Specific chromosomal control of the nucleolus and of the cytoplasm in wheat

Abstract

The effect on nucleolar and cytoplasmic growth of increased and decreased doses of various of the chromosomes of hexaploid wheat was studied. Removal of pairs of strong nucleolar chromosomes or of either arm of nucleolar chromosomes was found to have little or no effect on nucleolar volume, nucleolar dry mass, or cytoplasmic dry mass, or cytoplasmic RNA content. With respect to numbers of nucleoli produced, normally latent nucleolar chromosomes can compensate for almost 100 per cent of the nucleolar activity of a missing pair of strong nucleolar chromosomes. Cells tetrasomic for non-nucleolar chromosomes do not differ significantly from the normal in nucleolar volume, nucleolar dry mass, or cytoplasmic dry mass. On the other hand, cells tetrasomic for the strong nucleolar chromosomes show significant increases (about 50 per cent) in nucleolar volume, nucleolar dry mass, cytoplasmic dry mass, and cytoplasmic RNA content. Cells tetrasomic for the strong nucleolar chromosomes, but missing one pair of non-nucleolar chromosomes, so that the total chromosome number is the same as in normal cells, behave as regular tetrasomics for these nucleolar chromosomes. In the microsporocyte tissue studied, both nucleolar and cytoplasmic protein synthesis and cytoplasmic RNA content is under specific nucleolar chromosomal control. The increase in cytoplasmic mass may result from the increase in nucleolar mass, increase in the products synthesized by the nucleolus, or an increase in general nuclear RNA and protein brought about by the increased dose of nucleolar chromosomes. The increase in cell mass resulting from additional nucleolar chromosomes may have important implications in respect to the mitotic index.