NUCLEAR BEHAVIOR DURING CONJUGATION AND MEIOSIS IN THE FISSION YEAST SCHIZOSACCHAROMYCES POMBE

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The fine structure involved with the nuclear behavior occurring during conjugation and meiosis in the fission yeast Schizasaccharomyees pombe was studied by serial thin sectioning and electron microscopy. A pair of haploid cells conjugated via a cytoplasmic isthmus, where cytoplasmic vesicles accumulated. The fusion of the two nucleii was preceded by the fusion of respective nucleus-associated organelles (NAOs), and the prophase I nucleus, taking an elongated shape with the fused NAO at one end, contained strands of dense linear elements, whose number and distribution were not constant, as seen in the three-dimensional reconstruction of serial sections. The first meiotic division was carried out by the spindle, being a preserved nuclear envelope, in which chromosomal microtubules were found to associate with opposite NAOs connected by the pole-to-pole microtubules. The second meiotic division proceeded almost synchronously in the two separate nuclei, the formation of the forespore membrane beginning at the cytoplasmic side of the differentiated NAO.