Nucleotide sequence and genomic organization of a tandem satellite array from the rock vole Microtus chrotorrhinus (Rodentia)

Expansion of rDNA and pericentromere satellite repeats in the genomes of bank voles Myodes glareolus exposed to environmental radionuclides

Altered copy number of certain highly repetitive regions of the genome, such as satellite DNA within heterochromatin and ribosomal RNA loci (rDNA), is hypothesized to help safeguard the genome against damage derived from external stressors. We quantified copy number of the 18S rDNA and a pericentromeric satellite DNA (Msat-160) in bank voles (Myodes glareolus) inhabiting the Chernobyl Exclusion Zone (CEZ), an area that is contaminated by radionuclides and where organisms are exposed to elevated levels of ionizing radiation. We found a significant increase in 18S rDNA and Msat-160 content in the genomes of bank voles from contaminated locations within the CEZ compared with animals from uncontaminated locations. Moreover, 18S rDNA and Msat-160 copy number were positively correlated in the genomes of bank voles from uncontaminated, but not in the genomes of animals inhabiting contaminated, areas. These results show the capacity for local-scale geographic variation in genome architecture and are consistent with the genomic safeguard hypothesis. Disruption of cellular processes related to genomic stability appears to be a hallmark effect in bank voles inhabiting areas contaminated by radionuclides.

Complete Mitochondrial Genome of Three Species of the Genus Microtus (Arvicolinae, Rodentia)

The 65 species of the genus Microtus have unusual sex-related genetic features and a high rate of karyotype variation. However, only nine complete mitogenomes for these species are currently available. We describe the complete mitogenome sequences of three Microtus, which vary in length from 16,295 bp to 16,331 bp, contain 13 protein-coding genes (PCGs), two ribosomal RNA genes, 22 transfer RNA genes and a control region. The length of the 13 PCGs and the coded proteins is the same in all three species, and the start and stop codons are conserved. The non-coding regions include the L-strand origin of replication, with the same sequence of 35 bp, and the control region, which varies between 896 bp and 930 bp in length. The control region includes three domains (Domains I, II and III) with extended termination-associated sequences (ETAS-1 and ETAS-2) in Domain I. Domain II and Domain III include five (CSB-B, C, D, E and F) and three (CSB-1, CSB-2, and CSB-3) conserved sequence blocks, respectively. Phylogenetic reconstructions using the mitochondrial genomes of all the available Microtus species and one representative species from another genus of the Arvicolinae subfamily reproduced the established phylogenetic relationships for all the Arvicolinae genera that were analyzed.

Structural and Evolutionary Relationships in the Giant Sex Chromosomes of Three Microtus Species

The genus Microtus has high karyotypic diversity. The existence of notable differences in the length of its sex chromosomes contributes to this variation. Variations in size are attributed to the enlargement of their heterochromatin content, which is of such magnitude in some species that they are referred to as "giant sex chromosomes". Here, we perform an intra- and interspecific analysis of the molecular composition of the heterochromatic blocks in three species with giant sex chromosomes (Microtus chrotorrhinus, M. cabrerae and M. agrestis). Our results show that the heterochromatic content is very similar in both the X and Y chromosomes of M. chrotorrhinus, and that their molecular composition is more closely related to the heterochromatic blocks of M. agrestis than to the sex heterochromatin of M. cabrerae; however, species-specific differences do clearly exist. Interestingly, the euchromatic regions of the X chromosome of all three of these species share a homologous region composed of heterochromatic-related sequences. Our results therefore reinforce the idea that certain similarities in the original organization of these X chromosomes could have facilitated their later enlargement.

Epigenetic modifications in sex heterochromatin of vole rodents

The genome of some vole rodents contains large blocks of heterochromatin coupled to the sex chromosomes. While the DNA content of these heterochromatic blocks has been extensively analyzed, little is known about the epigenetic modifications controlling their structure and dynamics. To better understand its organization and functions within the nucleus, we have compared the distribution pattern of several epigenetic marks in cells from two species, Microtus agrestis and Microtus cabrerae. We first could show that the heterochromatic blocks are identifiable within the nuclei due to their AT enrichment detectable by DAPI staining. By immunostaining analyses, we demonstrated that enrichment in H3K9me3 and HP1, depletion of DNA methylation as well as H4K8ac and H3K4me2, are major conserved epigenetic features of this heterochromatin in both sex chromosomes. Furthermore, we provide evidence of transcriptional activity for some repeated DNAs in cultivated cells. These transcripts are partially polyadenylated and their levels are not altered during mitotic arrest. In summary, we show here that enrichment in H3K9me3 and HP1, DNA demethylation, and transcriptional activity are major epigenetic features of sex heterochromatin in vole rodents.

Molecular and physical characterization of the complex pericentromeric heterochromatin of the vole species Microtus thomasi

A new repeated DNA from Microtus thomasi, Mth-Alu2.2, was cloned and characterized and is presented here for the first time. Digestion of genomic DNA from M. thomasi with AluI restriction enzyme revealed a 2.2-kb repetitive DNA sequence with a high AT content (69%). This sequence consists of a tandemly repeated nonanucleotide of the consensus sequence CACAATGTA, which constitutes approximately 93-95% of the total unit length. The location of the Mth-Alu2.2 sequence in the karyotype was determined by FISH, demonstrating strong hybridization signals in the pericentromeric regions of all chromosomes and in the heterochromatin blocks of several X chromosome variants. In addition, the distribution of the 4 pericentromeric repeat sequences Msat-160, Mth-Alu900, Mth-Alu2.2, and interstitial telomeric repeats was analyzed by in situ hybridization in M. thomasi, in order to shed light on the complex composition of the chromosomal pericentromeric regions in this species. The order and organization of these sequences in the pericentromeric regions are conserved, with slight variations in both the degree of overlapping and the amount of each repeated DNA in the chromosomes. Specifically, Mth-Alu2.2 is localized in the terminal regions of the chromosomes, with Msat-160 occupying the immediately inner region, partially intermixed with Mth-Alu2.2. The sequence Mth-Alu900 is found in internal positions below Msat-160, and the interstitial telomeric repeats are located close to the long-arm euchromatin of the chromosomes.

Evolutionary story of a satellite DNA from Phodopus sungorus (Rodentia, Cricetidae)

With the goal to contribute for the understanding of satellite DNA evolution and its genomic involvement, in this work it was isolated and characterized the first satellite DNA (PSUcentSat) from Phodopus sungorus (Cricetidae). Physical mapping of this sequence in P. sungorus showed large PSUcentSat arrays located at the heterochromatic (peri)centromeric region of five autosomal pairs and Y-chromosome. The presence of orthologous PSUcentSat sequences in the genomes of other Cricetidae and Muridae rodents was also verified, presenting however, an interspersed chromosomal distribution. This distribution pattern suggests a PSUcentSat-scattered location in an ancestor of Muridae/Cricetidae families, that assumed afterwards, in the descendant genome of P. sungorus a restricted localization to few chromosomes in the (peri)centromeric region. We believe that after the divergence of the studied species, PSUcentSat was most probably highly amplified in the (peri)centromeric region of some chromosome pairs of this hamster by recombinational mechanisms. The bouquet chromosome configuration (prophase I) possibly displays an important role in this selective amplification, providing physical proximity of centromeric regions between chromosomes with similar size and/or morphology. This seems particularly evident for the acrocentric chromosomes of P. sungorus (including the Y-chromosome), all presenting large PSUcentSat arrays at the (peri)centromeric region. The conservation of this sequence in the studied genomes and its (peri)centromeric amplification in P. sungorus strongly suggests functional significance, possibly displaying this satellite family different functions in the different genomes. The verification of PSUcentSat transcriptional activity in normal proliferative cells suggests that its transcription is not stage-limited, as described for some other satellites.

Rapid, independent, and extensive amplification of telomeric repeats in pericentromeric regions in karyotypes of arvicoline rodents

The distribution of telomeric repeats was analyzed by fluorescence in situ hybridization in 15 species of arvicoline rodents, included in three different genera: Chionomys, Arvicola, and Microtus. The results demonstrated that in most or the analyzed species, telomeric sequences are present, in addition to normal telomeres localization, as large blocks in pericentromeric regions. The number, localization, and degree of amplification of telomeric sequences blocks varied with the karyotype and the morphology of the chromosomes. Also, in some cases telomeric amplification at non-pericentromeric regions is described. The interstitial telomeric sequences are evolutionary modern and have rapidly colonized and spread in pericentromeric regions of chromosomes by different mechanisms and probably independently in each species. Additionally, we colocalized telomeric repeats and the satellite DNA Msat-160 (also located in pericentromeric regions) in three species and cloned telomeric repeats in one of them. Finally, we discuss about the possible origin and implication of telomeric repeats in the high rate of karyotypic evolution reported for this rodent group.

Characterization of the satellite DNA Msat-160 from species of Terricola (Microtus) and Arvicola (Rodentia, Arvicolinae)

In the subfamily Arvicolinae (Cricetidae, Rodentia) the satellite DNA Msat-160 has been so far described in only some species from the genus Microtus and in one species from another genus, Chionomys nivalis. Here we cloned and characterized this satellite in two new arvicoline species, Microtus (Terricola) savii and Arvicola amphibius (terrestris). We have also demonstrated, by PCR and FISH, its existence in the genomes of several other species from both genera. These results suggest that Msat-160 already occurred in the common ancestor of the four genera/subgenera of Arvicolinae (Microtus, Chionomys, Arvicola, and Terricola). In Arvicola and Terricola, Msat-160 showed the basic monomer length of 160 bp, although a higher-order repeat (HORs) of 640 bp could have been probably replacing the original monomeric unit in A. a. terrestris. Msat-160 was localized by FISH mostly on the pericentromeric regions of the chromosomes, but the signal intensity and the number of carrier chromosomes varied extremely even between closely related species, resulting in a species-specific pattern of chromosomal distribution of this satellite. Such a variable pattern most likely is a consequence of a rapid amplification and contraction of particular repeats in the pericentromeric regions of chromosomes. In addition, we proposed that the rapid variation of pericentromeric repeats is strictly related to the prolific species radiation and diversification of karyotypes that characterize Arvicolinae lineage. Finally, we performed phylogenetic analysis in this group of related species based on Msat-160 that results to be in agreement with previously reported phylogenies, derived from other molecular markers.

Characterization of the satellite DNA Msat-160 from the species Chionomys nivalis (Rodentia, Arvicolinae)

The satellite DNA Msat-160 has been previously characterized in several species of the genus Microtus. Here we present the characterization of Msat-160 from Chionomys nivalis, a species with a very primitive karyotype. As in other Microtus species analyzed, C. nivalis Msat-160 is AT rich, has a monomer length of 160 bp, is undermethylated and is mainly located in all the pericentromeric heterochromatin of all autosomes and the X chromosome, but is completely absent from the Y chromosome. Hence, our results support the hypothesis that Msat-160 was initially distributed in the pericentromeric heterochromatin of all autosomes and the X chromosome. The taxonomic status of the genus Chionomys in relation to the genus Microtus is a very interesting issue, so we constructed phylogenetic dendrograms using Msat-160 sequences from several Microtus species. Although the results were not informative about this issue, the presence of Msat-160 in C. nivalis and Microtus species suggested that both genera are closely related and that this satellite DNA was present in the common ancestor. Studies of Msat-160 in different arvicoline species could help to determine the origin of this satellite and, perhaps, to establish the phylogenetic relationships of some arvicoline groups.

New families of site-specific repetitive DNA sequences that comprise constitutive heterochromatin of the Syrian hamster (Mesocricetus auratus, Cricetinae, Rodentia)

We molecularly cloned new families of site-specific repetitive DNA sequences from BglII- and EcoRI-digested genomic DNA of the Syrian hamster (Mesocricetus auratus, Cricetrinae, Rodentia) and characterized them by chromosome in situ hybridization and filter hybridization. They were classified into six different types of repetitive DNA sequence families according to chromosomal distribution and genome organization. The hybridization patterns of the sequences were consistent with the distribution of C-positive bands and/or Hoechst-stained heterochromatin. The centromeric major satellite DNA and sex chromosome-specific and telomeric region-specific repetitive sequences were conserved in the same genus (Mesocricetus) but divergent in different genera. The chromosome-2-specific sequence was conserved in two genera, Mesocricetus and Cricetulus, and a low copy number of repetitive sequences on the heterochromatic chromosome arms were conserved in the subfamily Cricetinae but not in the subfamily Calomyscinae. By contrast, the other type of repetitive sequences on the heterochromatic chromosome arms, which had sequence similarities to a LINE sequence of rodents, was conserved through the three subfamilies, Cricetinae, Calomyscinae and Murinae. The nucleotide divergence of the repetitive sequences of heterochromatin was well correlated with the phylogenetic relationships of the Cricetinae species, and each sequence has been independently amplified and diverged in the same genome.

A repeat DNA sequence from the Y chromosome in species of the genus Microtus

In most mammals, the Y chromosome is composed of a large amount of constitutive heterochromatin. In some Microtus species, this feature is also extended to the X chromosome, resulting in enlarged (giant) sex chromosomes. Several repeated DNA sequences have been described in the gonosomal heterochromatin of these species, indicating that it has heterogeneous and species-specific composition and distribution. We have cloned an AT-rich, 851-bp long, repeated DNA sequence specific for M. cabrerae Y chromosome heterochromatin. The analysis of other species of the genus Microtus indicated that this sequence is also located on the Y chromosome (male-specific) in three species (M. agrestis, M. oeconomus and M. nivalis), present on both Y and X chromosomes and on some autosomes in M. arvalis and absent in the genome of M. guentheri. Our data also suggest that the mechanism of heterochromatin amplification operating on the sex chromosomes could have been different in each species since the repeated sequences of the gonosomal heterochromatic blocks in M. cabrerae and M. agrestis are different. The absence of this sequence in the mouse genome indicates that its evolutionary origin could be recent. Future analysis of the species distribution, localization and sequence of this repeat DNA family in arvicolid rodent species could help to establish the unsolved phylogenetic relationships in this rodent group.

Isolation, cloning and characterization of two major satellite DNA families of rabbit (Oryctolagus cuniculus)

We report here the isolation, cloning and characterization of two abundant centromeric satellite sequences (Rsat I and Rsat II) what are not related to each other, and that of a divergent subfamily (Rsat IIE) of rabbit (Oryctolagus cuniculus). The Rsat I monomers had a 375 base pair (bp) average length, while repeat units Rsat II and Rsat IIE were approximately 585 bp long. Variable amounts of Rsat I were detected by FISH at the centromeric region of 11 chromosome pairs of the complement. Rsat II hybridized to the centromere of 12 different chromosomes, and two of these were labeled also with the Rsat IIE probe. Two-color in situ hybridizations with the satellite probes and rDNA revealed that the NOR chromosomes carried different satellites. Rsat I was abundant on chromosome 20 and 21, but it was undetectable on chromosomes 13 and 16. Large Rsat II arrays were found on chromosomes 16, 20 and 21, but reduced amount was detected on chromosome 13. The variant Rsat IIE was prominent on chromosome 16, but was absent from the other rDNA-bearing chromosomes. The rDNA signal on chromosome 21 was localized to the 21q(ter) region, what can be a useful cytological marker in comparative cytological studies. These data show that rabbit chromosomes form at least four distinct groups based on the satellite composition of their centromeres. The differences in the chromosomal distribution of satellite families will help easy FISH identification of individual chromosomes, as well as to unveil the evolutionary history of the Leporidae karyotype.

Chromosomal localization of six repeated DNA sequences among species of Microtus (Rodentia)

C-banding and fluorescence in situ hybridization (FISH) document the distribution of constitutive heterochromatin and six highly repeated DNA families (MSAT2570, MSAT21, MSAT160, MS2, MS4 and STR47) in the chromosomes of nine species of Microtus (M. chrotorrhinus, M. rossiaemeridionalis, M. arvalis, M. ilaeus, M. transcaspicus, M. cabrerae, M. pennsylvanicus, M. miurus and M. ochrogaster). Autosomal heterochromatin is largely centromeric and contains different repeated families in different species. Similarly, large C-band positive blocks on the sex chromosomes of four species contain different repeated DNAs. This interspecific variation in the chromosomal distribution and copy number of the repeats suggests that a common ancestor to modern species contained most of the repetitive families, and that descendant species selectively amplified or deleted different repeats on different chromosomes.

Comparative analysis of the MSAT-160 repeats in four species of common vole (Microtus, Arvicolidae)

The highly repeated tandemly arranged satellite DNA from the MSAT-160 family has been studied in 4 species of common vole (the Microtus arvalis group). All the monomer units analysed were classified into 4 subfamilies on the basis of similar nucleotide substitutions. The first 3 subfamilies do not show any species specificity since they combine monomers from several of the vole species examined; the fourth subfamily contains monomer units with substitutions specific to M. arvalis. Certain monomers of different species within the first 3 subfamilies display additional identical substitutions, making them more similar. Despite considerable similarity in monomer sequence within the subfamilies, specific features were found for each of the 4 species. A specific ratio of each type of monomer belonging to the corresponding subfamilies is typical of each species. In addition, the genomes of common vole species differ in the abundance of the MSAT-160 DNA, its pericentromeric location and organization. The mechanisms possibly involved in the evolution of the common vole MSAT-160 sequences are discussed.

Molecular and cytogenetic characterization of highly repeated DNA sequences in the vole Microtus cabrerae

The genus Microtus presents several species with extremely large sex chromosomes that contain large blocks of constitutive heterochromatin. Several cytogenetic and molecular studies of the repetitive sequences in species of the genus Microtus have demonstrated that the heterochromatin is highly heterogeneous. We have cloned and characterized a family of repetitive DNA sequences from M. cabrerae, a species with large heterochromatic blocks on the giant sex chromosomes. These repetitive sequences are 65.84% A-T rich, organized in tandem, with a 161-bp unit and are located on the centromeric region of autosomes and the X chromosome. In addition, this repetitive DNA is located throughout the entire heterochromatic block of the X chromosome and on three interstitial bands in the heterochromatic block of the Y chromosome. Comparative analysis of this family of repetitive sequences from three Microtus species revealed that the development of these sequences has occurred by concerted evolution. Our results support the hypothesis that the heterochromatic blocks from the sex chromosomes of different species are evolving independently and they probably have the genetic capacity to amplify and retain different satellite DNAs. For a topic related to the location of these repetitive DNA sequences on the Y chromosome of M. cabrerae, we propose a model to explain the origin of a length polymorphism previously described for this chromosome.

Retrieval of four adaptive lineages in duiker antelope: evidence from mitochondrial DNA sequences and fluorescence in situ hybridization

Independent molecular markers (mitochondrial DNA sequences from two genes and fluorescence in situ hybridization with satellite DNA sequences as hybridization probes) were employed to investigate phylogenetic relationships among duiker antelope. When analyzed singly or taken together, the molecular and cytogenetic data allowed for the delimitation of four adaptive groups: the conservative dwarfs which are basal, a savanna specialist which groups apart from the forest duikers, the giant duikers, and the red duikers. Within the latter, a further subdivision comprising an east African and a west African red duiker clade is evident. The placement of the endangered zebra duiker and Aders' duiker remains problematic. Several of the nomenclatural divisions in current use are questioned by our results. These include the recognition of Philantomba as genus name for the blue and Maxwell's duiker and that Harvey's duiker be relegated to a subspecies of the Natal red duiker. We place our results in a biogeographic context and argue that duiker speciation has been driven predominantly by habitat fragmentation which probably led to the disruption of gene flow between geographic populations.

Isolation of a species-specific satellite DNA with a novel CENP-B-like box from the North African rodent Lemniscomys barbarus

A species-specific satellite DNA (Lb-MspISAT) was isolated from the North African rodent Lemniscomys barbarus. This DNA is highly homogeneous in the sequence of different repeats and shows no internal repetitions. Filter and in situ hybridizations demonstrated that it is tandemly repeated at the centromeres of all chromosomes of the complement. A 19-bp CENP-B-like motif was found in Lb-MspISAT which conserves 12 of the 17-bp of the human CENP-B box, but only 5 of the 9-bp of the canonical sequence that is necessary to bind the CENP-B protein. Compared with the human CENP-B box, nucleotide substitutions and insertions increase the palindromic structure of this motif. The possibilities that it may be involved in centromeric function or in homogenization of the Lb-MspISAT sequence are discussed.

Repetitive DNA sequences in the common vole: cloning, characterization and chromosome localization of two novel complex repeats MS3 and MS4 from the genome of the East European vole Microtus rossiaemeridionalis

We have characterized two novel, complex, heterochromatic repeat sequences, MS3 and MS4, isolated from Microtus rossiaemeridionalis genomic DNA. Sequence analysis indicates that both repeats consist of unique sequences interrupted by repeat elements of different origin and can be classified as long complex repeat units (LCRUs). A unique feature of both repeat units is the presence of short interspersed repeat elements (SINEs), which are usually characteristic of the euchromatic part of the genome. Comparative analysis revealed no significant stretches of homology in the nucleotide sequences between the two repeats, suggesting that the repeats originated independently during the course of vole genome evolution. Fluorescence in situ hybridization analysis demonstrates that MS3 and MS4 occupy distinct domains in the heterochromatic regions of the sex chromosomes in M. transcaspicus and M. arvalis but collocalize in M. rossiaemeridionalis and M. kirgisorum heterochromatic blocks. The localization pattern of the repeats on the vole chromosomes confirms the independent origin of the two repeats and suggests that expansion of the heterochromatic blocks has occurred subsequent to speciation.

Organization and chromosomal localization of a B1-like containing repeat of Microtus subarvalis

A repetitive DNA sequence, MS2, was isolated from EcoRI-digested genomic DNA of the vole, Microtus subarvalis. The fragment was cloned and sequenced. Sequence analysis of this 1194-bp fragment revealed a 156-bp region demonstrating a 55% homology with the mouse B1 repeat. The remaining MS2 sequence shows no significant homology with other known GenBank sequences. The results of in situ hybridization of MS2 on vole metaphase chromosomes indicate the fragment is confined to heterochromatin blocks of the sex chromosomes in all but one species (M. arvalis). Distribution of MS2 sequences provides evidence for heterogeneity of the giant heterochromatin blocks of the XY Chromosomes (Chrs) in voles, for the unique cluster-like localization of MS2 within these blocks.

Identification of highly conserved loci by genome painting

Fluorescence in situ hybridization was used to identify patterns of DNA similarity among the genomes of several rodent taxa. Total genomic or Cot-1 DNAs were used as hybridization probes against metaphase preparations across different taxonomic levels, including three species of Microtus (suborder Sciurognathi), three species of Microtus (suborder Sciurognathi), Mus musculus (suborder Sciurognathi) and Ctenomys steinbachi (suborder Hystricognathi). The hybridization patterns of Mus or Peromyscus (sciurognath) DNA to Mus metaphases, which were consistent with what is known of the satellite sequences in these species, demonstrated the efficacy of this approach for molecular cytogenetics and evolutionary biology. Additional hybridizations to chromosomes of Ctenomys or Microtus identified loci consisting of highly conserved DNA sequences. This approach has proved useful in investigating genome homologies across divergent rodent lineages. Chromosome microdissection can be used to characterize these regions further.

Rapid, localized amplification of a unique satellite DNA family in the rodent Microtus chrotorrhinus

A novel satellite DNA family (called MSAT-2570) was isolated and characterized from the rodent Microtus chrotorrhinus. With a length of 2,570 bp the repeat unit is among the largest yet reported in mammals and comprises a series of short direct and inverted repeats. These repeat motifs may prevent nucleosome formation or represent an endless source of genetic variation. Restriction enzyme digestion using the two pairs of isoschizomers HpaII/MspI and MboI/Sau3AI demonstrated tissue specific differences in satellite DNA methylation that may reflect variable chromatin conformation or differences in patterns of gene expression. The sex chromosomes of M. chrotorrhinus are usually large in size among mammals, comprising 15%-20% of the karyotype and containing large blocks of heterochromatin. In situ hybridization of the satellite DNA revealed chromosomal localization predominantly to sex chromosome heterochromatin. A survey of related rodents including three congeneric species also with giant sized sex chromosomes demonstrated that MSAT-2570 is present only in the genome of M. chrotorrhinus. However, another previously reported satellite DNA also isolated from M. chrotorrhinus has been shown to reside on sex chromosome heterochromatin in one of the other three species, indicating that these giant blocks of heterochromatin are complex in structure and comprise multiple, unrelated satellite DNA families.

Heterogeneity in the concerted evolution process of a tandem satellite array in meadow mice (Microtus)

The evolutionary history of a 160-bp tandem satellite array, originally described from Microtus chrotorrhinus and called MSAT-160, was examined in related species of arvicolid rodents by sequence analyses, quantitative dot blotting, and Southern blotting. Results indicate that MSAT-160 is present in 12 of the 20 species and subspecies of Microtus assayed, but not in species belonging to any of the eight other genera examined. DNA from each species containing MSAT-160 was digested with 12 restriction endonucleases and restriction patterns were obtained reflecting the variable extent of homogenization of any given variant in different species. For example, with MboI digestion, M. chrotorrhinus produced a type A ladder pattern where most monomers contain the restriction site, M. ochrogaster generated a type B pattern where most monomers lack the site, and M. agrestis yielded a pattern intermediate between the A and B types. Further, dot blotting revealed copy-number differences between species. These findings indicate that changes in the periodic structure and amount of satellite DNA have occurred since these species last shared a common ancestor. In addition, various species-specific patterns were documented, illustrating that mechanisms other than genome-wide homogenization, such as stochastic mutation, out-of-register crossing over, deletion, and random amplification also play a role in structuring tandem arrays. Stochastic mutation and homogenization rates in satellite DNA, levels of species diversity, and magnitudes of chromosomal divergence differ significantly in Microtus, Mus and Ctenomys, the three rodent lineages examined.