Aberrant X chromosome dosage compensation causes hybrid male inviability in Caenorhabditis

Zygotic reproductive isolation frequently initiates with hybrid incompatibility in the heterogametic sex, such as males in XX/XY systems. The genetic basis of hybrid male incompatibility has long remained elusive. Here, we show that crosses of Caenorhabditis nigoni males with C. briggsae females result in insufficient expression of Cbr-xol-1, an X-linked master switch responsible for intimately linked sex determination and dosage compensation pathways, consequently triggering aberrant X-chromosome repression in males, and ultimately leading to embryonic inviability. In contrast, male embryos from the reciprocal cross maintain normal expression level of C. nigoni xol-1 genes, consistent with their viability. We further demonstrate that the cis-regulatory regions of Cbr-xol-1 and Cni-xol-1 have functionally diverged. Finally, X transcription is also aberrantly repressed in lethal hybrid male embryos from crosses between gonochoristic species C. latens and C. remanei. Our results suggest an evolutionary scenario in which incompatibility of the dosage compensation system leads to reproductive isolation.