Aspects of the ultrastructure of mitotic nuclei of the fungus Uromyces phaseoli var. vignae are described from both intercellular hyphae in the cowpea host and infection structures induced to differentiate in vitro. The interphase nucleus-associated organelle (NAO) consists of two trilamellar acircular disks connceted by an osmiophilic bar. The intranuclear spindle develops between these disks when they separate. The spindle contains pole to pole, interdigitating, chromosomal, and fragmentary microtubules arranged to form a central bundle along the surface of which lie the metaphase chromosomes. No metaphase plate is found. There are up to three microtubules per kinetochore and approximately 14 chromosomes on the haploid spindle. Telophase elongation appears to involve extension of pole to pole microtubules with no evidence for the remaining presence of interdigitating microtubules. Concomitantly, numerous cytoplasmic microtubules develop from each NAO disk where few or none are present in other phases. Reformation of the interphase NAO involves the formation of a sausage-shaped intermediate at late telophase. The nuclear envelope remains intact and the nucleolus persists throughtout division. Various aspects of the spindle and NAOs appear to be evolutionary intermediates between Ascomycetes and higher Basidiomycetes, thus supporting the theory of Basidiomycete evolution from the former group and demonstrating an encouraging correlation between mitotic characteristics and other phylogenetic markers.

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