Reproduction Abstracts

Introduction: In meiosis, homologous chromosomes pair, synapse, and recombine with their partners in parallel with the formation of the synaptonemal complex, a tripartite structure with two axial elements connected by transverse filaments. It has been demonstrated that two meiosis-specific cohesin subunits, RAD21L and REC8 are essential for the formation of the axial elements and homologous chromosome recombination. However, it is unknown how they are involved in synapsis and crossover recombination between homologous chromosomes. To get a clue about individual functions of different types of mieotic cohesins, in the present study, we tried to determine the detailed localization of RAD21L and REC8 on the synaptonemal complex.

Materials and methods: Mouse spermatocytes were fixed and immunofluorescently labeled with the antibodies against cohesin subunits (RAD21L and REC8), the synaptonemal complex proteins (SYCP3 and SYCP1), and a component of recombination intermediates (MSH4). The signals were observed by three-dimensional structured illumination microscopy (3D-SIM).

Results and discussion: At pachytene stage, two well-separated axial elements labeled with anti-SYCP3 antibody were observed by 3D-SIM. Numerous dot-like signals of RAD21L and REC8 were detected inside the synaptic axial elements rather than right along the axial elements. Some sinals of RAD21L, but not REC8, were seen as if they formed a bridge between axial elements. Furthermore, the ratio of overlapping areas between RAD21L and MSH4 signals was greater than the one between REC8 and MSH4. From these results, we propose a hypothesis that RAD21L might coordinate synapsis and recombination between homologous chromosomes by connecting non-sister chromatids from homologous chromosomes.