Synchronous mitosis is common in multinucleated cells. We analyzed a unique 
asynchronous nuclear division cycle in a multinucleated filamentous fungus, 
Ashbya gossypii. Nuclear pedigree analysis and observation of GFP-labeled 
spindle pole bodies demonstrated that neighboring nuclei in A. gossypii cells 
are in different cell cycle stages despite close physical proximity. Neighboring 
nuclei did not differ significantly in their patterns of cyclin protein 
localization such that both G1 and mitotic cyclins were present regardless of 
cell cycle stage, suggesting that the complete destruction of cyclins is not 
occurring in this system. Indeed, the expression of mitotic cyclin lacking 
NH(2)-terminal destruction box sequences did not block cell cycle progression. 
Cells lacking AgSic1p, a predicted cyclin-dependent kinase (CDK) inhibitor, 
however, showed aberrant multipolar spindles and fragmented nuclei that are 
indicative of flawed mitoses. We hypothesize that the continuous cytoplasm in 
these cells promoted the evolution of a nuclear division cycle in which CDK 
inhibitors primarily control CDK activity rather than oscillating mitotic cyclin 
proteins.