It is becoming increasingly clear that within the cell nucleus the chromatin has morphological and functional compartmentalizations related to the ribonucleoprotein and deoxyribonucleoprotein activities and to the nuclear organization of gene expression. This concept derives from Rabl and Boveri whose anticipatory idea was that chromosomes maintain their anaphase-telophase orientation, i.e. individual chromosomes in interphase nuclei tend to occupy exclusive territories rather than to intermingle.

We are investigating the spatial relationship of the chromosomal arms in Rb (Robertsonian) heterozygotes compared to those of all-telocentric and Rb homozygotes (Garagna et al., 1995). The possible changes in the chromosomal nuclear territories (due to the presence of Rb heterozygosities) may be correlated with the accuracy of the cytodifferentiative process of the male germ cells (Garagna et al., 2001).

The cytodifferentiation of the male gamete is being studied in mice with different karyotypes:

i) standard all-telocentric chromosomes;

ii) homozygous Robertsonian (Rb) metacentrics derived by whole-arm reciprocal translocations;

iii) heterozygous hybrids obtained from the cross between all-telocentric and Rb metacentric homozygous mice and between Rb homozygous races with different Rb chromosome arms.