Letter | Published:

‘Selfers’ and High Mutation Rate during Meiosis in Ascobolus immersus

Nature volume 204, page 809 (21 November 1964) | Download Citation



DEMEREC1–3 applied the term ‘selfers’ to “auxotrophic mutants of bacteria in which transduction with homologous phage gives rise to significantly larger numbers of wild-type variants than occur by spontaneous reversion in uninfected control bacteria”. Lissouba et al.4 found that some mutants affecting ascospore pigmentation in Ascobolus immersus produce wild-type ‘recombinants’ in homo-allelic crosses, whereas numerous other whitespore mutants when selfed never give wild-type revertants. However, this phenomenon has not been investigated in detail. We have examined one such mutant, ‘186’, more precisely, because it produced the ‘recombinants’ in relatively high frequency. Homo-allelic crosses of this mutant were carried out and tetrad analysis applied. It was characteristic that wild-type spores were found mainly in asci with 6 : 2 segregation, namely, 6 spores ‘186’ and 2 wild. In the case of normal back mutation before meiosis, asci with 4 mutant and 4 wild spores should be obtained. Among 127,964 tetrads examined, 38 of the 6 : 2 and 3 of the 4 : 4 asci were found (frequencies 0.000297 and 0.000023, respectively). Asci with segregation 4 : 4 in homologous cross may represent the normal rate of spontaneous back-mutations that correspond to Demerec's control experiments, because only one set of genes is present during mitosis. In turn, the 6 : 2 tetrads can originate only during meiosis. In meiosis two sets of genes are present in the nucleus, so that the situation is analogous to the transduction experiments with ‘homologous phage’. Assuming that the 4 : 4 asci result from back-mutations in mitosis and the 6 : 2 asci from back mutations in meiosis, it is obvious that the frequency of this phenomenon is about 10-fold higher in meiosis than in mitosis. Presumably this difference is even greater if we take into account the clonal effect which works only on back-mutations in mitosis, increasing the number of wild-type nuclei that could enter meiosis.

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  1. Department of General Genetics, Polish Academy of Sciences, Warsaw, al. Ujazdowskie 4, Poland.



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