Conservation of gene order between horse and human X chromosomes as evidenced through radiation hybrid mapping

A radiation hybrid (RH) map of the equine X chromosome (ECAX) was obtained using the recently produced 5000_rad horse X hamster hybrid panel. The map comprises 34 markers (16 genes and 18 microsatellites) and spans a total of 676 cR₅₀₀₀, covering almost the entire length of ECAX. Cytogenetic alignment of the RH map was improved by fluorescent in situ hybridization mapping of six of the markers. The map integrates and refines the currently available genetic linkage, syntenic, and cytogenetic maps, and adds new loci. Comparison of the physical location of the 16 genes mapped in this study with the human genome reveals similarity in the order of the genes along the entire length of the two X chromosomes. This degree of gene order conservation across evolutionarily distantly related species has up to now been reported only between human and cat. The ECAX RH map provides a framework for the generation of a high-density map for this chromosome. The map will serve as an important tool for positional cloning of X-linked diseases/conditions in the horse.