Neurospora nuclei in hyphae and conidia are small (~2 µm diameter), making observations of mitosis difficult. The genome is small, the nuclear envelope persists, cells are highly multinucleate, and divisions are asynchronous. These difficulties have been partially overcome by using stages when the nuclei and chromosomes are exceptionally large, as in the ascus, by observing synchronous mitoses in the enlarged ascogenous cells of the mutant Banana, and by using ethylene glycol to induce giant cells and nuclei. Mitotic divisions studied in these different cell types clearly resemble the prophase, metaphase, anaphase, and telophase stages of higher eukaryotes.
Ascus divisions. The ascus is an exceptionally large cell, and the four large postmeiotic nuclei show conventional division stages. The second postmeiotic mitosis occurs in the young, immature ascospores. Divisions within an ascus are synchronous. Following McClintock’s (1945) initial description of postmeiotic mitoses in the ascus, detailed observations were made by Singleton (1953). The spindles, nucleolus, and individual chromosomes are clearly resolved in favorable nuclei (Figs. 1-6; Raju 1980).
Croziers in the Banana mutant. The giant ascospore mutant Banana makes enlarged multinucleate crozier cysts that are particularly suitable for observing mitosis (Raju and Newmeyer 1977). In crosses of wild type x Ban, the first-formed croziers and asci develop normally. The later-formed precrozier and crozier cells, however, do not develop into asci. Instead they revert to mitosis, producing swollen cysts containing many nuclei. All nuclei in an individual cyst divide synchronously, providing replicate information for unambiguous identification of division stages (Figs 7-10).
Ethylene glycol-induced giant cells. When Neurospora conidia are cultured in liquid medium containing 3.22 M ethylene glycol, they grow without cell division, forming giant spheres with multiple nuclei (Bates and Wilson 1974). In a few giant cells, nuclear numbers remain small (1-3), but the nuclei become very large (Figs 11-25). Seven large chromosomes are seen in some nuclei suggesting polyteny, and 14 or more chromosomes are seen in other large nuclei, indicating polyploidy. Cell volume and nuclear volume are positively correlated in giant cells. Nuclear divisions are not synchronous within individual multinucleate giant cells. Spindles and spindle pole bodies, which are normally conspicuous in asci, are also seen in normal and reverted croziers, but they have not been clearly identified in the ethylene glycol-induced giant cells (Raju 1984).
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