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.