Comparative mapping in equids: the asine X chromosome is rearranged compared to horse and Hartmann's mountain zebra

The X chromosomes of the extant equids, in general, share morphology and banding pattern similarities. However, the donkey X is, in part, an exception because of significantly different centromeric index and variant banding patterns in the pericentromeric region. To verify the underlying molecular basis of this difference, twelve equine BAC clones were FISH mapped to donkey (EAS) and Hartmann's mountain zebra (EZH) metaphase spreads. Loci from the terminal region of Xp and distal to terminal regions of the Xq showed the same order and relative position in all three species, implying cross-species conservation of these chromosomal segments. However, loci from the proximal/pericentromeric regions of either arms showed similar FISH locations in horse and zebra but a slightly deviant location and relative position in the donkey. Three of the markers (tel-OTC, TRAP170 and (ps)ALDH2- cen) located on the short arm of ECAX and EZHX were found inverted on the long arm of EASX, along with the transposition of the centromere. This molecular evidence of a pericentromeric inversion helps define the likely evolutionary breakpoints causing the rearrangement. The breakpoints most likely correspond to the region between Xp16-->q12 in the horse and Xp12-->q13 in the donkey. The findings coupled with the highly conserved X-chromosome gene order between horse and outgroup species, human and cat, suggest that the equine type X is ancestral while the asine type X arose as a result of an independent inversion event. The study adds two new markers to horse, 11 to donkey and 12 to Hartmann's zebra gene maps, thus contributing to the expansion of comparative maps in the equids.