Mutational Interference and the Progression of Muller's Ratchet When Mutations Have a Broad Range of Deleterious Effects

Abstract

Deleterious mutations can accumulate in asexual haploid genomes through the process known as Muller's ratchet. This process has been described in the literature mostly for the case where all mutations are assumed to have the same effect on fitness. In the more realistic situation, deleterious mutations will affect fitness with a wide range of effects, from almost neutral to lethal. To elucidate the behavior of the ratchet in this more realistic case, simulations were carried out in a number of models, one where all mutations have the same effect on selection [one-dimensional (1D) model], one where the deleterious mutations can be divided into two groups with different selective effects [two-dimensional (2D) model], and finally one where the deleterious effects are distributed. The behavior of these models suggests that deleterious mutations can be classified into three different categories, such that the behavior of each can be described in a straightforward way. This makes it possible to predict the ratchet rate for an arbitrary distribution of fitness effects using the results for the well-studied 1D model with a single selection coefficient. The description was tested and shown to work well in simulations where selection coefficients are derived from an exponential distribution.