When closely related species mate, their offspring sometimes survive but cannot reproduce. This pattern often affects males first, with hybrid males frequently failing to produce functional sperm even when hybrid females remain fertile. In a new study published in Molecular Biology and Evolution, Whitehead Institute Member Yukiko Yamashita, graduate student in her lab Adrienne Fontan, and senior scientist in her lab Romain Lannes identify a cellular defect that contributes to this phenomenon in fruit flies. This finding may help explain how diverging species become reproductively incompatible.