Banana (Ban) was isolated as a spontaneous morphological mutant (Newmeyer and Galeazzi 1974). It shows four abnormal phenotypes: giant ascospores, abnormal ascogenous tissue, abnormal vegetative morphology, and female sterility. The first two phenotypes are dominant and the latter two are recessive. The accompanying 32 photographs illustrate abnormal ascospore development, reverted ascogenous tissue, and the interaction of two ascus mutants Banana and Peak.
Giant ascospores: In crosses of wild type x Ban, each ascus cuts out a single giant ascospore enclosing all four products of meiosis and their mitotic derivatives (Fig. 1; Raju and Newmeyer 1977). Early ascus development and meiosis are apparently normal. Spindles and spindle pole body plaques also appear normal through meiosis and the first postmeiotic mitosis. However, unlike in wild-type asci where all eight nuclei line up in single file prior to spore delimitation, the pairs of sister nuclei from each spindle fail to line up, but remain on opposite sides of the ascus. A single giant ascospore is now cut out that encloses all eight nuclei (Figs. 2-8). After a single mitosis in the just-delimited giant ascospores, most of the spores contain 16 nuclei (Figs. 9-11). However, ~ 10% of young ascospores contain 32 nuclei, perhaps because a second postmeiotic mitosis had occurred before the spore wall was formed (Fig. 12). Giant ascospores develop longitudinal striations as they pigment and mature (Fig. 13). Four or five additional mitoses occur in the mature, black ascospores.
Synchronous mitoses and probable endoreduplication in abnormal ascogenous cells: In older perithecia, after ~ 50 to 75 asci have been differentiated, most of the later-formed crozier and precrozier cells fail to differentiate new asci. Instead, they either and revert to mitosis (Figs. 14-24) or form enlarged nuclei (Figs. 25-27). Mitoses are synchronous in the reverted crozier and precrozier cells (Figs. 21-24). The nuclei and chromosomes are much larger than in vegetative hyphae providing superb material for observing the stages of mitosis. We have used these multinucleate cells to show that chromosome behavior in mitosis is essentially the same as that in higher eukaryotes (Raju 1984; Raju and Newmeyer 1977).
Ascospore inclusions: In the developing asci, ascospore-delimiting membranes are formed at the periphery of the ascus during interphase II. Following a postmeiotic mitosis, these membranes invaginate to sequester individual nuclei and the immediately surrounding cytoplasm into eight uninucleate ascospores. Since a single giant ascospore requires less spore-delimiting membrane than do eight normal-size ascospores, there appears to be a surplus. Elongated tubular inclusions are formed inside the giant ascospores, which we interpret as being the unused wall membranes (Figs. 28-30). Similar ascospore inclusions have also been found in the giant, round, ascospores that result from the interaction of Banana with a swollen ascus mutant Peak (Figs. 31, 32).
Use of Banana in cell biology and genetic analysis: The Banana mutation was used to rescue Spore-killer sensitive nuclei by enclosing them together with killer nuclei in the giant ascospores (Raju 1979; 1994). Ban was also used to show diffusion of GFP-tagged histone H1 protein across the cytoplasm of giant ascospores that are heterokaryotic for tagged and untagged nuclei (Freitag et al. 2004).
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