A morphological trait involved in reproductive isolation between Drosophila sister species is sensitive to temperature

Male genitalia are usually extremely divergent between closely related species, but relatively constant within one species. Here we examine the effect of temperature on the shape of the ventral branches, a male genital structure involved in reproductive isolation, in the sister species Drosophila santomea and D. yakuba . We designed a semi-automatic measurement pipeline that can reliably identify curvatures and landmarks based on manually digitized contours of the ventral branches. With this method, we observed that temperature does not affect ventral branches in D. yakuba but that in D. santomea ventral branches tend to morph into a D. yakuba-like shape at lower temperature. Our results suggest that speciation of D. santomea and D. yakuba was associated with a change in genitalia plasticity.