The beetle amnion and serosa functionally interact as apposed epithelia

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. Withdrawal is essential for development and has been a morphogenetic puzzle. Here, we use new fluorescent transgenic lines in the beetle Tribolium castaneum to show that the EE tissues dynamically form a basal-basal epithelial bilayer, contradicting the previous hypothesis of EE intercalation. We find that the EE tissues repeatedly detach and reattach throughout development and have distinct roles. Quantitative live imaging analyses show that the amnion initiates EE rupture in a specialized anterior-ventral cap. RNAi phenotypes demonstrate that the serosa contracts autonomously. Thus, apposition in a bilayer enables the amnion as 'initiator' to coordinate with the serosa as 'driver' to achieve withdrawal. This EE strategy may reflect evolutionary changes within the holometabolous insects and serves as a model to study interactions between developing epithelia.

Morphogenetic functions of extraembryonic membranes in insects

“…The mechanism that controls extraembryonic membrane rupture remains elusive, butthe unexpected finding that serosa rupture is preceded and apparently triggered by the formation of a hole in the amnion marks a potential breakthrough [25,51].Dissecting the cellular and molecular events that generate the serosa-sealed hole in the amnionmight be essential for understanding the process of membrane rupture. Furthermore, this discovery hints at the importance of the amniotic cavity, which may store factors that prepare for serosa rupture.…”

“…Knockdown of zen activity(Of-zen) suppresses the formation of the hole in the amnion and prevents the subsequent rupture of the overlying serosa [25].Instead, the serosa in these embryos ruptures and contracts ectopically at a later stage,leaving the amniotic cavity intact and thereby causing ventral closure of the lateral epidermis around the legs.Likewise in Tribolium, knockdown of a zen homolog, Tc-zen2, leaves the amnion intact [27]. Additional data from the Panfilio group indicatethat in this species, too, the amnion forms a hole at the site where the attached serosa will rupture [51].…”

Evolution of epithelial morphogenesis: phenotypic integration across multiple levels of biological organization

“…Here, precision in determining the site of EE opening involves morphological specialization in a cap of amniotic cells. Furthermore, preparation for rupture in Tribolium involves the formation of an amnion-serosa epithelial bilayer over most of the amnion’s surface area ( Koelzer et al, 2015 ), not just the narrow ring of amnion-serosa contact seen in Oncopeltus . These differences in local behavior of the amnion and in the amnion-serosa connection are all the more striking given that Of-zen and Tc-zen2 , the second Tribolium paralog, both act extraembryonically to ensure that EE rupture occurs (Figure 1 : green diamonds; van der Zee et al, 2005 ; Panfilio et al, 2006 ).…”

“…The planar (lateral–lateral) nature of amnion-serosa attachment in Oncopeltus allows the serosa to efficiently pinch off and draw the edges of the amnion together above it (Figure 2B ). In Tribolium , inter-tissue shearing is required so that the portion of the amnion over the serosa (apical-basal connection) can detach, enabling final serosal internalization ( Koelzer et al, 2015 ).…”