A recurrent inversion on the eutherian X chromosome.

Chromosomal inversions have an important role in evolution, and an increasing number of inversion polymorphisms are being identified in the human population. The evolutionary history of these inversions and the mechanisms by which they arise are therefore of significant interest. Previously, a polymorphic inversion on human chromosome Xq28 that includes the FLNA and EMD loci was discovered and hypothesized to have been the result of nonallelic homologous recombination (NAHR) between near-identical inverted duplications flanking this region. Here, we carried out an in-depth study of the orthologous region in 27 additional eutherians and report that this inversion is not specific to humans, but has occurred independently and repeatedly at least 10 times in multiple eutherian lineages. Moreover, inverted duplications flank the FLNA-EMD region in all 16 species for which high-quality sequence assemblies are available. Based on detailed sequence analyses, we propose a model in which the observed inverted duplications originated from a common duplication event that predates the eutherian radiation. Subsequent gene conversion homogenized the duplications, thereby providing a continuous substrate for NAHR that led to the recurrent inversion of this segment of the genome. These results provide an extreme example in support of the evolutionary breakpoint reusage hypothesis and point out that some near-identical human segmental duplications may, in fact, have originated >100 million years ago.