Microscopic analysis of Neurospora ropy mutants defective in nuclear distribution

Movement and distribution of nuclei in fungi has been shown to be dependent on microtubules and the microtubule-associated motor cytoplasmic dynein. Neurospora crassa mutants known as ropy are defective in nuclear distribution. We have shown that three of the ro genes, ro-1, ro-3, and ro-4, encode subunits of either cytoplasmic dynein or the dynein activator complex, dynactin. Three other ro genes, ro-7, ro-10, and ro-11, are required for the integrity or localization of cytoplasmic dynein or dynactin. In this report, we describe a microscopic analysis of N. crassa ro mutants. Our results reveal that defects in germination of conidia, placement of septa, and mitochondrial morphology are typical of ro mutants. Two classes of cytoplasmic microtubules are identified in wild-type and ro mutants. One class of microtubules has no obvious association with nuclei while the other class of microtubules connects spindle pole bodies of adjacent nuclei. The possible role of internuclear microtubule tracts in the movement and distribution of nuclei is discussed.