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.