Fig. 6. Interspecific comparisons of zygotic cell cycle checkpoints. (A) Summary model suggested by the data in Fig. 5. Regulatory processes are represented by magenta circles and broken magenta arrows (see text for details). (B) The present study suggests that regulation of zygotic development in F. serratus is accomplished by a system of checkpoints that lie intermediate between those of the tightly coupled budding yeast (C) and the loosely coupled Drosophila embryo (D). Zygotic polarization and cell cycle progression in F. serratus are coordinated by Ca²⁺ dependent processes, but there is no dependence of the cell cycle on polarization. (C) In budding yeast, cell cycle progression and polarization are made interdependent by the G1/S checkpoint and the morphogenesis checkpoint. (D) In Drosophila embryos, cell cycle progression and zygotic polarization are independent, run in parallel and may be uncoupled from each other by inhibitor treatments.