STRUCTURE OF THE RIBOSOMAL-RNA GENES IN THE HAMSTER SPERM NUCLEUS

We have examined the structure of the major ribosomal RNA (rRNA) genes in the hamster sperm nucleus, using fluorescent in situ hybridization (FISH). The rRNA genes are present as tandemly repeated clusters loca ted at the telomeric ends of the short arms of five pairs of acrocentr ic chromosomes in the Syrian golden hamster (as they are in humans). I n somatic cells, these five chromosome pairs come together to form the nucleolus, the site of rRNA synthesis. The nucleolus remains intact t hrough S phase of the cell cycle, breaking apart only during late G(2) and mitosis when the chromosomes condense. Mammalian sperm nuclei are the final products of meiotic division and morphological differentiat ion that includes a dramatic chromatin condensation. Consequently, it was not immediately obvious whether the rRNA genes would be condensed into a nucleolus-like structure in the mature spermatozoa, or separate d, as they are in mitotic chromosomes. We found that of 117 sperm nucl ei examined, 91.5% contained between two and five FISH signals for the rRNA gene clusters, and 64.0% contained four (29%) or five (35%) sign als. In decondensed hamster sperm nuclei, the rRNA hybridized signals were separated into independent strands. These data collectively indic ate that the chromosomes containing the rRNA genes are not bound toget her into a pre-nucleolar structure in fully condensed mammalian sperm nuclei.