DYNAMIC CHANGES IN RAD51 DISTRIBUTION ON CHROMATIN DURING MEIOSIS IN MALE AND FEMALE VERTEBRATES

Antibodies against human Rad51 protein were used to examine the distri bution of RadS1 on meiotic chromatin in mouse spermatocytes and oocyte s as well as chicken oocytes during sequential stages of meiosis. We o bserved the following dynamic changes in distribution of Rad51 during meiosis: (1) in early leptotene nuclei there are multiple, apparently randomly distributed, foci that by late leptonema become organized int o tracks of foci. (2) These foci persist into zygonema, but most foci are now localized on Rad51-positive axes that correspond to lateral el ements of the synaptonemal complex. As homologs synapse foci from homo logous axes fuse. The distribution and involvement of Rad51 foci as co ntact points between homologs suggest that they may be components to e arly recombination nodules. (3) As pachynema progresses the number of foci drops dramatically; the temporal occurrence (mice) and physical a nd numerical distribution of foci on axes (chickens) suggest that they may be a component of late recombination nodules. (4) In early pachyn ema there are numerous RadS1 foci on the single axis of the X (mouse s permatocytes) or the Z (chicken oocytes) chromosomes that neither pair , nor recombine. (5) In late pachynema in mouse spermatocytes, but not oocytes, the RadS1 signal is preferentially enhanced at both ends of all the bivalents. As bivalents in spermatocytes, but not oocytes, beg in to desynapse at diplonema they are often held together at these Rad 51-positive termini. These observations parallel observations that rec ombination rates are exceptionally high near chromosome ends in male b ut not female eutherian mammals. (6) From diakinesis through metaphase I, Rad51 protein is detected as low-intensity fluorescent doublets th at localize with CREST-specific antigens (kinetochores), suggesting th at Rad51 participates, at least as a structural component of the mater ials involved, in sister kinetochore cohesiveness. Finally, the change s in Rad51 distribution during meiosis do not appear to be species spe cific, but intrinsic to the meiotic process.