THE DEVELOPMENTAL STAGE OF INACTIVATION OF RYE ORIGIN RIBOSOMAL-RNA GENES IN THE EMBRYO AND ENDOSPERM OF WHEAT X RYE F1 HYBRIDS

To identify the developmental stage during which the preferential inac tivation of rRNA genes from the rye parent occurs in wheat x rye hybri ds, nucleolar activity was evaluated in the embryo and endosperm of de veloping seeds of the hybrids. The hybrids were obtained from crosses of euploid and aneuploid lines of hexaploid wheat, Triticum aestivum c v. Chinese Spring, with rye, Secale cereale cv. Centeio do Alto. The n umber of nucleolar organizing regions (NORs) and nucleoli present in t he embryo and endosperm cells of wheat, and wheat x rye F-1 hybrids, a t different times after fertilization was scored by silver staining. T he inactivation of rDNA of rye origin in F-1 hybrids occurs simultaneo usly in the embryo and in the endosperm between 4 and 5 days after fer tilization, when these have been through six and 10 cell cycles respec tively; We conclude that the genomic interactions leading to the inact ivation of the rye origin rDNA is a time-dependent process, related to the developmental stage and independent of the number of cell cycles (DNA replication rounds) they have been through.