Deficit of Mitonuclear Genes on the Human X Chromosome Predates Sex Chromosome Formation

dean, rebecca ; zimmer, fabian ; mank, judith, e.;

Two tax a studied to date, the therian mammals and Caen or habd it is elegans, display under representations of mito nuclear genes (mtN genes, nuclear genes who seproducts are imported to and act within the mitochondria) on their X chromosomes. This pattern has been interpreted as the result of sexual conflict driving mt-N genes off of the X chromosome. However, studies inseveral other species have failed to detect a convergent biased distribution of sex-linked mt-Ngenes, leading to questions over the generality of the role of sexual conflict in shaping the distribution of mt-N genes. Here we tested whether mt-N genes moved off of the therian X chromosome following sex chromosome formation, consistent with the role of sexual conflict, or whether the paucity of mt-N genes on the therian X is a chance result of an underrepresentation on the ancestral regions that formed the X chromosome. We used a synteny-based approach to identify the ancestral regions in the platypus and chicken genomes that later formed the therian X chromosome. We then quantified the movement of mt-N genes on and off of theX chromosome and the distribution of mt-N geneson the human X and ancestral X regions. We failed to find an excess of mt-N gene movement off of the X. The bias of mt-N genes on ancestral therian X chromosomes was also not significantly different from the biases on the human X. Together our results suggest that, rather than conflict driving mt-N genes off of the mammalian X, random biases on chromosomes that formed the X chromosome could explain the paucity of mt-N genes in the therian lineage