Control of mitotic synchrony in Physarum polycephalum. Phase shifting by fusion of heterophasic plasmodia contradicts a limit cycle oscillator model.

Fusion of two multinuclear plasmodia of Physarum polycephalum representing different stages of the mitotic cycle causes a rapid mitotic synchronization of "young" (A) and "old" (B) nuclei [2, 14, 19]. This communication reveals that even at large phase differences synchronization always occurs by advancing "young" and retarding "old" nuclei. This strongly contradicts a limit cycle oscillator model [8] which would permit synchronization on the shorter arc of the phase circle (advancing B and retarding A) at phase differences larger than 1/2 of the cycle. Density labelling of newly synthesized DNA in mixed plasmodia with BdUrd indicates that G2-nuclei become irreversibly committed for mitosis and DNA synthesis at a transition point located within only a few minutes of the visible onset of chromosome condensation.