Germinating conidiospores switch to polarized apical growth following an initial period of isotropic expansion. At the same time, they re-enter the nuclear division cycle. The relationship between spore polarization and nuclear division was investigated by testing the effect of cell cycle inhibitors and temperature-sensitive cell cycle mutations on spore morphogenesis. On rich media, it was found that spore polarization is delayed if completion of the first mitosis is blocked. The observed delay may be dependent upon the activity of the mitosis-promoting NIMA kinase. An additional mechanism appears to prevent polarization as the spore progresses through its first S phase. In contrast, on poor media, spore polarization does not require completion of the first mitosis. These observations suggest that spore morphogenesis is influenced by cell cycle signals in a growth-dependent manner.


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