Flies under stress: A test of fluctuating asymmetry as a biomonitor of environmental quality

Fluctuating asymmetry (FA) has been proposed as a measure of environmental quality. The premise is that increased levels of environmental stress (e.g., chemical pollutants) are reflected by increased levels of FA within populations of organisms developing at the site. We test this premise by examining the relationship between stress, fitness, and FA among laboratory populations of house fly (Musca domestica L.). Exposures from 0.00 to 1.00 mu L/L of the pesticide ivermectin during egg-to-pupal development caused less than or equal to 18-fold differences in levels of stress (measured as percentage pupation), and less than or equal to 18-fold differences in levels of fitness (measured as the average number of first-instar larvae produced by each female in the starting population) among populations. However, no differences were detected among population levels of FA for five wing traits examined. Power analyses estimated that the accuracy of the image analysis method used to obtain measurements would have detected a 10% difference in average FA among populations 95% of the time. Hence, the results of this study do not support the use of FA as a method for monitoring changes in environmental quality. The absence of a treatment effect on levels of FA may reflect the development of surviving flies in "refuges," thereby minimizing or avoiding their contact with ivermectin (the "refuge" hypothesis). Alternatively, flies surviving exposure to ivermectin may represent a "robust" subset of the starting population whose symmetries are relatively unaffected by the effects of stress (the "differential mortality" hypothesis). These two hypotheses have received little attention in the literature, yet may have important implications for interpreting the effect of environmental stress on FA.