Hybrid Drosophila cell lines show selective genome instabilities

Background: Post-zygotic reproductive isolation is thought to be caused by genomic conflict. However, the exact nature of these detrimental incompatibilities is largely unknown. To identify genomic regions that became incompatible during the formation of species and are therefore prone to get lost in hybrids, we established hybrid cell lines derived from an authentic interspecies cross between Drosophila melanogaster and Drosophila simulans. Results: All four independently derived lines selectively lost the D. melanogaster allele of the hybrid incompatibility gene Hmr, confirming its incompatibility with cellular viability. Moreover, each line underwent distinct but directionally biased erosion of additional D. melanogaster chromosomal regions, preferentially affecting the X chromosome, chromosome 4, and chromosome 3R. Loss of the D. melanogaster X was dosage compensated by upregulation of genes on the hemizygous D. simulans X, while D. melanogaster rDNA was paradoxically amplified across all lines. Nucleolar dominance was driven by relative rDNA copy number rather than species identity, overturning the prevailing view of fixed species-based dominance. Finally, the measurements of the species-specific proteome revealed species specific differences in the RNA to protein coefficient K_p as novel and yet undescribed sources of molecular conflict in hybrids. Conclusions: These findings reveal a hierarchical, constraint-driven mechanism of hybrid genome resolution.