Increase of the spontaneous mutation rate in a long-term experiment with Drosophila melanogaster

Abstract

In a previous experiment, the effect of 255 generations of mutation accumulation (MA) on the second chromosome viability of Drosophila melanogaster was studied using 200 full-sib MA1 lines and a large C1 control, both derived from a genetically homogeneous base population. At generation 265, one of those MA1 lines was expanded to start 150 new full-sib MA2 lines and a new C2 large control. After 46 generations, the rate of decline in mean viability in MA2 was approximately 2.5 times that estimated in MA1, while the average degree of dominance of mutations was small and nonsignificant by generation 40 and moderate by generation 80. In parallel, the inbreeding depression rate for viability and the amount of additive variance for two bristle traits in C2 were 2-3 times larger than those in C1. The results are consistent with a mutation rate in the line from which MA2 and C2 were derived about 2.5 times larger than that in MA1. The mean viability of C2 remained roughly similar to that of C1, but the rate of MA2 line extinction increased progressively, leading to mutational collapse, which can be ascribed to accelerated mutation and/or synergy after important deleterious accumulation.

Figure 1.

(A) Viability determination for MA2 chromosomes in homozygosis and in heterozygosis with control C2C chromosomes. (B) Viability determination for MA2 chromosomes in homozygosis and for control C2 chromosomes in panmixia. (C) Viability determination for MA2 and control C2 chromosomes in homozygosis. (D) Viability determination for control chromosomes (C1 and C2) in homozygosis and panmixia.

Figure 2.

Percentage of surviving MA lines (starting at ordinate 100) and estimates of the between-line variance (×10⁻³, starting at ordinate 0) plotted against generation number for the previous (Chavarrías et al. 2001, dashed lines) and present experiments (solid lines).