Synteny and regional marker order sssignment of 26 type I and microsatellite markers to the horse X- and Y-chromosomes

The hypothesis that the conservation of sex-chromosome-linked genes among placental mammals could be extended to the horse genome was tested using the UCDavis horse-mouse somatic cell hybrid (SCH) panel. By exploiting the fluorescence in-situ hybridization (FISH) technique to localize an anchor locus, X-inactivation-specific transcript (XIST) on the horse X chromosome, together with the fragmentation and translocation of the X- and Y-chromosome fragments in a somatic cell hybrid panel, we regionally assigned 13 type I and 13 type II (microsatellite) markers to the horse X- and Y-chromosomes. The synteny groups that correspond to horse X- and Y-chromosomes were identified by synteny mapping of sex-specific loci zinc finger protein X-linked (ZFX), zinc finger protein Y-linked (ZFY) and sex-determining region Y (SRY) on the SCH panel. A non-pseudoautosomal gene in the human steroid sulfatase (STS) was identified in both X- and Y-chromosome- containing clones. The regional order of the X-linked type I markers examined in this study, from Xp- to Xq-distal, was [STS-X, the voltage-gated chloride channel 4 (CLCN4)], [ZFX, delta- aminolevulinate synthase 2 (ALAS2)], XIST, coagulation factor IX (F9) and [biglycan (BGN), equine F18, glucose-6-phosphate dehydrogenase (G6PD)] (precise marker order could not be determined for genes within the same brackets). The order of the Y-linked type I markers was STS-Y, SRY and ZFY. These orders are the same arrangements as reported for the human X- and Y-chromosomes, supporting the conservation of genomic organization between the human and the horse sex chromosomes. Regional ordering of X- linked type I and microsatellite markers provides the first integration of type I and type II markers in the horse X chromosome.