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Rad51 immunocytology in rat and mouse spermatocytes and oocytes
1Department of Biology, York University, North York, Ontario, Canada M3J 1P3. <chromsma@yorku.ca>
Abstract:
On the assumption that Rad51 protein plays a role in early meiotic chromosomal events, we examine the location and time of appearance of immuno-reactive Rad51 protein in meiotic prophase chromosomes. The Rad51 foci in mouse spermatocytes appear after the emergence of, and attached to, short chromosomal core segments that we visualize with Cor1-specific antibody. These foci increase in number to about 250 per nucleus at the time when core formation is extensive. The numbers are higher in mouse oocytes and lower in rat spermatocytes, possibly correlating with recombination rates in those cases. In the male mouse, foci decrease in number to approximately 100 while chromosome synapsis is in progress. When synapsis is completed, the numbers of autosomal foci decline to near 0 while the X chromosome retains about 15 foci throughout this time. This stage coincides with the appearance of testis-specific histone H1t at mid- to late pachytene. Electron microscopy reveals that at first Rad51 immunogold-labeled 100 nm nodules are associated with single cores, and that they come to lie between the chromosome cores during synapsis. It appears that these nodules may be the homologs of the Rad51-positive early nodules that are well documented in plants. The reciprocal recombination-correlated late nodules appear after the Rad51 foci are no longer detectable. The absence of Rad51 foci in the chromatin loops suggests that in wild-type mice Rad51/DNA filaments are restricted to DNA at the cores/synaptonemal complexes. The expected association of Rad51 protein with Rad52 could not be verified immunocytologically.
Insights
Rad51 protein forms foci on meiotic chromosome cores in mouse spermatocytes, peaking during extensive core formation and decreasing as synapsis progresses. These foci appear to be restricted to chromosomal cores.
Area of Science:
- Meiosis and chromosome biology
- Molecular genetics and cell biology
Background:
- Rad51 protein is crucial for DNA repair and homologous recombination.
- Its role in early meiotic chromosomal events, particularly in mammals, requires detailed investigation.
Purpose of the Study:
- To investigate the temporal and spatial localization of Rad51 protein during meiotic prophase in mouse spermatocytes.
- To correlate Rad51 foci dynamics with chromosomal core formation and synapsis.
Main Methods:
- Immunocytology using anti-Rad51 and anti-Cor1 antibodies in mouse spermatocytes.
- Electron microscopy with immunogold labeling for Rad51.
- Comparative analysis in mouse oocytes and rat spermatocytes.
Main Results:
- Rad51 foci appear on chromosomal cores after initial core segment emergence, increasing significantly during extensive core formation.
- Foci numbers decrease during synapsis, with autosomal foci disappearing upon completion while the X chromosome retains foci.
- Electron microscopy shows Rad51 nodules associated with single cores, moving between cores during synapsis, and are absent from chromatin loops.
Conclusions:
- Rad51 protein localizes to chromosomal cores during early meiotic prophase, suggesting its role is restricted to core DNA.
- The observed Rad51 foci dynamics correlate with meiotic progression and may represent early recombination nodules.
- Further studies are needed to elucidate the precise function of Rad51 at meiotic cores.