Phenotypic plasticity of body pigmentation in Drosophila : correlated variations between segments

Phenotypic plasticity of body pigmentation (the last three abdominal segments and the mesothorax) was investigated as a function of growth temperature in Drosophila melanogaster and Drosophila simulans. Two populations of each species were analysed, from two French localities with different climatic conditions. For each population, ten isofemale lines were reared at temperatures ranging from 14 to 31 degree C. Two methods were used and compared to estimate genetic correlations (r sub(g)) between segments, a simple method using directly the family mean values (r sub(m)) and a theoretically better method correcting variances and covariances for family size (r sub(c)). Both methods produced very similar data but the first one (r sub(m)) was preferred because it allowed an estimate of r sub(g) in all cases. Genetic and phenotypic correlations decreased regularly with distance between body segments, revealing an antero-posterior gradient: the extension of dark pigmentation is determined by increasingly different genetic systems in more distant segments. Genetic correlations were substantially larger than phenotypic correlations, in opposition to Cheverud's conjecture, although the two sets of values were highly correlated.