Nuclear Segregation and the Growth of Clones of Spontaneous Mutants of Bacteria

Abstract

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. Rather the discrepancies indicated a difference between parents and new 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. This phenomenon, called segregation lag, is a source of error in the calculation of mutation rate by methods involving the number 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)/genera-tion.