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.
