CONTROL OF RIBOSOMAL RNA GENE MULTIPLICITY IN WHEAT

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Abstract
Ribosomal RNA gene multiplicities are reported for various monosomic, tetrasomic and nullisomic-tetrasomic compensating types of wheat in the ''Chinese Spring'' background. Several diploid and tetraploid species as well as 3 varieties of hexaploid Triticum aestivum also were studied for the level of DNA/rRNA hybridization at the 3 ploidy levels. All 4 nucleolus organizer chromosomes 1A, 1B, 6B and 5D of wheat contain rRNA genes. The chromosome 1B might be the only one that behaves in a relatively additive fashion in different aneuploid conditions and the remaining 5 homoeologous chromosomes of homoeologous sets 1, 5 and 6 and perhaps other chromosomes might contain factor(s) that control the total number of rRNA genes. The proportion of rDNA in diploid, tetraploid and hexaploid wheats was similar, suggesting that the number of rRNA genes may have increased proportionally with ploidy level apparently representing balanced conditions regarding the control of rRNA gene multiplicity. The number of rRNA genes in hexaploid wheat varieties, however, was genotype dependent; ''Atlas 66'' and ''Chinese Spring'' wheat varieties possessed more rRNA genes than ''Era''.