Characterization of the genomic Xist locus in rodents reveals conservationof overall gene structure and tandem repeats but rapid evolution of uniquesequence

The Xist locus plays a central role in the regulation of X chromosome inact ivation in mammals, although its exact mode of action remains to be elucida ted. Evolutionary studies are important in identifying conserved genomic re gions and defining their possible function. Here we report cloning, sequenc e analysis, and detailed characterization of the Xist gene from four closel y related species of common vole (field mouse), Microtus arvalis. Our analy sis reveals that there is overall conservation of Xist gene structure both between different vole species and relative to mouse and human Xist/XIST. W ithin transcribed sequence, there is significant conservation over five sho rt regions of unique sequence and also over Xist-specific tandem repeats. T he majority of unique sequences, however, are evolving at an unexpectedly h igh rate. This is also evident from analysis of Ranking sequences, which re veals a very high rate of rearrangement and invasion of dispersed repeats. We discuss these results in the context of Xist gene function and evolution .