The Integrity of the Speciation Core Complex is necessary for centromeric binding and reproductive isolation inDrosophila

Postzygotic isolation by genomic conflict is a major cause for the formation of species. Despite its importance, the molecular mechanisms that result in the lethality of interspecies hybrids are still largely unclear. The genusDrosophila, which contains over 1600 different species, is one of the best characterized model systems to study these questions. We showed in the past that the expression levels of the two hybrid incompatibility factorsHmrandLhrdiverged in the two closely relatedDrosophilaspecies,D. melanogasterandD. simulans, resulting in an increased level of both proteins in interspecies hybrids. This overexpression leads to mitotic defects, a misregulation in the expression of transposable elements and a decreased fertility. In this work, we describe a distinct six subunit Speciation Core Complex (SCC) containing HMR and LHR and analyse the effect ofHmrmutations on complex function and integrity. Our experiments suggest that HMR acts as a bridging factor between centromeric chromatin and pericentromeric heterochromatin, which is required for both its physiological function and its ability to cause hybrid male lethality.