Practice question

In Drosophila ovary, cap cells located at terminal filament tip secrete Decapentaplegic, activating BMP pathway in adjacent germline stem cells to maintain identity. Phosphorylated Mad-Medea complex directly silences transcription of bag-of-marbles, encoding differentiation-promoting factor required for cyst formation and meiosis entry. High local BMP concentration sustains self-renewal, prevents premature differentiation, and represses bam via heterochromatin modulation. Daughter cells displaced from niche experience BMP attenuation, de-repress Bam, and initiate four synchronous transit-amplifying divisions leading to oocyte and nurse cells, constituting bistable switch.