Genetic variation in the orientation of nuclear spindles during the development of asci in Neurospora.

Abstract

The effect of a single mutant gene, peak-2, in Neurospora crassa is to alter the ascus from one in which the eight spores are linearly arranged to one in which the eight spores are arranged nonlinearly. The same gene transferred to N. tetrasperma determines an ascus similar to that of the mutant N. crassa but in striking contrast to the normal ascus for N. tetrasperma, which includes four heterokaryotic spores, linearly arranged. A comparative cytological study has been made on asci from both species,-including material from zygotes that are homozygous normal, heterozygous for peak-2, and homozygous for peak-2. The observed morphogenetic differences are closely related to differences in nuclear spindle orientation, which in turn are correlated with the geometry of the cell. The observations are shown to bear on the basis of pseudohomothallism in N. tetrasperma. Speculations on the importance of the cell wall in relation to spindle orientation are presented. Differences of expression of peak-2 in the different species are discussed. THE SIGNIFICANCE of spindle behavior in the development of the ascus of Neurospora, particularly in determining the positions reached by the products of the different nuclear divisions in developing asci, has been emphasized by Dodge (1927, 1942), McClintock (1945), Dodge, Singleton, and Rolnick (1950), and Singleton (1953). These workers also have indicated how spindle behavior in a developing cellular system determines the major morphological difference between the species of Neurospora that have eight-spored and those that have four-spored asci. The morphological difference is not merely a matter of spore number but is associated with profound genetic consequences.