Mitotic trigger waves and the spatial coordination of the Xenopus cell cycle

Despite the large size of the Xenopus laevis egg (~1.2 mm diameter), a fertilized egg rapidly proceeds through mitosis in a spatially-coordinated fashion. Mitosis is initiated by a bistable system of regulatory proteins centered on Cdk1(1,2), raising the possibility that this spatial coordination could be achieved through trigger waves of Cdk1 activity(3). Using an extract system that carries out cell cycles in vitro, we show that mitosis does spread through Xenopus cytoplasm via trigger waves, propagating at a linear speed of ~60 µm/min. Perturbing the feedback loops that give rise to Cdk1’s bistability changes the speed and dynamics of the waves. Time lapse imaging of intact eggs argues that trigger waves of Cdk1 activation are responsible for surface contraction waves, ripples in the cell cortex that precede cytokinesis(4,5). These findings indicate that Cdk1 trigger waves help ensure the spatiotemporal coordination of mitosis in large eggs. Trigger waves may be an important general mechanism for coordinating biochemical events over large distances.