Presence of additional RNA and DNA during microsporogenesis of Sikkim Primitive

Abnormal meiotic patterns were found in some cells during our study of the chromosome morphology of Sikkim Primitive. These cells contained additional DNA and, in some cases, a second nucleolus. The modified Stockinger and Kellner staining procedure was used to demonstrate the presence of two nucleoli, while presence of additional DNA was shown with the Feulgen stain. The amount and morphology of this additional DNA is extremely variable, ranging from a tight heterochromatic knot to small and irregular fragments scattered in the cytoplasm. However, this additional DNA did not have the appearance of supernumerary chromosomes. We estimate that the cells with additional DNA occurred at an overall rate of 8% in our samples while cells which had two nucleoli were much less common. Our observations at pachytene indicate that the cells containing two nucleoli have in general two functional nucleolar organizers, one found at chromosome 6 and the second one on a small fragment of additional DNA. When present, the two nucleoli seem to be in synchrony in the sense that their size in a given cell is identical but some significant exceptions were found and most of the cells with smaller nucleoli did not show the presence of the second nucleolar organizer. It was interesting to discover the persistence of a nucleolus at metaphase. At this stage the nucleolus is rarely found associated with DNA. A vesicle, which appears to be a nucleolus, is present in some cases at anaphase where severe disruptive effects seem to be caused by its presence. Our data indicate that the abnormal cells are seen most commonly before the first meiotic division and possess in general a volume larger than the cells which appear normal. In these cells, it is not uncommon to find some irregular, non-staining structures which have not yet been characterized. The fate of the additional DNA is difficult to trace through meiosis, but we expect to find more information on this question in the mitotic study of the selfed plants. Persistent nucleoli and additional DNA levels during cell division are two characteristics which have been frequently observed in cancer cells but remain essentially unexplained events. The etiology and significance of these findings is currently under investigation in our laboratory.

John P. Peeters

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