Abstract

The size and shape of the chromo- somes, as well as the chromosomal domains that compose them, are determinants in the distribution and interaction between the bivalents within the nucleus of spermato- cytes in prophase I of meiosis. Thus the nuclear architecture characteristic of the karyotype of a species can be modified by chromosomal changes such as Robertsonian (RB) chromosomes. In this study we analysed the meiotic prophase nuclear organization of the heterozygous spermato- cytes from Mus musculus domesticus 2n=26, and the synaptic configuration of the hexavalent formed by the dependent Rb chromosomes Rbs 6.16, 16.10, 10.15, 15.17 and the telocentric chromosomes 6 and 17. Spreads of 88 pachytene spermatocytes from two males were studied and in all of them five metacentric bivalents, four telo- centric bivalents, one hexavalent and the XY bivalent were observed. About 48% of the hexavalents formed a chain or a ring of synapsed chromosomes, the latter closed by synapsis between the short arms of telocen- tric chromosomes 6 and 17. About 52% of hexavalents formed an open chain of 10 synapsed chromosomal arms belonging to 6 chromosomes. In about half of the unsy- napsed hexavalents one of the telocentric chromosome short arms appears associated with the X chromosome single axis, which was otherwise normally paired with the Y chromosome. The cluster of pericentromer- ic heterochromatin mostly determines the hexavalent’s nuclear configuration, drag- ging the centromeric regions and all the chromosomes towards the nuclear envelope similar to an association of five telocentric bivalents. These reiterated encounters between these chromosomes restrict the interactions with other chromosomal domains and might favour eventual rearrangements within the metacentric, telocentric or hexavalent chromosome sub- sets. The unsynapsed short arms of telocen- tric chromosomes frequently bound to the single axis of the X chromosome could fur- ther complicate the already complex segre- gation of hexavalent chromosomes.