Temperature shock during development fails to increase the fluctuating asymmetry of a sexual trait in stalk–eyed flies

Abstract

The fluctuating asymmetry (FA) of bilateral traits is claimed to be a general indicator of environmental stress. Exaggerated sexual ornaments are thought to show elevated levels of FA and a greater response to stress than other traits. Previous work with stalk–eyed flies (Cyrtodiopsis dalmanni) has shown that the FA of the sexual trait (male eye stalks), wing length and wing width were unaffected by a continually applied food stress. Here we tested whether a transient stress (24–h heat shock at 31°C during development) affected the FA of these traits. A second experiment tested the combined stresses of transient heat shock at 31°C with continuous exposure to desiccation. In each experiment, temperature shock reduced the trait size, confirming that the treatments were stressful. However, stress had no effect on the FA of individual traits or the FA summed across all traits. Exposure to the combined stresses significantly elevated mortality and reduced trait size compared to the single–stress regime. However, FA did not differ significantly between flies from the two experiments. We found no evidence that FA in sexual and non–sexual traits reflects transient stress during the development of C. dalmanni.