Gap Phase Introduction in Every Cell Cycle of C. elegans Embryogenesis

Early embryonic cell cycles usually progress with S and M phases without any gap phase. How the gap phases are introduced spatiotemporally over development remains poorly defined. To establish the cell-specific introduction of gap phases in embryo, we generate multiple fluorescence ubiquitin cell cycle indicators (FUCCI) in C. elegans. Time-lapse 3D imaging followed by lineal expression profiling reveals sharp and differential dynamics of the FUCCI reporters, allowing the precise demarcation of cell cycle phases throughout embryogenesis. Surprisingly, most cells divide with S and M phase only until they have completed their last round of division, after which they arrest in the G0 or G1 phase with two exceptions. Only a single cell pair divides with full cycle and the E2 and E4 cells temporarily acquire the G2 phase. In summary, we provide a first global picture of gap phase introduction in a metazoan embryo with the newly developed FUCCI reporters.