Cytological detection of crossing-over in BUdR substituted meiotic chromosomes using the fluorescent plus Giemsa technique

Abstract

THE relationship between chiasmata and crossing-over has long been of interest, for if chiasmata are the cytological manifestations of recombination events they represent an easily accessible and reliable source of information on the frequency and distribution of crossing-over throughout the entire genome. Autoradiographic studies on meiotic chromosomes have proved that chiasma formation involves breakage and reunion of homologous, non-sister chromatids^1–4. However, the inherent lack of precision in autoradiography has demanded that the stages of meiosis investigated, usually anaphase I or later, are remote from the time of crossing-over, and after homologous chromosome pairs have disjoined. This has precluded a comparison of chiasma distributions with cross-over positions, with one exception. In the grasshopper Stethophyma grossum, the proximal localisation of chiasmata at metaphase I has been shown to correspond perfectly with the exchange positions of autoradiographic label at anaphase I (ref. 4).