat ligands that activate Thickveins receptors on adjacent GSCs, leading to phosphorylation of Mad and suppression of bag-of-marbles differentiation factor. Repression of bam keeps stem cells undifferentiated and proliferative. As daughter cystoblast moves away from niche, BMP concentration drops, bam is derepressed and differentiation proceeds through four transit divisions. Loss of BMP causes premature stem loss, while ectopic BMP expands stem pool, illustrating niche signaling sustaining self-renewal.