The nucleolar organizer activity and the presence of rDNA zones were studied in Tricepiro Don René INTA, a synthetic forage crop (2n=42, 6x, AABBRR with introgression of Thinopyrum in 6A chromosome). The use of pTa71 probe revealed the presence of six rDNA zones but Ag-NOR indicated the presence of only four NORs (nucleolus organizer regions). Differential amphiplasty phenomenon could explain the inactivity of some rDNA zones. Simultaneous use of FISH (pTa71 probe) and DAPI banding analysis

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... cated that the chromosomes with rDNA zones belong to two rye and to four wheat chromosomes. The maximum number of nucleoli observed in somatic cells and the number of chromosomes with secondary constrictions agreed with the four active NORs detected with the Ag-staining method. The use of the pSc119.2 probe allowed to recognize that chromosomes with secondary constrictions are the wheat chromosomes 1B and 6 B. The results obtained using classical and molecular cytogenetics lead us to conclude that this crop has a similar behavior to that observed in hexaploid triticales in relation to the phenomenon of amphiplasty. Furthermore, the complexity of genetic interactions between wheat and rye genomes described in triticales, are not modified by the presence of Thinopyron introgression in Tricepiro Don René INTA.