Factors regulating the onset of epithelial-like polygonal architecture in the syncytial Drosophila embryo.

Abstract

Epithelial cell shapes in metazoans have a conserved hexagon-dominated polygon distribution. However, the geometric constraints and mechanisms that result in the spherical to polygonal transition remain to be elucidated. Here, we use syncytial Drosophila embryos to characterize the temporal onset of polygonal plasma membrane organization.We find that circular to polygonal plasma membrane shape transition occurs when the length of the lateral membrane increases beyond a threshold from interphase to metaphase in each nuclear division cycle. DE-cadherin levels increase while Myosin II levels decrease during the threshold ingression. DEcadherin depletion leads to decrease in furrow length and increase in circularity. Increased Myosin II activity results in complete loss of lateral membrane extension, thereby, giving rise to spherical plasma membrane architecture. Decreased myosin activity, on the other hand, leads to the transition to polygonal shape at a length below the threshold length. Our study, thus, elucidates the role of a balance between DE-cadherin and Myosin II across the lateral domain length in stable formation of polygonal architecture in syncytial Drosophila embryos. It further highlights the importance of fine tuning Myosin II-based contractility for achieving this threshold length.

We also look at the onset of epithelial-like polarity in the syncytial Drosophila embryos and find that the onset of epithelial-like polarity occurs at nuclear division cycle 12. This is coincident with the onset of hexagon dominance. Knockdown of polarity proteins, like Bazooka and Peanut, results in delayed onset of this hexagon dominance, while DE-cadherin depletion results in loss of hexagon dominance.

Taken together, we show that the syncytial Drosophila embryo shows epithelial-like characteristics in terms of shape distributions and polarity despite lacking a basal domain.