Characterization of the genomic Xist locus in rodents reveals conservation of overall gene structure and tandem repeats but rapid evolution of unique sequence.

The Xist locus plays a central role in the regulation of X chromosome inactivation in mammals, although its exact mode of action remains to be elucidated. Evolutionary studies are important in identifying conserved genomic regions and defining their possible function. Here we report cloning, sequence analysis, and detailed characterization of the Xist gene from four closely related species of common vole (field mouse), Microtus arvalis. Our analysis reveals that there is overall conservation of Xist gene structure both between different vole species and relative to mouse and human Xist/XIST. Within transcribed sequence, there is significant conservation over five short regions of unique sequence and also over Xist-specific tandem repeats. The majority of unique sequences, however, are evolving at an unexpectedly high rate. This is also evident from analysis of flanking sequences, which reveals a very high rate of rearrangement and invasion of dispersed repeats. We discuss these results in the context of Xist gene function and evolution.

Comparative chromosome and mitochondrial DNA analyses and phylogenetic relationships within common voles (Microtus, Arvicolidae).

The four species of common voles within the genus Microtus--M. kirgisorum, M. transcaspicus, M. arvalis, and M. rossiaemeridionalis--are so closely related that neither morphological features nor paleontological evidence allow clarification of their phylogeny. Analysis of vole karyotypes and mitochondrial DNA sequences, therefore, is essential for determining their phylogenetic relationships. A comparison of high resolution GTG-banding patterns allows us to ascertain the similarity between the karyotypes of these species, revealing that they are composed of rearrangements of the same chromosomal elements. Based on this analysis, we propose possible routes of chromosomal divergence involved in speciation within this group of voles and construct a phylogenetic tree of their karyotypes. We suggest that two different karyotypic variants existed during the course of vole evolution--one resulting in M. rossiaemeridionalis and M. transcaspicus, the other, M. kirgisorum and M. arvalis. As an alternative approach FITCH and KITSCH computer programs were used to construct a phylogenetic tree of vole molecular evolution based on a pairwise comparison of mitochondrial cytochrome b sequences and the divergence time of the species was determined. The correlation between the trees constructed using karyologic and molecular approaches is discussed in the context of other available data.

Restriction endonuclease analysis of highly repetitive DNA as a phylogenetic tool.

Multiple band patterns of DNA repeats in the 20-500-nucleotide range can be detected by digesting genomic DNA with short-cutting restriction endonucleases, followed by end labeling of the restriction fragments and fractionation in nondenaturing polyacrylamide gels. We call such band patterns obtained from genomic DNA "taxonprints" (Fedorov et al. 1992). Here we show that taxonprints for the taxonomic groups studied (mammals, reptiles, fish, insects-altogether more than 50 species) have the following properties: (1) All individuals from the same species have identical taxonprints. (2) Taxonprint bands can be subdivided into those specific for a single species and those specific for groups of closely related species, genera, and even families. (3) Each restriction endonuclease produces unique band patterns; thus, five to ten restriction enzymes (about 100 bands) may be sufficient for a statistical treatment of phylogenetic relationships based on polymorphisms of restriction endinuclease sites. We demonstrate that taxonprint analysis allows one to distinguish closely related species and to establish the degree of similarity among species and among genera. These characteristics make taxonprint analysis a valuable tool for taxonomic and phylogenetic studies.

Variations in the expression of the gene Pgd due to the effect of chromosomal rearrangements in Drosophila melanogaster.

The effects of chromosomal rearrangements on the expression of the gene Pgd coding for 6-phosphogluconate dehydrogenase (PGD) was studied in D. melanogaster. Of 21 chromosomal rearrangements examined, 2 produced complete loss of PGD activity, 10 markedly decreased it, 3 slightly increased it, and 6 had no appreciable effect. The effect of some rearrangements can be restricted to the larval stage. The results of histochemical staining and the quantitative analysis of the electrophoretic PGD patterns allowed us to identify chromosomal rearrangements whose inhibition of PGD activity was due to mosaic expression of the Pgd gene. The rearrangements whose effects are variegated represented 50% of those decreasing the expression of the marker gene or about 25% of those we considered.