Generations of mutation accumulation

Figure 5.2 The results of the classic Drosophila melanogaster mutation accumulation experiment carried out by Mukai et al. (1972). The experiment maintained three distinct sets of mutation-accumulation populations with 25 lines each. The left-hand panel shows the change in mean viability over time and the right-hand panel shows the change in the variance among replicate independent lines. Each point is the value obtained from one set of mutation-accumulation populations. Mutation of any type makes the lines diverge genetically and increases the variance. Mean viability declines over time as deleterious mutations are more common than advantageous mutations. Redrawn from Figure 2 in Mukai et al. (1972).

that maintain the second chromosome without recombination in many replicate homozygous families or lines over many generations. Mutations of all types occur on this non-recombining chromosome and are fixed by genetic drift within each line of flies due to a single male founder for each generation. At intervals of 10 generations, the flies in all of the different independent lines were assayed for viability in comparison with a control line that did not experience any mutation due to chromosomal inversions (again accomplished with special breeding techniques). The change in average viability and variance in viability found by Mukai et al. (1972) is shown in Fig. 5.2. The variance in viability among the replicate lines has increased because the second chromosome of each line diverged due to the occurrence and fixation of mutations. In addition, the average viability has declined as expected if deleterious mutations are more common than beneficial mutations. The results are consistent with deleterious mutations that cause an average reduction in viability of 5% or less when homozygous. Therefore, this experiment and others like it motivated a view of the mutation fitness spectrum as drawn in Fig. 5.1. However, mutation-accumulation experiments have been carried out in only a relatively few organisms

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