SUMMARY: An investigation of the rate of increase of spontaneous mutants and the subsequent increase in mutant proportion in a bacterial culture revealed discrepancies between the observed results and those expected on the assumption that mutant and parent grew with equal rates. These discrepancies could not be accounted for in terms of a selective difference between established mutants and their parents since, when the two were mixed together in reconstruction experiments, they fared equally well for hundreds of generations. Rather the discrepancies indicated a difference between parents and mutants. The data were consistent with the hypothesis that the mutation occurred independently among one of four mutable units (nuclei) and that the mutant nucleus was ‘dominant’ over its sister non-mutant nuclei in the heterocaryon so formed. As a consequence, a delay of two generations ensued before the mutant unit segregated into the homocaryotic ancestor of a mutant clone. This process delayed the onset of an increase in mutant numbers after mutation. The accurate prediction of the pseudo-equilibrium level of mutants, based on verifiable assumptions of periodic selection, mutation and segregation lag, is added evidence for the occurrence of a two-generation delay before the increase of the mutant clone. This phenomenon, called segregation lag, is a source of error in the calculation of mutation rate by methods involving the numbers of mutants found in bacterial cultures. Furthermore, because bacteria may be multinucleate, the rate of mutation/bacterium/generation is not the same as the rate of mutation mutable unit (nucleus)/generation.


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