Practice question

Interdigital sculpting divergence between ducks and chicks illustrates differential BMP antagonism controlling webbing. In chick, BMP2 BMP4 BMP7 in interdigital mesenchyme activate Smad signaling, upregulate Msx genes and drive caspase-dependent apoptosis, freeing digits into separate units. In duck, persistently elevated Gremlin1, a DAN family BMP antagonist, is maintained in interdigital tissue, binding BMPs and blocking apoptotic cascade, retaining webbing for swimming adaptation. Ectopic Gremlin in chick mimics webbing, Gremlin inhibition restores apoptosis. SHH maintains Gremlin loop, Wnt7a dorsalizes, FGF10 drives outgrowth.