Lock‐and‐key structural isolation between sibling Drosophila species

Drosophila santomea Lachaise & Harry, which is endemic to the African island of São Tomé, and its sibling D. yakuba Burla comprise a new model system of speciation. They are morphologically distinguishable only by slight differences in the male genitalia and body coloration. As a previously undescribed difference, the aedeagus of D. yakuba bears a pair of stout spines (the ventral branches of the basal processes (VB)), instead of the paired humps found in D. santomea . Here, we show that this difference works as a lock‐and‐key isolating mechanism between the siblings. During conspecific copulation, D. yakuba females receive the spines in a pair of pocket‐shaped structures, which are protected by hardened plates, in the genitalia. The females of D. santomea , which lack such pockets, are wounded by the spines of the VB when mated with D. yakuba males. This genital mismatching resulted in leakage of the ejaculate, making 80% of the matings infertile and causing a prolonged struggle to separate pairs glued together by the ejaculate.