Fig. 1.

Fitness functions. Additional parameters: n = 50, $s_b=0.1,\hspace{0.17em}{s}_d=0.1$, γ = 5. (A) The three fitness functions used in this study in the case of beneficial mutations only. The selection coefficient is defined such that $1-s_b$ represents the fitness of a cell with zero beneficial substitutions (a cell with $n\gamma$ beneficial substitutions has a fitness of 1, where n is the number of cytoplasmic genomes and γ is the number of substitutions each cytoplasmic genome must accumulate before the simulation is terminated). In this example, where n = 50, $s_b=0.1$, and γ = 5, a cell’s fitness is 0.9 when its cytoplasmic genomes carry no beneficial substitutions, and its fitness is 1 when each cytoplasmic genome in the cell carries an average of five substitutions ($50\times 5=250$ beneficial substitutions in total). (B) The deleterious fitness function. Here, a cell with no deleterious substitutions has a fitness of 1, while a cell with $n\gamma$ substitutions has a fitness of $1-s_d$. We only examine a concave down decreasing function for the accumulation of deleterious substitutions (unless we are comparing cytoplasmic genomes to free-living genomes, in which case we use a linear fitness function). (C) One of the fitness functions used in the model with both beneficial and deleterious mutations. The beneficial substitution portion of the function can take any of the forms in panel A while the deleterious substitution portion takes the form in panel B (unless we are comparing cytoplasmic genomes to free-living genomes, in which case both the beneficial and deleterious fitness functions are linear). In this example, the fitness surface combines a linear function for beneficial substitutions with a concave down fitness function for deleterious substitutions. The color represents the fitness of a cell carrying a given number of deleterious substitutions (x-axis) and beneficial substitutions (y-axis). Equations for the fitness functions can be found in supplementary section S3.2 (A), section S4 (B), and section S5.2 (C), Supplementary Material online.