Dominance of a NOR (nucleolar organizer region) over its allele and over its sister NOR after asymmetric 5-azacytidine substitution in plant chromosomes

Dominance of a 5-azacytidine-substituted nucleolar organizer region (AZA NOR) over another with normal DNA when confined to a common nucleus or to different nuclei in a common cell was studied in Allium cepa L. root meristems. AZA administration took place for one S period. The assessment of NOR dominance was done either in early Gi (nucleologenesis) or later in interphase (mature nucleoli). At the mitosis immediately preceding the second interphase after AZA, the segregated AZA and normal chromatids of each chromosome of the pair of homologues were confined to a single binucleate cell by inhibiting the formation of the cell plate. Anaphase segregation of the homologous AZA NORs was seen to occur at random. The AZA NORs completed nucleologenesis after a shorter time than normal NORs, independently of the intranuclear or intracellular presence of other NORs. After nucleologenesis was over in the AZA NORs, a few of the normal NORs, when in a common nucleus, remained permanently inefficient in developing a full-size mature nucleolus or any nucleolus at all (partial or total dominance, respectively). Dominance of an AZA NOR over its sister or allelic NOR with normal DNA led to the new nucleolar patterns of asymmetry and inverse symmetry, respectively, making their appearance in these binucleate cells.