Fixation of mutators in asexual populations: the role of genetic drift and epistasis

We study the evolutionary dynamics of an asexual population of nonmutators and mutators on a class of epistatic fitness landscapes. We consider the situation in which all mutations are deleterious and mutators are produced from nonmutators continually at a constant rate. We find that in an infinitely large population, a minimum nonmutator-to-mutator conversion rate is required to fix the mutators but an arbitrarily small conversion rate results in the fixation of mutators in a finite population. We calculate analytical expressions for the mutator fraction at mutation-selection balance and fixation time for mutators in a finite population when the difference between the mutation rate for mutator and nonmutator is smaller (regime I) and larger (regime II) than the selection coefficient. Our main result is that in regime I, the mutator fraction and the fixation time are independent of epistasis but in regime II, mutators are rarer and take longer to fix when the decrease in fitness with...我们针对一类上位适合度景观（epistatic fitness landscape），研究了由非突变体（nonmutator）与突变体（mutator）构成的无性繁殖种群的进化动力学。我们考虑所有突变为有害突变，且非突变体以恒定速率持续产生突变体的情形。研究发现，在无限大种群中，需达到最小的非突变体-突变体转换速率才能使突变体得以固定；而在有限种群中，任意微小的转换速率均可使突变体发生固定。我们推导了突变-选择平衡下的突变体占比解析表达式，以及当突变体与非突变体的突变率差异小于（模式I）与大于（模式II）选择系数（selection coefficient）时，有限种群中突变体的固定时间。我们的核心结果为：在模式I中，突变体占比与固定时间均不受上位性（epistasis）影响；而在模式II中，突变体更为稀少，且其固定所需时间更长，当适合度随……