Trade-offs within and between sexual traits in stalk-eyed flies (Diptera: Diopsidae)

The allocation of limited resources to competing body parts during development may affect both the absolute and relative sizes of physical traits, creating potentially dramatic consequences for the evolution of morphology. While negative correlations between the sizes of body parts ("trade-offs") arising from the distribution of finite resources have long been anticipated, empirical support is relatively rare. In this study, I use two related species of stalk-eyed flies that differ in morphology to investigate whether eye-span exaggeration results in trade-offs with other body parts. One species, Cyrtodiopsis dalmanni, is sexually dimorphic, with males having exaggerated eye-span while the other, C. quinqueguttata, is sexually monomorphic with both sexes having approximately equivalent and relatively unexaggerated eye-span. I utilize complementary approaches including artificial selection, application of exogenous juvenile hormone, and diet manipulation to alter the absolute and relative length of the eye-stalks in order to reveal correlated changes in other physical traits. The results of these experiments suggest that exaggerated eye-span in male C. dalmanni is accompanied by a decrease in two other features of head morphology, eye-bulb size and eye-stalk width, as well as compromised testis growth and sperm production. No trade-offs were observed in females of either species or male C. quinqueguttata. These results are consistent with hormonally-mediated trade-offs arising from the allocation of limited resources to an exaggerated trait and suggest that those trade-offs may act as costs of developing exaggerated eye-span.