[Functional and mutational variations of heterochromatic region in the maize nucleolus organizer (NO) (variation of the NO heterochromatic region)].

The following facts are considered in connection with the problem of population polymorphism at heterochromatic regions of maize chromosomes: (a) variation (1-3 microm) of the heterochromatic region of nucleolus organizer (NO knob) in pollen mother cell at the pachytene stage; (b) presumably function-dependent variation of the degree of its compaction (from a compact state in the majority of plants to a puff-like state); (c) the presence of rearrangements in the NO knob region (duplications and deletion); and (d) homozygous (in all cases) state of the NO knob. Deletion is combined with alterations in the structure of chromosomal NO and the overall karyotype. It is assumed that inbreeding and MGEs influence the mutability of the NO locus and activation of the gene set controlling cytokinesis, chemical reduplication, and, possibly, rDNA amplification. The mutation was classified as a systemic mutation. The mechanisms of NO knob homozygotization in meiosis (mitosis) and the mechanisms of maintenance of the polymorphism at functionally inactive chromosome knob regions in maize populations are compared.