Sister chromatid exchanges in mammalian meiotic chromosomes

Meiotic cells have been, and remain, a relatively difficult tissue in which to study SCEs. Experimental evaluations have had to contend with various problems unique to germ cells, i.e., poor in vitro growth, unusual cytotoxicities (from BrdUrd), trying chromosome morphologies, and confusion between chromosome label patterns generated by SCEs, COs, and replication kinetics. Nevertheless, BrdUrd differential staining techniques in insects and rodents have progressed to the point where SCE frequencies can be reliably determined. In the described Armenian hamster system, SCEs may be directly resolved in selected primary spermatocyte bivalents and indirectly assessed in secondary spermatocyte cells. SCE frequencies determined from spermatogonial, primary spermatocyte, and secondary spermatocyte cells are in good agreement. The system should be applicable for studies of possible mechanistic similarities between SCE and CO exchange, and for evaluating genotoxic effects. Experimental evidence in Drosophila (33) argues against a very close molecular relationship between these forms of breakage and recombination. However, it is likely that new information can be gained from cells in which normal SCE and CO frequencies and distributions can be characterized, and from studies of the consequences to these events after various mutagenic perturbations to premeiotic and meiotic DNA.