Bivalent 15 regularly associates with the sex vesicle in normal male meiosis

Using fluorescent in-situ hybridization, we investigated the positioning of different human bivalents at the pachytene stage of normal male meiosis. W e showed that, in about 35% of nuclei, the pericentromeric region of bivale nt 15 is closely associated with the sex vesicle (SV). This behaviour may b e linked to the presence of three domains in the pericentromeric region of chromosome 15: a large imprinted domain, a nucleolar organizing region (NOR ), and a heterochromatic block. In order to define the domains of chromosom e 15 involved in this association, we analysed the meiotic behaviour of oth er bivalents with similar domains: human bivalent 11 and mouse bivalent 7, bearing imprinted domains, other human acrocentric bivalents bearing a NOR, and the human bivalents 1, 9 and 16 containing a heterochromatic region. N one of these bivalents were as frequently associated with the SV as the hum an bivalent 15. Nevertheless, we suggest that the bivalent 15 heterochromat in may be responsible for the association because of two properties: its te lomeric location on chromosome 15 and its strong sequence homology with the Yq heterochromatin. This phenomenon could explain the high frequency of tr anslocations between the chromosome 15 and the X or Y chromosomes.