COMPARATIVE MAPPING OF X-CHROMOSOMES IN VOLE SPECIES OF THE GENUS MICROTUS

Comparative mapping of X-linked genes has progressed rapidly since Ohn o's prediction that genes on the X chromosome should be conserved as a syntenic group in all mammals. Although several conserved blocks of h omology between human and mouse have been discovered, rearrangements w ithin the X chromosome have also been characterized. More recently, so me exceptions to Ohno's law have been reported. We have used fluoresce nce in situ hybridization (FISH) to map five genes, Gla, G6pd, Hprt, P gk1 and Xist, to two of the largest conserved segments of X material i n five members of the genus Microtus (grey vole) and show that vole X chromosomes demonstrate greater homology to human than to mouse. Cytog enetic analysis indicates a relatively high frequency of rearrangement during vole evolution, although certain blocks of homology appear to be highly conserved in all species studied to date. On this basis we w ere able to predict the probable location of the rat X inactivation ce ntre (Xic) based solely on high-resolution G-banding. Our prediction w as then confirmed by mapping the rat Xist gene by FISH. The possible s ignificance of conserving long-range chromosome structure in the vicin ity of the Xic is discussed with respect to the mechanism of X inactiv ation.