[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.

[Heterochromatic regions in corn A- and B- chromosomes]. / Geterokhromaticheskie raiony A- i B-khromosom kukuruzy.

A collection of maize forms from the Vavilov All-Russia Institute of Plant Breeding (VIR) was studied. We compared variation in the following traits: the number and size of heterochromatic knob regions (HKRs) of chromosomes of mother pollen cells at pachytene depending on the presence (B+) or absence (B0) of B-chromosomes; size of B-chromosomes (general, of heterochromatic part); and the frequency of B+ plants in groups of forms contrasting in the HKR number (10-15, 4-10, 2-7, 1-4). It was shown that B chromosomes had statistically significantly different effects on HKR polymorphism, relative heterochromatin "content" in the cell of multiknob and knobless forms, selection of plants for early ear flowering in these forms. The combination of maximum (and minimum) sizes of heterochromatic regions of A- and B-chromosomes was established; the genomic level of control of the trait is suggested. The role of the relationship of polymorphism at heterochromatic regions of A- and B-chromosomes between them and with the systems of maize reproduction is considered as a mechanism of maintenance of optimum plant heterozygosity via adaptive ontogenetic redistribution of heterochromatin among loci, chromosomes, and gametes.