Landmarks in the early duplication cycles of Aspergillus fumigatus and Aspergillus nidulans: polarity, germ tube emergence and septation.

When the spores of filamentous fungi break dormancy, nuclear division is accompanied by a series of ordered morphological events including the switch from isotropic to polar growth, the emergence of a second germ tube from the conidium and septation. Correlation of these morphological events with nuclear number allows them to serve as duplication cycle landmarks. Early duplication cycle landmarks have been characterized in Aspergillus nidulans, but not in other filamentous fungi. To learn more about duplication cycle control in filamentous fungi, a study was undertaken to compare the timing of landmarks in Aspergillus fumigatus and A. nidulans. Nuclear duplication took approximately 45 min in A. fumigatus, with mitosis occupying roughly 5% of this period. Under the same conditions, nuclear duplication in A. nidulans took approximately 60 min, with mitosis occupying roughly 4% of this period. In A. fumigatus the isotropic to polar switch preceded the first mitosis in 22% of cells, while in A. nidulans the isotropic to polar switch did not occur until after the first mitosis. In both A. fumigatus and A. nidulans the earliest emergence of a second germ tube from the conidium occurred after the third mitotic division. However, by the fifth mitosis only 19% of A. fumigatus conidia had a second germ tube, compared to 98% of A. nidulans conidia. In both A. fumigatus and A. nidulans, formation of the first septum occurred after the fourth mitotic division. In all experiments a few cells lagged behind the others in nuclear number. In this delayed group, it was common to see landmark events at an earlier mitotic division. Differences in nuclear number when identical landmarks occur in A. fumigatus versus A. nidulans, and uncoupling of mitotic division and landmarks in delayed cells suggest that nuclear division and morphogenesis lie in parallel pathways, perhaps coordinated by checkpoints.