Three evolutionary schemes to explain gene localization between X and autosomes (denoted as A). The grey box indicates the preferential localization of male-biased (including testis-specific) genes on the autosomes and away from the X chromosome. In each scheme, a different force drives this biased localization. The dotted arrow indicates a step that may or may not follow. Scheme I: MSCI precedes and drives biased gene localization through selective pressure against X-linked spermatogenesis genes, which would fail to be properly expressed in spermatocytes. Scheme II: sexual antagonism leads to biased gene localization through selective pressure for X-linked female-advantageous but male-disadvantageous mutations (and against the opposite). If this leads to all spermatogenetic genes on the X chromosome being selected against, conditions are right for MSCI to possibly arise. Scheme III: improper regulation of tissue-specific genes on the X chromosome leads to biased gene localization through selective pressure against tissue-specific genes on the X chromosome. We suggest that a plausible mechanism for the X/A dependence of tissue-specific expression reported by Mikhaylova and Nurminsky may be dosage compensation (indicated by an asterisk). Again, this leads to conditions in which it is possible for MSCI to arise.
Lu and Wu BMC Biology 2011 9:30 doi:10.1186/1741-7007-9-30