Cdk1 and Plk1/Plx1 activation leads to their inactivation through negative 
feedback loops. Cdk1 deactivates itself by activating the APC/C, consequently 
generating embryonic cell cycle oscillations. APC/C inhibition by the mitotic 
checkpoint in somatic cells and the cytostatic factor (CSF) in oocytes sustain 
the mitotic state. Plk1/Plx1 targets its co-activator Bora for degradation, but 
it remains unclear how embryonic oscillations in Plx1 activity are generated, 
and how Plk1/Plx1 activity is sustained during mitosis. We show that 
Plx1-mediated degradation of Bora in interphase generates oscillations in Plx1 
activity and is essential for development. In CSF extracts, phosphorylation of 
Bora on the Cdk consensus site T52 blocks Bora degradation. Upon fertilization, 
Calcineurin dephosphorylates T52, triggering Plx1 oscillations. Similarly, we 
find that GFP-Bora is degraded when Plk1 activity spreads to somatic cell 
cytoplasm before mitosis. Interestingly, GFP-Bora degradation stops upon mitotic 
entry when Cdk1 activity is high. We hypothesize that Cdk1 controls Bora through 
an incoherent feedforward loop synchronizing the activities of mitotic kinases.