CHROMOCENTER ASSOCIATION IN PLANT-CELL NUCLEI - DETERMINANTS, FUNCTIONAL-SIGNIFICANCE, AND EVOLUTIONARY IMPLICATIONS

Owing to close associations of different chromosomes at their heteroch romatic regions, nuclear repatternings consisting of changes in the nu mber and size of the chromocenters may occur with tissue differentiati on in plant species. By studying these changes in the nuclear structur e in a number of Magnoliopsida with chromocentric nuclei, we found: (i ) that chromocenter association is affected by the degree of partition ing of the genome, since the extent of the phenomenon correlates posit ively with the number of chromosomes in the complement, the level of p loidy, and the number of chromocenters in meristematic nuclei, whereas the correlation with the mean DNA content per chromosome is negative; (ii) that this kind of nuclear repatterning plays a role in or is a m arker of events that have a part in the control of nuclear activity, s ince a negative correlation was found to exist, as a rule, between the extent of chromocenter association and that of RNA synthesis in the n ucleus. and (iii) that this large-scale control mechanism of gene expr ession is exploited in cell differentiation from its early stages. Ind eed, chromocenter association takes place at the base of the meristems and remains unchanged during further tissue development. In certain s pecies, this phenomenon has not been observed; its presence or absence may represent a character of entire families, and may be linked to th e position of a species on an evolutionary scale.