Abstract
Lahn and Page previously observed that genes on the human X chromosome were physically arranged along the chromosome in “strata,” roughly ordered by degree ofdivergence from related genes on the Y chromosome. They hypothesized that this ordering results from a historical series of suppressions ofrecombination along the mammalian Y chromosome, thereby allowing formerly recombining X and Y chromosomal genes to diverge independently. Here predictions ofthis hypothesis are confirmed in a nonprimate mammalian order, Rodentia, through an analysis ofeight gene pairs from the X and Y chromosomes ofthe house mouse, Mus musculus. The mouse X chromosome has been rearranged relative to the human X, so strata were not found in the same physical order on the mouse X. However, based on synonymous evolutionary distances, X-linked genes in M. musculus fall into the same strata as orthologous genes in humans, as predicted. The boundary between strata 2 and 3 is statistically significant, but the boundary between strata 1 and 2 is not significant in mice. An analysis ofsmaller fragments of Smcy, Smcx, Zfy, and Zfx from seven species of Mus confirmed that the strata in Mus musculus were representative ofthe genus Mus.