Analysis of Meiotic Exchange by Tritium Autoradiography

Abstract

MITOTIC sister chromatid exchanges (SCE) are currently the subject of research and controversial discussion^1,2. In contrast, there is uncertainty as to whether this type of exchange ever occurs in meiotic chromosomes, but cytological studies on ring-rod heterozygotes in Zea³ and Antirrhinum⁴ certainly suggest that sister as well as non-sister crossovers occur during meiosis. A few direct attempts at this problem have been made using the technique of tritium autoradiography^5–7, but by and large the results have proved inconclusive. The detection of meiotic exchanges by this method depends on the fact that germ line chromosomes, like those of somatic cells, replicate in a semi-conservative manner. Thus chromosomes labelled with ³H-thymidine during the pre-meiotic S-phase enter meiosis with both chromatids labelled, while chromosomes labelled one cell cycle earlier, that is, during the S-phase of the last gonial mitosis, enter meiosis having one labelled and one unlabelled chromatid. This system clearly provides an opportunity to test for the occurrence of sister chromatid exchanges in meiotic chromosomes using procedures analogous to those widely applied to mitotic chromosomes. On a simple view this would involve the detection of label switches involving sister chromatids, but in practice the situation is far more complex and the detection and estimation of meiotic SCEs are made difficult by a number of complicating factors. The most obvious of these is the occurrence of another class of exchanges in meiotic cells, namely, the crossovers which involve non-sister chromatids of homologous chromosomes. The most irritating feature of these crossovers is that in suitable combinations they can mimic the effect of SCE by producing label switches which are reciprocal between sister chromatids of AI or MII chromosomes (see Fig. la). This fact led Taylor⁵ to conclude that “… when exchanges are observed they cannot readily be classified as SCEs or as crossovers”. The results presented here permit a more optimistic view of the situation.