The association between fluctuating asymmetry, trait variability, trait heritability, and stress: A multiply replicated experiment on combined stresses in Drosophila melanogaster

A number of hypotheses have been proposed about the association between dev elopmental stability, phenotypic variability, heritability, and environment al stress. Stress is often considered to increase both the asymmetry and ph enotypic variability of bilateral traits, although this may depend on trait heritability. Empirical studies of such associations often yield inconsist ent results. This may reflect the diversity of traits and conditions used o r a low repeatability of any associations. To test for repeatable associati ons between these variables, multiply replicated experiments were undertake n on Drosophila melanogaster using a combination stress at the egg, larval, and adult stages of reduced protein, ethanol in the medium, and a cold sho ck. Both metric and meristic traits were measured and levels of heritable v ariation for each trait estimated by maximum likelihood and parent-offsprin g regression over three generations. Trait means were reduced by stress, wh ereas among-individual variation increased. Fluctuating asymmetry (FA) was increased by stress in some cases, but few comparisons were significant. On ly one trait, orbital bristle, showed consistent increases in FA. Changes i n trait means, trait phenotypic variability, and developmental stability as a result of stress were not correlated. Extreme phenotypes tended to have higher levels of FA, but only the results for orbital bristles were signifi cant. All traits had low to intermediate heritabilities except orbital bris tle, which showed no heritable variation. Only traits with low heritability and high levels of phenotypic variability may show consistent increases in FA under stress. Overall, the independence of phenotypic variability, plas ticity, and the developmental stability of traits extend to changes in thes e measures under stressful conditions.