KARYOKINESIS OF THE SOMATIC NUCLEUS OF ASPERGILLUS NIDULANS. I. THE JUVENILE CHROMOSOME CYCLE (FEULGEN STAINING)

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Abstract
Somatic karyokinesis in the hyphae of Aspergillus nidulans can be divided into at least 2 distinct chromosome cycles. The 1st, or juvenile cycle, is a mode of karyokinesis which is restricted to young fast growing hyphae. During resting stage the nucleus in the conidium is contracted and irregular to spherical in shape. On incubation the nucleus unfolds and forms a nuclear filament to which a terminal extra-chromosomal body is attached. The nuclear filament is particulate in nature containing approximately nine Feulgen-positive bodies. It is assumed that these chromatin bodies represent the 8 chromosomes of A. nidulans together with a DNA-containing centriole. The nuclear filament can attain several configurations such as linear filamentous, crescent- or horseshoe-shaped and ring-shaped. It is believed that crescent-, horseshoe-, and ring-shaped nuclei are nuclear filaments wound around a disc-like "central body" (nucleolus), and that these configurations arise as a result of nuclear streaming and nucleolar development. Prior to nuclear division, the centriole divides. In ring-shaped nuclei, rotation of the daughter centrioles through 180[degree] can be observed. After the centrioles have taken up polar positions the nucleus divides. No spindle apparatus can be detected and it is thought that cytoplasmic streaming forces separate the daughter nuclei. After division the daughter nuclei can be either filamentous, crescent- or ring-shaped. Subsequent nuclear divisions occur in rapid succession without intermediate resting stages. After 14 hr. of incubation the nuclei condense into a resting-stage-like structure. On further incubation this structure enlarges and yields an extremely large nuclear filament which belongs to the 2nd chromosome cycle (maturation cycle). This cycle provides the nuclear material necessary for the development of special vegetative structures such as the sterigmata and conidia.