Somatic nuclear division in the sporidia of Ustilago violacea. I. Acetic orcein staining.

Abstract

The course of somatic nuclear division was investigated in randomly and synchronously dividing populations of Ustilago violacea. Interphase nuclei with diffuse chromatin are found in the sporidia just before budding, but distinct chromatin granules form at the onset of bud formation. These granules become more condensed and appear to be connected by lightly staining fibers. When the bud cell reaches about half the size of the parent cell, the granules aggregate into two parallel 'bars' or 'tracks' of chromatin. This double structure migrates towards the isthmus between parent cell and bud cell and passes through completely, leaving the parent cell enucleate. The nucleus in the bud cell becomes highly condensed and is composed of two lobes which separate from each other to form two daughter nuclei in the bud cell. One nucleus returns to the parent cell and immediately enters interphase, while the other is retained in the bud cell in telophase. The nucleus in the bud cell remains in telophase until the cell reaches the full size of a parent cell, when it then enters interphase. Counts of the chromatin granules before the formation of the 'double-bar' stage show that the number of visible elements may be as many as 20 in both haploid and diploid strains, and it is suggested that previous claims that n = 2 in U. violacea are based on misinterpretation of aggregates of chromosomes. The similarity in the number of visible elements in haploid and diploid strains is considered to be a result of somatic association of homologous chromosomes in the diploid strain.