ABSTRACT
Unlike passive rupture of the human chorioamnion at birth, the insect extraembryonic (EE) tissues – the amnion and serosa – actively rupture and withdraw in late embryogenesis. Despite its importance for successful development, EE morphogenesis remains poorly understood. Contradicting the hypothesis of a single, fused EE membrane, we show that both tissues persist as discrete epithelia within a bilayer, using new tissue-specific EGFP transgenic lines in the beetle Tribolium castaneum. Quantitative live imaging analyses show that the amnion initiates EE rupture in a specialized anterior-ventral cap, while RNAi manipulation of EE tissue complement and function reveals that the serosa is autonomously contractile. Thus the bilayer efficiently coordinates the amnion as initiator and serosa as driver to achieve withdrawal. The novel bilayer architecture may reflect evolutionary changes in the EE tissues specific to holometabolous insects. More generally, tissue apposition in a bilayer exemplifies a high degree of functional interaction between developing epithelia.
