The rate of mutation and the homozygous and heterozygous mutational effects for competitive viability: a long-term experiment with Drosophila melanogaster.

The effect of 250 generations of mutation accumulation (MA) on the second chromosome competitive viability of Drosophila melanogaster was analyzed both in homozygous and heterozygous conditions. We used full-sib MA lines, where selection hampers the accumulation of severely deleterious mutations but is ineffective against mildly deleterious ones. A large control population was simultaneously evaluated. Competitive viability scores, unaffected by the expression of mutations in heterozygosis, were obtained relative to a Cy/L(2) genotype. The rate of decline in mean DeltaM approximately 0.1% was small. However, that of increase in variance DeltaV approximately 0.08 x 10(-3) was similar to the values obtained in previous experiments when severely deleterious mutations were excluded. The corresponding estimates of the mutation rate lambda > or = 0.01 and the average effect of mutations E(s) < or = 0.08 are in good agreement with Bateman-Mukai and minimum distance estimates for noncompetitive viability obtained from the same MA lines after 105 generations. Thus, competitive and noncompetitive viability show similar mutational properties. The regression estimate of the degree of dominance for mild-to-moderate deleterious mutations was approximately 0.3, suggesting that the pertinent value for new unselected mutations should be somewhat smaller.

The lack of mutational variance for fluctuating and directional asymmetry in Drosophila melanogaster.

Starting from a completely homozygous population of Drosophila melanogaster, lines were derived and independently maintained by a single brother-sister mating in each generation. Two bilateral traits--sternopleural bristle number and wing length--were individually scored on the right-(R) and left-hand (L) sides. Directional (DA) and fluctuating (FA) asymmetries were represented by the signed (R-L) and unsigned magnitude of R-L difference, respectively. Mutational variances (the mutational rate of input of genetic variation) and heritabilities (the mutational variance scaled by the environmental variance) of R, L, (R-L) and magnitude of R-L were calculated from the between-line divergence after a number of generations of mutation accumulation (bristle number: 171 lines, 122 generations; wing length: 148 lines, 170 generations). Mutational heritabilities of R and L were all significant, ranging from 0.73 x 10(-3)-2.10 x 10(-3). Those of (R-L) and magnitude of R-L were two orders of magnitude smaller and nonsignificant, ranging from -1.95 x 10(-5)-5.49 x 10(-5). These results imply that mutations affecting the DA or FA of bristle number and wing length have not been fixed in the lines or alternatively, that their effects were too small to be detected. In the population under study, the data strongly suggest that FA reflects only developmental noise due to non-genetic processes.