THE LACK OF MUTATIONAL VARIANCE FOR FLUCTUATING AND DIRECTIONAL ASYMMETRY IN DROSOPHILA-MELANOGASTER

Starting from a completely homozygous population of Drosophila melanog aster, lines were derived and independently maintained by a single bro ther-sister mating in each generation. Two bilateral traits--sternople ural bristle number and wing length--were individually scored on the r ight- (R) and left-hand (L) sides. Directional (DA) and fluctuating (F A) asymmetries were represented by the signed (R-L) and unsigned \R-L\ difference, respectively. Mutational variances (the mutational rate o f input of genetic variation) and heritabilities (the mutational varia nce scaled by the environmental variance) of R, L, (R-L) and \R-L\ wer e calculated from the between-line divergence after a number of genera tions of mutation accumulation (bristle number: 171 lines, 122 generat ions; wing length: 148 lines, 170 generations). Mutational heritabilit ies of R and L were all significant, ranging form 0.73 x 10(-3)-2.10 x 10(-3). Those of (R-L) and \R-L\ were two orders of magnitude smaller and non-significant, 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, t hat their effects were too small to be detected. In the population und er study, the data strongly suggest that FA reflects only developmenta l noise due to non-genetic processes.