The condition-dependence of male genital size and shape

The male genitals of internal fertilizers evolve rapidly and divergently, and sexual selection is generally responsible for this. Many sexually selected traits are condition-dependent - with their expression dependent upon the resources available to be allocated to them - as revealed by genetic or environmental manipulations of the condition. However, it is not clear whether male genitals are also condition-dependent. Here we manipulate the condition in two ways (via inbreeding and diet) to test the condition-dependence of the genital arch of Drosophila simulans. We found that genital size but not genital shape suffered from inbreeding depression, whereas genital size and shape were affected by dietary manipulation of the condition. The differential effects of these treatments likely reflect the underlying genetic architecture that has been shaped by past selection: inbreeding depression is only expected when traits have a history of directional selection, while diet impacts traits regardless of historical selection. Nonetheless, our results suggest genitals can be condition-dependent like other sexually selected traits.