Systematics of the Zygnemataceae (Chlorophyceae). IV. Nuclear Cytology of Sirogonium melanosporum, a Species with Large Chromosomes

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Abstract
The nuclear cytology for 3 strains of the green alga S. melanosporum is described. Interphase nuclei contain 1-6 nucleoli, a nucleolar-organizing track, and 23-26 chromocenters. During cell division, nucleoli are transformed into nucleolar substance which coats the chromosomes and is transported through mitosis on them. Chromatin strands with chromocenters condense in prophase to produce 6 rod-shaped chromosomes varying in length from 4.3-6.0 .mu.m. Two of these are nucleolar chromosomes. At metaphase, chromosomes are packed tightly within the nuclear envelope; at anaphase, separating chromatids are parallel; and at telophase, chromosomes lengthen with the nucleolar substance organized into discrete units which fuse to form one or more nucleoli. Cell division terminates with the reorganization of 2 nuclei that lie close to the newly formed septum or crosswall. No centromeres were observed, but the waviness of chromosome at anaphase suggests that they may be polycentric.