THE NUCLEOLUS ORGANIZER REGION OF MAIZE (ZEA MAYS L.): TESTS FOR RIBOSOMAL GENE COMPENSATION OR MAGNIFICATION

R. L. Phillips ¹, D. F. Weber ¹, R. A. Kleese ¹, and S. S. Wang ¹

¹ Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, Minnesota 55101 Department of Biological Sciences, Illinois State University, Normal, Illinois 61761

Ribosomal gene compensation and magnification that might be detected on a whole-plant basis was not found in maize. Plants monosomic for chromosome 6 (the NOR chromosome) were compared with monosomic-8 and monosomic-10 plants, disomic sibs, and parental lines. Assuming no rDNA compensation, monosomic-6 plants showed approximately the decrease expected in rRNA cistron number. Monosomic-8 had a normal ribosomal gene number, while monosomic-10 showed a decrease; but further documentation is needed. Besides demonstrating the absence of gene compensation, the results document our previous conclusion that maize chromosome 6 carries DNA complementary to ribosomal RNA. Further documentation was provided from studies with trisomic chromosome 6 plants showing proportional increases in ribosomal gene number. Progeny of the monosomic plants crossed as males to a standard singlecross hybrid possessed expected ribosomal gene numbers suggesting the lack of ribosomal gene magnification.—The ragged (rgd) mutant of maize, suspected of being deficient in rRNA cistrons, had a normal number.