The number of DNA genes coding for ribosomal RNA in a number of species and karyotypes of the subtribe Triticinae was determined through rRNA/DNA in vitro hybridization. One of the Nucleolar Organizing Regions in wheat is carried by the short ann of chromosome 5D. The combinations studied included aneuploid lines with different dosages of chromosome 5D or of its long arm (5D L) or short arm (5DS). At the hexaploid level, the deletion of the short arm of this chromosome induced different alterations in total rDNA depending on the morphological structure of the long arm (ditelosomic 5D L, 68%; mono-isosomic 5D L, 95%; di-isosomic 5D L, 84%). In tetrasomic 5D plants the rRNA gene number was decreased by 30% in contrast with nullisomic 5D-tetrasomic 5A or 5B which only showed a slight reduction (10%). At the tetraploid level, the presence of a segment of 5D L in the genotype of T. durum or the addition of two chromosomes 5D to the genotype of T. dicoccum increased the number of rRNA genes by 19 and 33% respectively.
These results show not only that the short ann of chromosome 5D carries very few rRNA genes in its NOR, but that there is no correlation between the number of rRNA genes and of NOR's in 5D. The existence of a regulatory mechanism, located in 5D L, which modulates the number of rRNA genes present in the NOR's of wheat chromosomes, is suggested.