THE INFLUENCE OF B-CHROMOSOMES ON RDNA ORGANIZATION IN RYE INTERPHASENUCLEI

Patterns of rye rDNA organization in interphase nuclei were studied th rough the use of in situ hybridization in spreads of root meristem cel ls from plants with and without B chromosomes (Bs). In cells from plan ts without Bs each rDNA locus is organized as a single perinucleolar k nob of condensed chromatin with decondensed chromatin inside the nucle olus. In plants with Bs there is a marked modification of the pattern, found in more than 23% of nuclei, which involves several regions of c ondensed chromatin interspersed with decondensed chromatin inside the nucleolus. This B-induced alteration in rDNA interphase organization s uggests a change in expression of the rRNA genes located on the A chro mosomes probably related to the reduction in nuclear RNA observed prev iously in plants with Bs. The influence of the Bs on the expression of A chromosome genes, through rearrangement of interphase chromatin, co uld provide the basis of an explanation for some of the known phenotyp ic effects of B chromosomes in rye.