Figure 1

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.