Frequency and distribution of chiasmata in Syrian hamster spermatocytes studied by the BrdU antibody technique.

The frequency and distribution of chiasmata and the nature of terminal "associations" was re-examined in Syrian hamster spermatocytes using the 5-bromo-2'-deoxyuridine (BrdU) antibody technique (BAT) for differential chromatid labelling. Differential chromatid substitution was achieved by BrdU incorporation at the penultimate pre-meiotic S-phase followed by one of three different staining protocols: (i) fluorescence plus Giemsa (FPG), (ii) acridine orange staining or (iii) BAT. For analysis of chiasmata frequency and localization in the diplotene/diakinesis stages the resolution of FPG and acridine orange staining was comparable to that of BAT. In metaphase II chromosomes BAT was more informative than FPG and acridine orange staining and revealed small, terminal crossover exchanges. This finding proves that many terminal associations of meiotic chromosomes actually represent chiasmata at the end of the first meiotic division. Some crossover exchanges were localized in the constitutive heterochromatin of autosomes. Using BAT we also detected crossover exchanges in close vicinity to each other. This observation is reminiscent of the fact that crossing over interference means a reduction in frequency and does not imply total inhibition.