Figure 4.

Evolutionary dynamics of Δctb-1 mtDNA under competitive conditions. (a) Population-level frequencies of the Δctb-1 mtDNA heteroplasmy in five 1G replicate lines under non-competed (grey lines) versus competitive conditions (coloured lines). In competitive assay populations established with equal ratios of Δctb-1 and WT mtDNA bearing worms (50 Δctb-1 bearing 1G hermaphrodites+50 WT mtDNA-bearing N2 hermaphrodites), the population dynamics of the Δctb-1 mitotype displays a steep decline in frequency with time. The Δctb-1 mtDNA heteroplasmy remains in high frequency (approx. 1.0) within each replicate line across generations under noncompetitive (control) conditions. In general, there is a significant reduction in the frequency of the Δctb-1 mitotype from 0.5 to less than 0.2 within two generations under competitive conditions, and eventual loss (undetectable via PCR) from the population between generations 3–8. The Δctb-1 mitotype remained undetected between generations 9–16. (b) A linear regression of the change in log[f(Δctb-1/WT mtDNA)] with time (generations). The single data point for each generation represents the average values of the Δctb-1 mutant mitotype and WT mtDNA across five independent replicates of line 1G (1G.C, 1G.L, 1G.N, 1G.T and 1G.U). The relative fitness, w, of the Δctb-1 mutant mitotype was calculated from the slope of the regression line and estimated to be 0.48, implicating a large deleterious fitness cost of the ctb-1 deletion bearing mtDNA and its gradual eradication in large competitive populations via purifying selection. (Online version in colour.)