RAD51 IMMUNOCYTOLOGY IN RAT AND MOUSE SPERMATOCYTES AND OOCYTES

On the assumption that Rad51 protein plays a role in early meiotic chr omosomal events, we examine the location and time of appearance of imm une-reactive Rad51 protein in meiotic prophase chromosomes. The Rad51 foci in mouse spermatocytes appear after the emergence of, and attache d to, short chromosomal core segments that we visualize with Cor1-spec ific antibody. These foci increase in number to about 250 per nucleus at the time when core formation is extensive. The numbers are higher i n mouse oocytes and lower in rat spermatocytes, possibly correlating w ith recombination rates in those cases. In the male mouse, foci decrea se in number to approximately 100 while chromosome synapsis is in prog ress. When synapsis is completed, the numbers of autosomal foci declin e to near 0 while the X chromosome retains about 15 foci throughout th is time. This stage coincides with the appearance of testis-specific h istone 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 d uring synapsis. It appears that these nodules may be the homologs of t he Rad51-positive early nodules that are well documented in plants. Th e reciprocal recombination-correlated late nodules appear after the Ra d51 foci are no longer detectable. The absence of Rad51 foci in the ch romatin loops suggests that in wild-type mice Rad51/DNA filaments are restricted to DNA at the cores/synaptonemal complexes. The expected as sociation of Rad51 protein with Rad52 could not be verified immunocyto logically.