Background: Meiotic pairing is essential for the proper orientation of chromosomes at the metaphase plate and their subsequent disjunction during anaphase I. In male Drosophila melanogaster, meiosis occurs in the absence of recombination or a recognizable synaptonemal complex (SC). Due to limitations in available cytological techniques, the early stages of homologous chromosome pairing in male Drosophila have not been observed, and the mechanisms involved are poorly understood.
Results: Chromosome tagging with GFP-Lac repressor protein allowed us to track, for the first time, the behavior of meiotic chromosomes at high resolution, live, at all stages of male Drosophila meiosis. Homologous chromosomes pair throughout the euchromatic regions in spermatogonia and during the early phases of spermatocyte development. Extensive separation of homologs and sister chromatids along the chromosome arms occurs in mid-G2, several hours before the first meiotic division, and before the G2/M transition. Centromeres, on the other hand, show complex association patterns, with specific homolog pairing taking place in mid-G2. These changes in chromosome pairing parallel changes in large-scale chromosome organization.
Conclusions: Our results suggest that widespread interactions along the euchromatin are required for the initiation, but not the maintenance, of meiotic pairing of autosomes in male Drosophila. We propose that heterochromatic associations, or chromatid entanglement, may be responsible for the maintenance of homolog association during late G2. Our data also suggest that the formation of chromosome territories in the spermatocyte nucleus may play an active role in ensuring the specificity of meiotic pairing in late prophase by disrupting interactions between nonhomologous chromosomes.